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It might be supposed that the estimations would agree better when the angles of position are the same for both arcs compared together, than when they are different. But this supposition is not borne out by my observations; for after correcting them by the above formula, the average deviation from the truth in the case of the careful comparisons is 4-4 per cent, when the angle of position of both arcs on the retina is the same, or within 10° of the same; and 4-i per cent, when it differs more than 10"; while in the case of the instantaneous comparisons these numbers are 79 and 6'3 respectively.

When the lower arc is horizontal, or nearly so, it is (on the average) esiimated as being shorter than when in a vertical position, but the difference is so slight that it is doubtful whether it would not disappear with a larger number of observations. The best correction formula 1 have obtained for this is to apply the factor

(104 - 048 COi (/)

to the result already obtained: 1/ being the deviation of the lower arc from the horizontal. But the application of this factor only reduces the sum of the squares of the differences between calculation and observation in the case of the careful compai isons from 1163 to mi.

The angle of position of the upper arc seems to make no difference in the results. T. W. Backhouse.

West Hendon House, Sunderland, October 24.

Proper Motions of the Stars.

Miss Ci.erke, in her very interesting article (nature, vol. xliv. p. 572) on the motion of th: sun in space, seems to think that we have only the two alternatives of supposing that the biighlness of a star is independent of its distance, or that the motions of the stars increase with their distance. I suspect that, when the proper motions of all stars down to the 9th magnitude have been tabulated, the necessity of adopting either alternative will disappear. My object in writing this letter, however, is to call the attention of spectroscopists to the question thus raised. The spectroscope, whf n used in connection with a powerful telescope, ought to be able to show whether the fainter stars as a rule move more rapidly in the line of sight than the brighter ones; for if the average motion in the line of sight is the same in both cases, astronomers »ill be slow to accept an explanation of phenomena which supposes a different average velocity on the whole. But even instruments incapable of deciding this question may throw light on the subject. It now appears certain that if a Sirian and a solar star of the same mass were placed at the same distance from us, the Sirian star would appear more than one magnitude brighter. Hence, before we can use magnitudes as in any sense a test of distance, we must ascertain the relative proportion of Sirian and solar stars in the groups which we are comparing. It would also be very desirable that the magnitudes of the stars employed by Profs. Eastman, Boss, and Stumpe, should be photometrically determined. The photometer has at all events the advantage over the eye that its results are in all cases (allowing for errors of observation) comparable. W. II. S. Mu^CK.

Dublin, October 17.

California Foxes. In Nature of September 10 (p. 452), there is a paragraph in praise of the intelligence of the (English) fox, with examples in proof. Permit me to say that his California cousin is next door to a fool. My young son has amused himself for the past three summers in trapping (in large box-traps) the small California foxes which infest these mountain;, and which live on a mixed diet of Manzanita berries and astronomer's chickens. I pass over the fact that each trap has painted over its door " Danger to all who enter here ! ", and I proceed to show that our California foxes are barely one remove from idiots. When they are caught, my boy is in the habit of fastening a small leather collar about their necks, and of chaining them with light chains to stakes near the Observatory buildings. Many of them have escaped by parting the chains (by dint of strength, not of intelligence), and have been again caught within two or three days in the same traps! One of them was caught three times in quick succession! I presume an English fox, once caught, would emigrate to North Britain, or at least to the next county. My own ideas of the intelligence of the fox, until I came here, were derived from Goldsmith's "Animated Nature," and, generally, from English writings.

I have now becon.e s-atisfied that the California fox is airier/. Either the struggle for existence is not sharp here, or he has made up his mind that enistence is not worth struggling for.

Lick Observatory, October 8. Edward S. Holden.

A Plague of Small Frogs

My wine-cellar has been visited during the recent rains v. ith a curious plague of small frogs (Rana ttmporaria) all the same size, about one inch long. There would be nothing surprising in this visitation were it not for the apparent absence of any means of communication from outside, the level of which is six feet above that of the cellar ; there is no drain near that part of the house. There is a step up before you go down into the wine-cellar from the adjacent cellar, against which the door closes, leaving no crack any animzl so large could squeeze through. The cellar has solid stone walls and a bricked floor. During the recent floods the water stood some three or four inches deep there, apparently oozing through a tiny hole level with the floor on the outside wall, into which the point of a pencil could only penetrate for an inch. Even had it been possible for these little creatures to come in that way they must have burrowed down six feet from the outside level. Only one or two were found in the cellar adjacent, which is lighted by a grating into the garden, whereas in the wine-cellar two or three dozen were caught, many of them drowned by the flood.

Is it not unusual for bats to fly in the day-time? Here one has been doing so for two afternoons, coming out about 2.30, and flying backwards and forwards after insects in most brilliant sunshine. The gardener tells me he has never observe! them do so before ; and having sometimes caught them in the day-time, always noticed that when thrown into the air they would drop at once, and run instead of flying.

R. Hah; Thomas.

BOTANY OF THE EM IN RELIEF
EXPEDITION.

THE botanical exploration of Tropical Africa leaves so much to desire that it was somewhat disappointing to find that Mr. Stanley brought nothing back which would give any idea of the nature of the dense forests which he traversed. The conditions under which such an expedition is necessarily executed make naturalhistory-collecting extremely difficult. Travellers, however, often suppose that because they cannot make extensive collections they can do nothing to add to our knowledge. Yet to fill a small portfolio with well-selected and significant specimens is not a very difficult matter. And these may often furnish the basis of useful and important conclusions as to the general nature of the flora. Sir Joseph Hooker was able to give the first account of the vegetation of Kilimanjaro from a small parcel of plants collected by a missionary, the Rev. Mr. New, who was supplied for the purpose by Sir John Kirk, with "a bundle of old Guardians." An officer of the Ashanti Expedition brought from Comassi the fruit of what proved to be a new species of Dubosria. And quite lately Lord Lamington sent to Kew a small parcel of plants collected by himself in an expedition through the Shan States, which contained good specimens of an interesting plant only known previously from imperfect material collected by Griffith. It has now been worked out and figured in the Kew " Icones Plantarum."

Nor is it so difficult as it might be supposed to do even more than this. And I am not sure that a little careful and intelligent plant-collecting would not be a healthy and useful distraction to the tedium and strain of an arduous journey. Nothing could probably exceed the difficulties under which Joseph Thomson travelled in Masailand; yet he managed, notwithstanding, to get together a tolerably extensive and most valuable botanical collection. Upon this Sir Joseph Hooker was able to base the first attempt at a rational theory of the geographical relations of the high-level flora of Eastern Equatorial Africa. Nothing, again, could be more admirable than the collections made by Brigade-Surgeon Aitchison when attached to the Kuram Field Force under Sir Frederick Roberts in Afghanistan. And the Government of India has now arranged—and it is an indication of the sympathy for science which animates its members—that, as part of the organization of the Botanical Survey of India, a botanist shall for the future be .attached to all frontier expeditions.

Major Jephson, who accompanied Mr. Stanley, seems, however, to have had his eyes about him. A correspondent has sent me a copy of the October number of the Mayflower, a small monthly horticultural periodical published in New York, which contains (pp. 155, 156 a short paper by him on the "Plants of the I ark African Wilderness." This seems to me worth polling on record in the pages of Nature, where it will be at least more accessible for future reference. At my request, Mr. Baker, the Keeper of the Kew Herbarium, h»» had the paper annotated with such critical comments • • »ere possible.

To Major Jephson's paper Mr. Stanley has prefixed a brief introduction, which adds nothing of importance. He remarks :—

"In this branch of science I fancy we were all but amateurs, and considering what very little time any of us could devote from the engrossing business of marching, and seeking for food to sustain life, Mr. Jephson shows what might have been done by him had circumstances been more favourable."

This is, however, erring a little on the side of modesty. As I have already shown, amateurs can do very useful »'>rk without much difficulty, if they are content to do only a little, but to do that little carefully. Some further observations are open to more serious criticism :—

"Africa is yet too young and too crude for the scientific botanist. We hnve only been pioneers to stake the highway to make ready for those who shall come after us. When the rails have been laid in pairs of iron lines across the swamp and desert, and the engined boat cleaves the rel bosoms of the great rivers, and furrows the dead green face of the fresh-water seas, then the tender-nurtured botanist, conveyed from point to point without danger to his valuable life, may be trusted, wiih his enthusiasm and devotion, to bring to us results worthy of science and the age. Of those who have given us an insight into the botanic treasures of the African world, Schivernfuth (sic) is by far the best, but he has also laboured under such disadvantages and discomforts that he was not able to do for Equatorial Africa a tenth part of what Bates did for the Amazon.''

One cannot but wonder a little at the ignorance of the literature of African travel which this paragraph displays. Men like Grant, Speke, Kirk, Welwitsch, Mann, Vogel, Barter, and Thomson, to mention only a few of those to whom we owe our knowledge of the African flora, would have thought it comical to be described as "tendernurtured' botanists. The work of Schweinfurth was admirable; yet no one would, I think, be more surprised than that distinguished naturalist, Mr. Bates, to learn th*t the botanical collections which he never even professed to make, were ten times better.

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trees, lilies,1 and Begonias. There is, however, a great wealth of different kinds of ferns, such as I have often seen cultivated in hot-houses in England. In many places the damp ground was covered by a thick growth of filmy ferns and Lycopodium of the most beautiful description.

"Here is a short extract from my journal, which will give some idea of the every-day sights we saw on the banks of the Lower Congo, 1700 feet above the sea, and 250 miles distant from it:— '"' At the bottom of a piece of swampy ground I came to a small stream, on the banks of which were growing Osmunda regalis"- or Royal fern. It was slightly stunted in growth, being not more than 2 feet in height. It is the first I ever have yet seen in the tropics. Close by the stream was growing a group of beautiful ground orchids,' in form like a Hyacinthus candicans. There were clusters of great pink flowers with yellow centres: the whole had a very gorgeous effect. Here, also, was a profusion of Lycopodium.4 It is of a kind I have not yet seen ; it creeps up and over everything in great bluegreen masses; its long tendrils creep up the tree trunks like ivy, to a height, in some cases, of 4 feet. There were quantities, also, of the ribbon fern, exactly like the Davallia pentaphylla? which has been introduced into English hot-houses from the Malayan Arc lipelago What would not florists at home have given for an acre of this ground?'

"In the forest there were two kinds of lilies which were common. One, which grew in swampy ground, was in form like an Amaryllis.6 It was white, with a deep crimson centre, and had a delicious but heavy scent. The other was a lily,7 which grew everywhere through the whole length of the forest. It was of a brilliant scarlet colour, and was formed of several hundreds of small flowers, forming a round ball like a huge Guelder rcse, four inches in diameter. It was of such a brilliant scarlet that it looked almost metallic, growing in the darkest recesses of the forest. One of the commonest and most striking of all the ferns we saw was the Platycerium alciiorne? It is an extremely interesting fern, one of a singular genus of epiphytal plants, growing on the branches of trees. Our Zanzibaris called it 'elephant ear,' from its curious shape. There was another of the same family, Platycerium Stemmaria, which we found growing upon rocks in the open country. Both these ferns grew at altitudes from 1000 to 5000 feet. Treeferns" of the ordinary kind we found growing in all the gullies and streams on the slopes of the mountains above the Albert Nyanza. The altitude was from 5000 to 6000 feet above the level of the sea, and I noticed especially that the flora here was remarkably like that in the Central Province of Ceylon, which is an altitude of 2500 to 4000 feet above the sea.

"By far the most common plant which we saw in the jungle was the Amomum, or wild cardamom.10 It was almost precisely the same in form as the cardamom which is cultivated in Ceylon. It grew almost throughout the whole of Central Africa. It has a large purple flower, which grows in clusters on the ground at the root of the plant, and from it a bright scarlet fruit forms, of a pear shape, and about the size of a small fig. It is divided into four quarters, and contains some white, fleshy pulp, very juicy and acid. This pulp is full of

1 Qinum.

3 Osmund* rtgah's is cosmopolitan, but in tropical xone is high up only

1 Mr. Kolfe cannot suggest anything better than Litsochilus.

* Sflarinella icandtns, no dr.ubt.

5 "Ribbon fern" wuuld suggest Opkiogltntttm fcndulitm or Vittaria, but they are not like Davallia ptntaphylla.

6 Criniim seylanuHm. 7 Bruntvigia toxicari*.

* /'/<«/>'< triMmaUUtirmtxs not African, but F. Stemmaria is widely spread. 9 No doubt Cyatkta Tktuuwiii, Baker, which is very near C. Drtgti of

the Cape.

,,J There are a large number of Amomunu in West Tropical Africa. The fruits ire 3- not 4 celled. See A. lianie/lii. etc., in Oliver and Hanbury* rmper in Journ. t ion Soc , vii. 109.*

small black aromatic tasting seeds like those of the cultivated cardamom. If ever planters go into Africa, the cardamom will be an important product of the soil for commerce, for there are vast tracts of forest with the climate, soil, and checkered shade which are necessary for the cultivation of the cardamon. Orchilla weed should also become a valuable article of commerce; it grows in many parts of the forest. I consider, however, that when the great forest of Central Africa is opened up to civilization, by far the most valuable article of commerce will be india-rubber, the want of which is increasingly felt in the civilized world. Nowthat electricity is so much used for various purposes, the demand for india-rubber grows larger and larger: the supply which is shut up in the African forest is practically unlimited. There are various trees of the fig tribe which yield this product, but by far the greatest amount is contained in the india-rubber vines* which abound in the forest and hang from almost every tree. In cutting our way through the forest in some places, we got covered with the milky glutinous sap, which dropped upon us from the vines we cut through. “The natives know its value, and use it largely for smearing the inside of their buckets in order to make them hold \vater. They use it largely also for covering the ends of their drum-sticks. Theindia-rubber obtained is of a clear, yellowish colour, like glue, and is of the most elastic description. “ln the forest region I saw no water-lilies, but in Emin Pasha’s Province, in the Bari country, I saw two kinds? They were both about the size of an ordinary white water-lily, and the leaves and flowers floated on the surface of the water, but the stalks and formation of the leaves and flowers was finer and more slender. One was of a pink coral-like colour, not white like the Zanzibar lily, and the other of a pale bluish lavender. They were growing in small clear pools only a few miles apart in the valley of the Nile, at an altitude of about 3000 feet above the sea. “One of the most interesting botanical discoveries I made in the forest was the discovery of a wild orangetree. During our march through the forest l had continually come upon trees varying from 8 to IS feet high. They had double leaves ofa peculiar shape, which had a delicious smell like orange leaves; the branches were covered \vith long sharp thorns, and I at once pronounced them to be orange-trees. My fellow officers smiled incredulously, and exclaimed: ‘Orange-trees3 in the middle of the forest !’ But I held to my opinion, and iust before reaching the open country, I came upon a tree with both flowers and fruit upon it. The flowers were exactly the same as the tlouers of a cultivated orangetree. The fruit, which was green, was about the size of a marble. On cutting through it with a knifel found it had the same divisions as an ordinary orange, but each division was full of small seeds, which were very bitter and aromatic. On reaching Emin’s Province I told him about it, and he regretted very much that I had not brought a specimen with me, for he was a good botanist, and wished to add it to his collection of dried plants. He told me my discovery was doubly interesting, as many years before a German had penetrated the forest on the west coast ofAfrica, and reported that he had found wild orange-trees. His story was discredited, and now our discovering the orange-tree in the forest pointed that his report was after all true. “ 1 have not space to speak much about the flowers we saw in the open country, but will say a few words about those flowers which we found at a high altitude on the slopes of Ruwenzori, or the Mountains of the Moon.

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l

f Landolphia.
‘ .Axfflf/114 .rlellntn and .\'. La/ur are both plentiful in Upper N»le~land.
3 This reads like a tree Citrus, and if so is an interesting discovery, as
no species is hitherto known there.

Lieutenant Stairs who made the ascent of the mountains,
gives the following facts in his report :-
“‘The barometer stood at 21'1o, thermometer 70° F.
Ahead of us and rising in one even slope stood a peak,
in altitude 1200 feet higher than we were. This we now
started to climb, and after going up a short distance
came upon three heaths. Some of these must have been
20 feet high, and as we had to cut our way foot by foot
through them our progress was necessarily slow. Here
and there were patches of inferior bamboos, almost every
stem having holes in it made by some boring insect, and
quite destroying its usefulness. Under foot was a thick
spongy carpet of wet moss, and the heaths on all sides of
us we noticed were covered with “Old Man’s Beard ”
(Umrfrz). \Vc found great numbers of blue violets which
had no smell, and from this spot l brought away some
specimens of plants for Emin Pasha to classify. The
altitude was 8500 feet. \\'e found blueberries and black-
berries 1 at an altitude of 10,000 feet. The following 2 are
the generic names of the plants collected as named by

Emin Pasha :

Clematis.
Viola.
Hibi-cus.
Impatiens.
Tephrosia.
Glycine.
Rubus.
V accinium.
Begonia.
Peucedanum.
Gnaphalium.
Helichrysum.
Senecio.
Sonchus.

Erica aréarra. l

Landolphia.
Heliotropium.
Lantana.

Moschosma.
Lissochilus.
Luzula.
Carex.
Anthistiria.
Adiantum.
Pellrea.
[Ver/1: ayui/1'/ra
Asplenium.
Aspidium.
Polypodium.
Lycopodium.
Selaginella.
Marchantia.
Parmelia.
Dracazna.
L7snea.
'l`ree Fern. '

“These were just a few specimens Lieutenant Stairs
brought down with him. But the slopes of Ruwenzori
will, when properly explored, yield numbers of unknown
treasures to be added to the Botanical Encyclopaedia.
"For many \veeks \ve drank coffee which \ve made
from the berries of the wild coftee-trees which abound
on the highlands round the great lakes of Central Africa.
The Arabian coffee was originally supposed to have come
from KaH`a,in Abyssinia. That which we found in Karagwe,
Ankori, and Uganda is equal in flavour to the Enest Ara-
bian coffee, and will, when Central Africa is opened up,
be another of the chief articles of commerce.
“ I. A. M. JEPHSON."

TOPVN FOGS AND THEIR EFFECTS.”

U NTIL 1880 the formation of fog was looked upon as arising simply from the separation of liquid water, probably in the form of hollow vesicles, from an atmosphere saturated with aqueous vapour; but in that year Aitken showed that really the determining cause of the separating out of liquid water, and consequent formation of fog, was dust present in the air. He pointed out that a change of state, a gas passing to a liquid, or zt liquid to a solid, really always occurred at what he terms a “ free

' lt would be very interesting to have these identified. The two highestknown species of Rubns are /[mur/u.r and n:g'r`d'u.r, :it 5000-6000 feet. ’ This list is in Stanley’s book. The Vrbla is no doubt aby.r:r'/dm, common to the mountains of Madagncar, Abyssinia, the Cameroons. and Fernando Po. There are three heaths lrnown on the high mountains of Central Africa, viz. E rrrrr aréor-za, Eririnal/rr .'llanniz`, and B/nrria .r/hula. There is no I 'G{{l}ll.f{lll known before in'1`1opical Africa; though three or four are plentiful in Madagascar, and there is one on the Drakensberg, so that its occurrence is most probable. The fems of Tropical Africa are nearly all species widely spread in other continents. 3 The paper by Dr. \V. J. Russell, F.R.S., introducing the discussion on Town I-`ogs at the Hymenic Congress.

surface"; that as long as a molecule of liquid water is surrounded by like molecules, and the same with gaseous water, we do not know at what temperature, or whether at any temperature, they would change their state; but if in contact with a solid then at the surface, where they meet, the change will occur; if the solid be ice it may become liquid or the liquid may become solid, and the same kind of action occurs when the liquid is in contact with its own vapour; in fact, that what we call the freezing and boiling-points of a body are the temperatures at which these changes take place at such free surfaces. The dust always present in the atmosphere offers this free surface to the gaseous water, and thus induces its condensation. This specific action of dust varies very considerably, first with regard to its composition, and second with regard to the size and abundance of the particles present. Sulphur burnt in the air is a most active fog-producer, so is salt. Many hygroscopic bodies form nuclei having so great an affinity for water that they can cause i ts condensation from an unsaturated atmosphere. At the same time nonhygroscopic bodies, such as magnesia and many others, are powerful fog-producers; no doubt their activity may in part be attributed to their being good radiators of heat, and thus becoming cooled, induce condensation. Mr. Aitken also shows that the products of combustion, even when the combustion is perfect, are powerful fog-producers, a fact which has important bearing on the production of town fogs. One other point in these experiments 1 cannot omit mentioning, it is the exceedingly minute amount of matter capable of inducing fog. In his first series of experiments Mr. Aitken showed that , J0 of a grain of iron wire, however often it was heated, evolved on each heating sufficient dust to cause a visible fog, and afterwards.with still more delicate apparatus, that •HMio of a grain of either iron or copper, when treated in the same way, gave a similar result, and from these last experiments he infers that even rooVaa grain of either wire, if only slightly heated, would yield sufficient nuclei to cause a visible amount of fog. It is of much importance and interest, seeing how small a quantity of dust is required to produce fog, to know that even this small amount may be filtered out of the air by passing it through cotton wool, and thus an air be obtained in which a fog cannot be produced. Mr. Aitken's description of such an atmospheie is at first most alluring, for he says, if there was no dust in the air there would be no fogs, no mists, and probably no rain ; but he goes on to state that when the atmosphere became burdened with supersaturated vapour, it would convert everything on the surface of the earth into a condenser ; every blade of grass and every branch of a tree would drip with moisture deposited by the passing air; our dresses would become wet and dripping, and umbrellas useless ; but o-.ir miseries would not end here, for the inside of our houses would become wet, the walls and every object in the room would run down with moisture. I think, if we picture to ourselves this state of things, we may be thankful that there is dust and fog. Dust in its finer forms is invisible to us ; but as its delicate particles become loaded with moisture, it becomes a fine mist, dense if the number of particles are many; if, however, the dust-particles are fewer, and the amount of aqueous vapour the same, each particle will have a larger amount of condensed moisture to carry, and it will give rise to a more coarsegrained fog; the moisture, or some of it, will be more feebly attached to its nuclei, producing then what is known as a wet fog, whereas at least a most important fact in the production of a dry fog is the strong affinity between the nuclei and the condensed vapour. As most of you are no doubt aware, Mr. Aitken has invented a most ingenious method for counting the number of dustparticles in air, and has obtained most interesting and valuable results. I can only mention here that some of

these results deal with the clearness of air in relation to the number of dust-particles present, and other results show how little effect rain has in diminishing the amount of the finer dust in air. Evidently towns will supply dust of all kinds, and therefore offer every inducement for fogs to form there, and that at least some of the particles will he capable of causing the condensation of moisture even from an atmosphere which is not saturated with aqueous vapour. This condensation of moisture is a very complete process for removing all kinds of impurities from the air. Floating particles are free surfaces, and become weighted by the moisture they condense and tend to sink, and even the gaseous impurities in the air will be dissolved and carried down by the moisture present.

Experiment confirms this, for it has been proved how correctly the impurities of an air can be ascertained bydetermining the composition of dew, even if it be artificially and locally formed. It is of importance to note that even the purely gaseous emanations front our towns cannot pass away when a fog exists, as is shown by the accumulation of carbonic acid which takes place during a fog. Taking 4 volumes in 10,000 volumes as the normal amount of carbonic acid in London air, some years ago I found that it increased in the case of a dense fog to as much as I4'l volumes, and often to double the normal amount, which must represent a very serious accumulation of the general impurities in the air.

A fog in this way becomes a useful indicator of the relative purity of the atmosphere in which it forms. If pure aqueous vapour be condensed it gives a white mist— a country fog, a sea fog—and a white light seen through it is not converted into a red light ; but in town fogs the whiteness of pure mist disappears and becomes dark, in some cases almost black in colour, the change being produced by the foreign matters floating in the air, and by far the most abundant colouring matters of our town fogs are the products generated by the imperfect combustion of coal ; but in addition to these bodies, many others must obviously find their way into the air over a town. Especially will there be dust from the universal grinding and pounding going on in street traffic and many mechanical operations, from the general disintegration of substances and the decomposition of perishable materials—all will add something to the air, and it will become an integral part of the fog. However, although it is often said that a town fog is so dense that it may be cut with a knife, still it is difficult to condense so much of it that it can be subjected to a searching chemical analysis. In 1885, by washing foggy air, I was able to determine the amount of sulphates and chlorides present, and as indicators of organic matter the quantity of carbon and nitrogen in the fog. The results showed strikingly how largely the amounts of organic matter and ammonia salts in the air varied with the weather ; no case of dense fog occurred when the experiments were being made ; but the mean of several experiments clearly showed ihat in foggy weather the amount of organic matter was double as much as existed in the air in merely dull weather, and that the amount of sulphates and chlorides increased under like conditions, but not to the same extent. Fog may, however, be made to give its own account of its constituents, for we have only to collect and analyze the deposit which it leaves to learn what its more stable constituents are, and we have to thank the air-analysis committee of the Manchester Field Naturalists' Society for the most complete analysis of such a deposit which has yet been made. The deposit analysed occurred during the last fortnight in February of this year (1891), and was obtained from the previously washed glass roofs of the plant-houses at Kew, and Messrs. Veitch's orchid-houses at Chelsea. At Kew 20 square yards of roof yieldeJ 30 grammes of deposit. At Chelsea the same area gave 40 grammes, which represents 22 lbs. to the acre or 6 tons

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These analyses give, 1 believe, for the first time, a definite account of the composition of fog-deposit. Soct and dust are by far its principal constituents, rendered sticky and coherent by hydrocarbons, but 1 should like to give you the striking description which Prof. Thiselton Dyer has sent me of this deposit, collected at Kew. He says: "It was like a brown paint, it would not wash off with water, and could only be scraped off with a knife. It thickly coated all the leaves of the evergreens, and upon what have not yet been shed it still remains." In the above analysis it is curious to note the large amount of metallic iron and magnetic oxide of iron.

The details with regard to these very interesting analyses we shall hear from a member of the Manchester Committee, and I will only ask you to note how large a proportion of these deposits arises from the imperfect combustion of coal. We also learn from the Manchester Committee some interesting facts with regard to fogdeposits which occurred last winter in their city. This deposit which was collected from Aucuba leaves contained as much as 6 to 9 per cent, of sulphuric acid, and 5 to 7 per cent, of hydrochloric acid, mostly, of course, in a state of combination, but the deposit was, they say, "actually acid to the taste." Also, that three days' fog deposited per square mile of surface, in by no means the worst part of Manchester, ij cwt. of sulphuric acid, and even as far out of the city as the Owens College, on the same area, over 1 cwt. of acid and 13 cwt. of blacks.

There is still one other point characteristic of town fogs to be noted : it is their persistency in an atmosphere considerably above the dew point. A country fog under such circumstances directly passes away; a town fog apparently does not do so. There seem to me to be two reasons for this: one is that the moisture is protected, and its evaporation to a large extent hindered, by the presence of oily matter; and secondly, when the moisture has really gone, the soot and dust remain, and produce a haze.

The great distance to which fogs will travel is also remarkable, for they have on many occasions been traced to a distance of at least 25 to 35 miles from London, and I believe I might say to 50 miles.

I have so far discussed the production and composition of town fogs, and before considering their effects, would say a word on the question of whether in London they are increasing in frequency and density. A complete and accurate record of fogs in London is not kept; several stations are required, and a correct method of registering the density and distinguishing the difference between haze and fog is necessary; but fortunately there is a fair approximation to this complete registration of London fogs published by the Meteorological Office in their daily reports. The observations are made every morning at Brixton, and every afternoon at Victoria Street, and from a paper by Mr. Brodie, on " Some Remarkable Features in the Winter of 1890-91," published in the Journal of the Royal Meteoro

logical Society, I learn that the number of fogs thus registered which have occurred each winter since 1870 is as follows, winter being represented by the months December, January, and February. I have divided these 20 years into four groups of 5 years each:—

Between 1870 and 1875, 93 fogs occurred.
„ 1875 and 1880, 119 ,, ,,

,, 1880 and 1885, 131 ,, ,,

,, 1885 and 1890, 156 ,, ,,

It appears, then, that during the last twenty years there has been a steady increase in the number of winter fogs. I am not aware of any data to prove whether the density of these fogs has increased, but it is probable that the increase of number of fogs largely depends upon an increase of atmospheric impurity, and the conversion of haze and mist into obvious fog; and as the great colouring matter of fogs arises from the combustion of coal, I have drawn up the following table from information which has been kindly furnished to me by Mr. G. Livesey and Mr. J. B. Scott, of the Coal Exchange. It gives the amount of coal really consumed annually in London ; it does not include the coal used by the different gas companies. For the first five years, the amount given in the table is rather too high, as the quantity used by the suburban gas companies could not be ascertained and deducted. The quantities apply to what is known as the London district—an area, on an average, of 15 miles round London. The table shows an absolute increase, during the last fifteen years, of 2,000,000 tons of coalthat is, half as much again is now burnt as was burnt in 1875

Coal consumed in London (that used by Gas Companies dedueled).

[table]

Supposing only I per cent, of sulphur in this last yearly amount is converted into sulphuric acid (H.,S04) and passes into the air; this would give 195,720 tons of this acid.

The five years' averages of winter fogs, we have seen, give a steady increase, but obviously the number each winter will vary much with the atmospheric conditions: for instance, last winter was remarkably favourable for the development of fog; for, again taking the last twenty years, the average number of days of fog during the winter is 25, but last winter the actual number was 50.

The general atmospheric conditions which induce fogs are a still and moist air and a high barometer— a state ot the air most usual under anticyclonic conditions. The immediate determining cause, however, of a fog is usually a sudden and considerable fall of temperature. Mr. Brodie also points out that last winter was a time of calms; the percentage of such days on the average for the last twenty years is 97, but last winter the number was 22. Emphatically, he says, it was an anticyclonic winter.

A form of fog, well termed a " high fog," now frequently occurs in London. The lights in a street during this form of fog are often as visible as on clear nights, but above hangs a fog so dense that the darkness of night may prevail during the day. This particular form of fog appears

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