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perature of the cooled column is erroneously stated to In the chapter on the Observatory a very good account be the temperature of the air. A little more detail, es- is given of the transit circle and its accompanying errors ; pecially in connection with the suspension of the tube in but of course, without spherical trigonometry, many points Chapman Jones's boiling-point apparatus, would have of great importance with regard to the reduction of obbeen advisable.

servations have had to be omitted. The chapters on the Bunsen's method of calibrating a eudiometer is given, earth, sun's apparent motion and time, all contain lucid but none of the methods for utilizing the results to obtain and concise explanations, which are well illustrated by the volume at any point is mentioned.

figures showing the great circles involved. Many inThe indiscriminate use of both English and French teresting problems are worked out in the chapters on the units throughout the book does not seem to have any moon and eclipses, while that on the planets contains a advantage ; indeed, to give the dimensions of a tube as good account of the stationary points in their apparent “I millimetre in bore, . . .8 inches in length,” may be motion. practical enough, but it is hardly scientific.

The Distances of the Sun and Stars" is the title of the The meaning of expressions such as

“ water .

chapter that concludes the non-dynamical section, and in drawn back . .. by the partial vacuum," " liquid ” of it the interesting problems on finding the parallax of the a given “gravity," “ ammonia condensed in hydrochloric sun are discussed, together with the various results that acid," "the tension of aqueous vapour,” might be con- ensue from the aberration of light. veyed in language free from objection.

Coming now to the second part of the book, the rotaThe book is almost free from typographical errors; tion of the earth and the resulting consequences are first only two were noticed. On p. 227, cadmium sulphate dealt with, in which the proofs of the former are clearly is printed for cadmium sulphide, and the letter (a) described ; while many problems relating to pendulum should be replaced by (6) at the foot of p. 349.

oscillations, variation of gravity at different places, &c., The table of contents, referring to page and para- are fully expounded. The following two and concluding graph, and the index are particularly complete. A useful chapters are devoted wholly to the laws of universal appendix giving results of typical analysis, constants gravitation, and to the multiple applications to which for calculating results, &c., is added, and a list of works they are subjected. These chapters are perhaps the of reference is given in the introductions to the different best in the volume, and contain, of course, sonie most parts. The relative importance of different estimations is important problems, such as the determination of the indicated by difference in the type, and cross-references density of the earth, precession, tides, &c. are frequently introduced. These points alone go a long The examples and examination-papers, which are by way to indicate the pains taken by the authors to meet no means few in number, will be found to be both orithe wants of the student.

J. W. R. ginal and well selected ; and this is really important, for

a sound knowledge of this subject can be obtained only

by the continual practice of working them out. ASTRONOMY.

In conclusion, we may state that altogether the work

is one that is sure to find favour with students of astroElementary Mathematical Astronomy. By C. W. C.

Barlow and G. H. Bryan. “University Correspondence nomy, and will be found useful to those for wþom it is College Tutorial Series.” (London : W. B. Clive and volume that we have received which is published in this

especially intended. This is by no means 'the first Co., 1892)

Tutorial Series, and the present work is a good example 'HE task of writing a book on astronomy which shall of the excellent text-books of which it is

composed. enable a beginner to grasp all the fundamental principles and methods without entering into elaborate details of mathematics is by no means a light one. What

OUR BOOK SHELF. the authors have done, and we may say very successfully too, has been to strike a mean between the numerous non

Practical Fruit Culture. By J. Cheal, F.R.H.S., mathematical works and those which involve high mathe

Member of Fruit Committee, Royal Horticultural matics, using just enough to enable the reader, if he

Society, &c. Illustrated. (London: George Bell and

Sons, 1892.) wishes, to proceed to the more advanced treatises. To simplify matters further, all investigations which

WITHIN the last few years farmers and others have

often been advised to take to the cultivation of fruit, and require a knowledge of the elements of dynamics have there can be little doubt that much of the profit conbeen collated together at the end under the title of nected with fruit-growing-now absorbed by foreign dynamical astronomy, thus separating them from those traders-might, under certain conditions, be kept in the of descriptive astronomy, which only needs elementary hands of our own people. The most important of these geometry, trigonometry, and algebra. Some of the chief

conditions is that persons who think of devoting attention

to fruit culture shall obtain sound information about the properties of the ellipse which are of astronomical im- work they propose to undertake, and that at all the later portance are contained in the appendix, while for the stages of their enterprise they shall act under the guidance properties of the sphere an introductory chapter has been of trustworthy authorities. In the present little volume, inserted.

which forms one of Bell's “ Agricultural Series,” Mr. Cheal This summary will give an idea of the range over

has brought together, and carefully arranged, a great which the student will have to extend his mathematical

many facts which cannot fail to be of service to intelligent

fruit-growers. He begins with a chapter on the general abilities, and after all it is by no means an extensive one,

prospects of culture for profit. Then come chapters on considering the ground which this work covers.

the selection of soil and situation, the preparation of the

THE

ground, what to plant, planting, pruning, cost and returns Egypt. The rainfall was violent, as the sides of narrow ridges per acre, the renovating of old orchards, gathering, pack- of rock, which cannot have collected much, are grooved into ing, and distributing, storing and preserving, grafting, deep Autings all along; and the waterfalls from basins of only budding, and stocks. These chapters form the first part

one or two square miles, are wide and steeply cut. of the book. The second part consists of counsels on

(5) Rainfall

, and depression forming an estuary. Up to various subjects to private growers, and in the

third the

about 300 feet above the present Nile, remains of perfectly author deals with insect pests and disease. The work

horizontal beds of débris may be found clinging to the sides of

the ravines, which must be subaqueous deposits. In front of is essentially practical, and will tend to stimulate the

each of the ravines are foot-hills of débris, evidently washed out development of what ought to become a more and more

of the ravines, and deposited in an estuary. There cannot have important British industry.

been much current in the main valley, as these foot-hills extend

sometimes two miles outward ; yet there was some current, as Blowpipe Analysis. By J. Landauer. Authorized English they are always on the lower side. This appears to be in the

Edition. By James Taylor. Second Edition. (London : human age, from the rolled river paläolith which I found at Macmillan and Co., 1892.)

Esneb, and which cannot have belonged to a later time. These DR. LANDAUER'S work presents so much sound know

estuarine beds occur as far up as Tel el-Amarna or further. ledge in so compact a form that the fact of the English levels of foot-hills in the estuary. To this belong the chipped

(6) Rainfall, gradual and intermittent elevation, leaving various version of it having reached a second edition is in no way Aints of the débris beds in front of the ravine of the kings at surprising. The text, we are informed in the preface, has Thebes, as man was probably inhabiting that valley, for these to been completely revised, and new methods of approved have been washed out of it. The rainfall continued until the value have been incorporated. The author and the estuary was completely dried, as the watercourses have cut translator call especial attention to the fact that some down to the present Nile level. Nile mud began to be brought additional details of manipulation will be found of value down and deposited while the water was yet 30 feet above the by readers who are working up the subject without a present, either as a river or estuary. teacher.

(7) Complete desiccation, throughout the historic age. The roads marked out with stones on the plain at Tel el-Amarna in 1400 B.C. are only destroyed in the very lowest lines of the

watercourses. The ancient buildings in Egypt only show the LETTERS TO THE EDITOR.

effects of rare storms, and not of continued rain. The mud

deposits throughout this age are at an average rate of 4 inches [The Editor does not hold himself responsible for opinions ex- per century in the old bed of the Nile.

pressed by his correspondents. Neither can he undertake The sequence described here seems to be tolerably clear, to return, or to correspond with the writers of, rejected though much more detail needs to be filled in as yet, manuscripts intended for this or any other part of NATURE.

W. M. FLINDERS PETRIE. No notice is taken of anonymous communications.] Prehistory of Egypt.

Aphanapteryx in the New Zealand Region. THE evidences of denudation are so striking in the ravines of I SENT you a short note some weeks ago announcing the the Egyptian deserts, that I make the following notes, hoping discovery of a species of the Mauritian genus Aphanapte yrthat some trained geologist will be induced to do more for the which I had named Aphanapteryx hawkinsi-in the Chatham subject. The successive periods which I have noted are as Islands. I have just returned thence from a visit made exfollow :

pressly for the purpose of searching for further remains of this (1) Rainfall, before much excavation of the Nile bed; pro- bird, of which I received at first only the cranium. I have ducing an enormous river with rolled gravels and blocks up to been very fortunate in my search, and have procured severa) 3 feet long. It is unlikely that this was merely a beach, as the most perfect crania, with tibia and femora, which I have no gravels extend for many miles north and south ; nor would it be doubt belong to the same bird, as more than once I discovered estuarine, as the flow must have been rapid. The only parts of these bones in the immediate neighbourhood of the crania. The these beds that I have seen are on the tops of the hills dividing bones have been disinterred from the lower beds of a sandbank the Nile from the Fayum, which are entirely composed of these facing the shore. Some years ago, a great storm, followed, as I gravels. The great mass of these beds has been denuded away am told, by a tidal wave of great height, broke the Eurybiaby the later Nile and rainfall, leaving these ridges 200-300 feet protected shore bluff ; and now the wind, having carried off the above the present Nile.

upper bed of light-coloured sand into the lands behind, is (2) Rainfall and elevation. All over the desert plateau of continually planing down more and more the brown lower bed limestone, the strata of which are usually horizontal, there are in which these bones seem mostly to be entombed. They are sharp depressions and faults. These are usually of small area, in a most perfect state of preservation, and very complete, a quarter to half a mile across, with a drop of over 200 feet. though deprived of all their animal matter. of the Aphana. The strata are at the same level, and horizontal, on the opposite pteryx crania some are considerably larger and some much sides of these depressions, but are steeply curved and faulted smaller than A. Bræckei, the larger reaching to within a inch down into the hollows. The only cause seems to be the falling of 6 inches from the top of the slender arched upper mandible in of immense caverns in the limestone, for there is no trace of to the occiput. The tibia and femora vary in size corresponding thrust or upward folding anywhere. Such caverns must have to the differences in the crania ; but they present the main been produced by great rainfall, and high elevation to allow of characters of the bones figured by M. Milne-Edwards in his the water draining off at so low a level, below that of the “Oiseaux fossiles de la France." It may yet turn out that present Nile.

more than one species of Aphanapteryx inhabited the Chatham (3) Disturbances. It is impossible to suppose that the great Islands. It is very singular that, among the thousands of bones gravel beds of the Nile in period (1) were deposited along the that have been collected from different swamps, Maori middens, steep edge of a basin 400-500 feet deeper; hence the deep and caves in various parts of New Zealand, not a single bone of Fayum basin must have been depressed (as it can scarcely have this bird should have come to light. In one refuse heap from a been eroded) subsequently to the period (1), and probably during Moriori feast laid bare by the wind on the beach of Petre Bay, the same disturbances which shook down the strata into the I found several specimens of Aphanapteryx hawkinsi, along caverns of (2).

with crania and other bones of ducks, swan, sea-birds, (4) Great rainfall and elevation continued, during which the seals, whales, &c. Swan-bones were everywhere very present Nile bed has been eroded, and the ravines graved out in abundant in this brown sand bed; some of them indicating its sides, reaching back for many miles through solid rock. This birds equal in size to Chenopis atrata, others considerably was subsequent to (3), as the ravines were partly determined by exceeding it. In one very ancient midden the remains i the subsidences, and have cut through them. This was a long dug up consisted almost entirely of swan-bones, with the period to allow of 200-300 feet of rock to be cut out of the Nile | intermixture of only a few duck-bones. The Aphana peryr bed. The elevation was probably the same as in (3), as the | must, I think, be the wingless bird spoken of by the Morioris as rock bed of the Nile is at a great depth under deposits in Lower | Mehiriki, although those to whom the skull has been

shown fail to recognize the bird at all. They speak also the group which Mr. Coste labels as doubtfully pigmentary ; but of another flightless bird by the name of Mehunui. This for the present I have sufficiently trespassed upon your space. bird, one old and very intelligent Moriori informed me, was the

F. GOWLAND HOPKINS, same as the Maoris called Kakapo. Mr. Alexander Shand, Guy's Hospital, S.E., April 9. Gull Research Student, an old resident in the Chatham Islands, and the sole European living versed in Moriori customs and traditions, and capable of

“C.G.S. System of Units." speaking their language with Auency, also confirmed this information, and told me that the Kakapo (according to the Morioris) The new edition of Prof. Everett's “C.G.S. System of was very abundant in the islands p.ior to 1836. He himself in Units” contains, at the very beginning, two misleading statethe early days had seen their burrows often. I had observed, ments, based seemingly on a misapprehension of facts. In so while collecting, several Psittacine bones, and on learning this valuable a work, such errors are to be deplored. fact I felt sure that those I had picked up and packed away must Pp. xiii. and xiv, give an account of weighings made at the belong to Stringops. On my arrival here, however, I find so far International Bureau of Weights and Measures between certain that there are no Kakapo bones in the collection, ihe Psittacine standard pounds” and the international standard of mass. bones being the head and beaks of Nestor notabilis, the Kea. From the statement as given, it would be inferred that there I have as yet had time to do no more than run through the is room for doubt as to the relation between the British standard collection I have brought back ; but there appear to be in it of mass and the international kilogramme. several large Ralline tibiæ of species unknown to me. I am The real facts are, that the standard pounds were only looking forward to another opportunity of thoroughly exploring nominally " pounds”; they were standards with known correcthese interesting islands with more time at my disposal than i tions, which, however, have not been applied to the equivalents could afford on this occasion.

HENRY O. FORBES. given on p. xiv. Canterbury Museum, February 23.

The true relation of the Imperial pound to the international kilogramme is given in the Board of Trade Report of Proceedings under the Weights and Measures Act, 1884 (p. 4), according

to which the Imperial pound = 453-5924277 grammes. Pigments of Lepidoptera.

On p. 34 of "C.G.S. System of Units,” Mr. Chaney's results A LETTER of mine on the subject of butterfly pigments was pub- of the weight of a cubic inch of water are discussed, and the lished so recently in NATURE (December 31, 1891, p. 197) that I

conclusion is reached that Mr. Chaney's result differs by 0'00125 hesitate to ask for further space at the present time.

But the from the theoretical relation between volume and mass of water appearance of Mr. Perry Coste's articles, together with the tone in the metric system. of some remarks made by him at the close of the last article,

This result is obtained by comparing Mr. Chaney's results, lead me to venture upon a few words, partly in criticism of a without reduction to vacuo, with the mass of a cubic centimetre theory he advances, and partly (though this is less important) in of water. claim of priority, since Mr. Coste does not do me the honour

Mr. Chaney states that the standard air to which his result is to refer to my work on the subject.

reduced weighs o‘3077 grains per cubic inch. Therefore his The chief generalization which Mr. Coste bases upon his

result reduced to vacuo is: one cubic inch of water at 62° F. experiments is that which he terms the “reversion effect," --that weighs 252.286 + 0*308 = 252-594 grains. is to say, the production of yellows from reds by the action of

If we take the value for the thermal expansion of water, in acids, and the restoration of the former by neutralization and

terms of the hydrogen thermometer scale, as determined at the other means. The theory that he advances to explain these International Bureau, we find the density of water at 16°•667 C., phenomena-namely, that the red body acts as a base, and forms

= 62° F., referred to its maximum density = 0.998861. with acids salts which are yellow-is quite untenable. As I have

Using the equivalents I metre = 393700 inches, and I shown (Proceedings of the Chemical Society, June 1889; vide gramme = 15:43235639 grains, we obtain: one cubic inch of NATURE, vol. xl. p. 335), the soluble yellows are themselves

water at 62° F. weighs in vacuo 252·6045 grains ; while Mr. acid bodies of quite definite composition. Indeed, as far back as

Chaney found 252'594 as above given, a discrepancy of one 1871, Prof. Meldola called attention to the fact that the pigment part in 25,000 only, as compared with one part in 800, given by of G. rhamni was soluble in water, and showed that its aqueous

Prof. Everett. It is not clear from Mr. Chaney's statement solution had an acid reaction. Mr. Coste has worked with D.

whether his weight in air is against brass or other weights ; eucharis ; if he will dissolve the red pigment from the border

therefore the vacuum reduction above applied is uncertain by a of the hind wing of this insect in pure wa'er, he will find that

small amount.

O. H. TITTMANN. a yellow solution is the result, but that, if the solution be

Washington, D.C., March 10. evaporated to dryness, the solid residue os pigment is red once more; showing that there is either the question of hydration to MR. TITTMANN thinks the true relation is, without doubtconsider, or a weak combination of the yellow acid body with a base, which is dissociated in aqueous solution. At any rate, I

I pound 453'5924277 grammes. have obtained from this red pigment of eucharis a silver com- Prof. W. H. Miller determined it to be pound which contains a percentage of metal exactly equal to that from the pigment of G. rhamni.

I pound = 453-59265 grammes, In 1889 I was able to predict possible constitutional formulae which is the value given on the Card of Equivalents published for the acid yellow pigments, and am happy to say that recent

by the Board of Trade. If the determination quoted by Mr. careful combustions of their silver salts to a large extent confirm Tittmann from a Board of Trade Report of 1884 was made my original ideas. My results will be shortly published in under such conditions as to render it authoritative, it is a pity it extenso.

has been allowed to remain for eight years buried in a BlueMr. Perry Coste's experiments are very useful as forming a book. One would have expected some intimation of it to be method of classifying these lepidopterous pigments; but, if he given to scientific men through the Royal Society or in the pages will forgive me for saying so, they are of far too empirical a

of NATURE. nature for any consideratiuns as to ibe constitution of the bodies As regards the three "standard pounds" which were comto be based upon them. As one who has for many years past

pared with standards at the Bureau International in 1883, Mr. spent a large portion of his time and no inconsiderable portion Tittmann says they had known corrections. This is not stated of his substance in obtaining a sufficiency of these pigments for in the Travaux et Vémoires, where the account of the comanalysis and investigation, Mr. Coste will forgive me if I do not parison is given. There is, however, in the case of the two respond to his invitation to leave him is to continue his which are of gilded bronze, a reference to a description of them researches alone.” It is hardly well for one investigator to say by Prof. Miller in his paper on the standard pound (Phil. Trans. “hands off” to another, and I shall not imitate Mr. Coste in 1856), and, on turning to it, I find that their errors, as stated by this matter ; but will only express a hope that in his future work him, do not agree even approximately with the determinations he will not confine himself to the immersion of the wings of his made at the Bureau. They differ even in the first significant insects in strong and destructive reagents.

figure of the error, which is the sixth figure of the entire value; I have lately been working at the genesis of these pigments in

so that, as far as this evidence goes, the five figures 45359 are the pupæ, and might say something with regard to the nature of all that are certain.

As regards the other matter referred to, Mr. Tittmann does meteorological problems which deal with clouds and prenot mention the publication in which “Mr. Chaney states that cipitation, solar and terrestrial radiation, and in a general the standard air to which his result is reduced weighs 0-3077 way with the diurnal and annual variations in the temgrains per cubic inch.” The only publication known to me is

perature and pressure of the atmosphere. Mr. Aitken's Mr. Chaney's paper in the Proceedings of the Royal Society,

work in originating this branch of science, and in making and it does not contain any such statement.

and discussing numerous observations of the number of I have always been taught to regard a standard weight as a

dust particles in the air of various places in this country, standard of mass, and therefore independent of such conditions

as well as on the Continent, at various altitudes, is pretty as temperature, pressure, and the material in the other scale

well known already (see NATURE, vol. xli. p. 394). Mr. pan ; whereas, it appears that Mr. Chaney, by direction of the Board of Trade, has made a determination which is only true

Aitken's results and conclusions were looked upon as for a particular density of the surrounding air, and a particular being of such importance as to warrant some of our density of the weights in the other pan.

leading meteorologists to apply to the Research Fund of For scientific purposes a standard of reference should be free the Royal Society for a grant to enable them to equip from variable elements, and should be of the utmost attainable the Ben Nevis Observatory with Aitken's dust-counting simplicity. For commercial purposes seterminations to six apparatus. The application was successful, and instrufigures are frivolous.

ments were at once ordered, and in due time erected at Mr. Tittmann's reductions appear to contain two errors. the Observatory. Instead of adding the weight of a cubic inch of air, he ought to

The apparatus consists of two dust counters, one a have added the difference between this and the weight of the air

portable form for use in the open air, and the other a displaced by the weights in the opposite pan. Again, he takes

laboratory form for use inside the Observatory. The the metre as 39'3700 inches, whereas Clarke's value is 39'370432, and Kater's 39-37079.

latter is fixed in the middle room of the tower, and has I have had some correspondence with Mr. Chaney since the pipes leading out to the free air, so that it is possible to publication of my new edition, and have had an erratum slip

observe with it in almost all sorts of weather and at any printed, which I trust you will allow me to subjoin, as it may hour day or night. The principle on which these instrube useful to several of your readers J. D. EVERETT. ments are constructed, so as to make the tiny invisible 5 Princess Gardens, Belfast, March 28.

particles of dust visible and easily countable, is pretty Addenda and Corrigenda.

well known already. Briefly it is this. To make the par

ticles visible, the air containing them is saturated with Page 63. In reducing Cailletet's experiments, .000 0026

water vapour, and by a stroke of an air-pump it is thereshould have been added instead of 000 0039. Page 77. Add-Violle's determination of velocity of sound

after cooled so much as to cause a condensation of the is 331 -10 + 01. Ann, de Chim. XIX. March, 1890.

vapour on the particles, whereby these are thus made Page 176, line 10. For Wuilleumeier, 1890, read Wuilleumier,

visible. Ordinary air is so dusty that if the receiver were 1890, Lippmann method.

full of such air it would be impossible to count the parAt end of page 164, add-Expressing C in amperes, R in ticles, and to make them easily countable the following ohms, and T in seconds, the heating effect in gramme-degrees process is resorted to. First, the chamber or receiver, is CoRT/4'2. 24CʻRT.

whose capacity is accurately known, is filled with pure Page 35. Mr. Chaney's determination here quoted was not in- dustless air by means of an air-pump and filter. Then tended as a determination of the density of water, but of the a fifth, a tenth, or any other fractional part of the amount apparent weight of water when weighed in air of density

of pure air inside is taken out, and the same amount ‘0021684 against brass weights of density 8'143. The cor- of dusty air allowed in. In this way the density of the recting factor for deducing the weight in vacuo or true density is shower caused by condensation is completely under the I'OOL 0687, which will change the value '998 752 obtained in

observer's control. A small graduated stage is placed the text into '999 82, to compare with Tralles' .999 88. Mr. Chaney's result is for distilled water deprived of air, and

one centimetre from the top of the receiver, so that all Tralles' appears to be for ordinary distilled water. According

the dust above this falls on to it, and by means of a to results recently obtained by the Vienna Standards Commis. magnifying glass all the particles on one or more of the sion (Wied. Ann., 1891, Part 9, p. 171), water deprived of air small squares of the stage are easily counted. Then, by has the greater density, the difference being '000 0032 at o° C., multiplying by the reciprocals of the various fractions and .000 0017 at 62° F. These differences are too small to affect used we arrive at the number of dust particles in a unit the above comparison.

of the free dusty air. In making an observation, the mean

of ten such tests is taken as the number of particles Influenza in America.

present for that time. In my copy of “ Johnson's Dictionary of the English Language

Observations were begun at the Ben Nevis Observatory in Miniature, to which are added an alphabetical account of the

with the portable instrument in February 1890, and with heathen deities and a copious chronological table of remarkable

the other instrument in the following summer. During events, discoveries, and inventions, by the Rev. Joseph Hamilton,

the whole of that year the work done was mainly preM.A., second American edition, Boston, 1806” (12mo, pp. 276), liminary, as great difficulty was experienced in getting I find on p. 275, “Influenza in North America, 1647, 1655, the dust work to fit into the general routine of Observatory 1697-98, 1732, 1737, 1747, 1756-57, 1761, 1772, 1781, 1789-90, work. The dust inquiry is not like some other special 1802.”

inquiries, that can be prosecuted for a certain time, and It is quite possible that these dates are well known, but they then discontinued after definite positive or negative are new to me, and may be of interest in connection with the

conclusions thereanent have been arrived at, but must, on recent epidemic.

EDWARD S. HOLDEN. Mount Hamilton, March 29.

the other hand, be carried on side by side with the other observations of meteorological phenomena, as pressure,

, temperature, humidity, &c., with any of which it is of DUST COUNTING ON BEN NEVIS.

equal importance, and having once been admitted into

the general routine of meteorological observations it WITHIN the last few years quite a new factor has must be kept on. This fact was soon seen on Ben Nevis

been introduced into the study of meteorology- from the extraordinary variations that were observed in namely, that which treats of the dust particles in the the dustiness of the air with changes of weather; and it atmosphere, of the number of dust particles present in was attempted to make continuous hourly observations of the air at any time, and the effect of dust in the air on the dust as of the other elements. It was found, however, climate and weather changes. It is now beginning to be that this could not be done without crippling the general recognized that the study of dust and its behaviour in the routine, this being as much as the two observers at the air forms the stepping-stone to the study of almost all Observatory could well cope with. In February 1891

Feb.

Means
Maxima ,

2,300

a system of three hourly observations of the dust par- Here there is a minimum indicated (526) at 4 a.m., and ticles was started, and this has been kept up with but few a maximum (1438) at 4 p.m. All the forenoon values are nterruptions since. The dust observation is made below the mean, and the evening values above it. It immediately after the usual hourly set is completed, and would appear that during the forenoon the summit of it can thus be studied along with all the other hourly | Ben Nevis is above the first or lowest cloud or dust records in their relation to the prevailing weather.

stratum. About noon this stratum rises to the level of A great many observations have in this way been the summit, and during the afternoon hovers above it, accumulated during the past two years, but we have not and falls again late at night. From this it might be had time for studying them in detail yet. A mere in- inferred that the summit is oftener clear of cloud in the spection, however, brings out some interesting points. One early morning, and oftener enveloped in the afternoon. of these is the enormous variation that is observed in the A table showing the number of times the top was clear number of dust particles, not only in the course of the during the last seven years shows that only about 30 per year, but often in the course of a few hours. At sea-level cent. is clear weather in which the summit is free from the number of dust particles in the air at any time depends fog ; but it does not show a daily variation as indicated very much on the locality and on the wind, whether by the dust values, what little it does show being quite blowing from a polluted district or not. The mean of a the reverse-namely, a maximum of clear weather in the number of observations made by Mr. Aitken at Kingair- middle of the day and a minimum at night. This clearly loch, in the west of Scotland, is 1600 particles per cubic indicates that when the dust layer falls below the summit centimetre. In London, on the other hand, he found at night, radiation at once forms an independent cap on 100,000 per cubic centimetre, and in Paris rather more. the hill-top; and again in the afternoon, although the dust On Ben Nevis the mean is 696 per cubic centimetre, the stratum envelopes the summit, the opposite radiation maximum being 14,400 per cubic centimetre, and on warms it up and prevents condensation, or rather evaposeveral occasions the minimum fell to o. A general rates the watery particles of the cloud. So that, contrary mean does not convey a fair idea of the dustiness of the to public opinion, the best time to visit the summit for the air at the mountain-top, although it may do so for places sake of the “ view” is the middle of the day, and not the at sea-level, because there is at the former place a great early morning. During fine settled weather the rise and daily range in the number of dust particles, depending on fall of this cloud stratum can be followed, but in average the rise and fall of the air past the place of observation. weather the effect of radiation completely masks it. The If there is any marked variation at sea-level it is entirely effect of solar radiation and nocturnal radiation on dust, of a different character. Below are the means, as well as as particles and as strata, is a problem that has to be the maxima and mimima, of all the months that have a studied and worked out. Very little is definitely known fairly representative number of observations.

about it at present. Number of Dust Particles per cubic centimetre on Ben Nevis.

In the study of the nature and motions of clouds the

dust observations will be of great value. When a fog 1890-91. Mar. 'April. May June July. Aug.

Nov.

envelopes the summit, the number of dust particles observed may vary greatly without any apparent change

in the thickness of the fog, but as a rule dry thick fog (1,515)' 1,037

(700) (588)| (606)
6,350 12,862 14,400
3,850

contains a great amount of dust, while thin wet mist conMinima ,

67

tains very little. It is when a thin drizzling mist en

velopes the summit that the lowest values are always The above table shows that the Ben Nevis air contains obtained, and then there is a distinct difference between most dust in spring, and it is probable that sea-level air the conditions at sea level and those at the summit, the is in this respect similar ; the cause of this greater amount

winds at the latter place differing in direction by 90° or of dust in spring than at any other time of the year being

more from the winds below, and sometimes the upper the great annual westward motion of the whole atmo

winds are blowing straight out from the centre of a shallow sphere, or at least of a considerable depth of it, at that time low-pressure area, and the dust that rises with the slight of the year. In a recent paper on “'The Winds of Ben ascending currents of the lower strata is almost entirely Nevis” (Trans. R.S.E., vol. xxxvi. p. 537), it has been

filtered out before reaching a height of 4400 feet. One of shown that this is one of the very few points in which the

Mr. Aitken's conclusions may briefly be put as follows: high- and low-level winds agree, viz. in the excess of Much wind, little dust ; much dust, little wind. That easterly winds in spring. The above means for summer

dust seems to accumulate in the quietest places is fully are probably too low, as that summer was exceptionally borne out by the Ben Nevis observations. This is true cold, and the general circulation was very abnormal, and not only horizontally, but also vertically, and it seems that in the direction which would tend to give low dust probable that this is what chiefly determines the position values. The maximum, 14,400, was observed at 1 p.m. on of cloud strata at all heights. And from this we may April 11, 1891 ; and, as an instance of how very much

infer that the motion of clouds is much slower than that the values change in a short time, at 8 a.m. that morning

of the general aërial currents; and again, since clouds the number was only 350 per cubic centimetre, and by tend to form between currents, and may have as direction midnight it had again fallen to 600 per cubic centimetre.

of motion the resultant of the directions of these currents, The daily variation is fairly well indicated from the it follows that as indices to the motions of the upper air three-hourly observations. For the months of March, the velocity and motion of clouds are very unsatisfactory. April, and May, 1891, the following are the means for the

Observations of the apparent haziness of the atmoeight hours of observation :

sphere are made whenever it is possible, and the relations

between the haziness of the air, the humidity, and the Number per cubic centimetre.

number of dust particles, have been found to be the same

as what Mr. Aitken pointed out. Briefly, he found that Hour.

Day.

with a constant humidity the haziness increased or

diminished with the number of dust particles, and with a Means. 526 1438

constant number of dust particles the haziness depended Difference

on the humidity (at least when the air was within 10 or 584

15 per cent. of saturation); for with increase of humidity Below, 118 284

the air became thicker, because apparently condensation

begins on the dust particles before the air reaches its Brackcted values are fo: 1890 only.

point of saturation.

418

1,757 4,940

4

1,286

4.000

O

3,150

12

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