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lı might be supposed that the estimations would agree better I have now becon.e satisfied that the California fux is arriéré. when the angles of position are the same for both arcs compared Either the struggle for existence is not sharp here, or he has together, than when they are different. But this supposition is made up his mind that existence is not worth struggling for. not borne out by my observations ; for after correcting them by Lick Observatory, October 8. EDWARD S. HOLDEN. the above formula, ihe average deviation from the truth in the case of the caresul comparisons is 4:4 per cent. when the angle of position of both arcs on the retina is the same, or within 10°
A Plague of Small Frogs. of ihe same ; and 4'i per cent. when it differs more than 10°; while in the case of the instantaneous comparisons these numbers
My wine cellar has been visited during the recent rains with a are 7'9 and 6-3 respectively.
curious plague of small frogs (Rana temporaria) all the same When the lower arc is horizontal, or nearly so, it is (on the size, about one inch long. There would be nothing surprising average) estimated as being shorter than when in a vertical posi- in this visitation were it not for the apparent absence of any tion, but the difference is so slight that it is doubtsul whether it
means of communication from outside, the level of which is would not disappear with a larger number of observations. The six feet above that of the cellar ; there is no drain near that part best correction formula 1 have obtained for this is to apply the of the house. There is a step up hefore you go down into the factor
winé-cellar from the adjacent cellar, against which the door (1.04 - 048 cos d)
closes, leaving no crack any animal so large could squeeze
through. The cellar has solid stone walls and a bricked foor. to the result already obtained : d being the deviation of the During the recent floods the water stood some three or four lower arc from the horizontal. But the application of this factor inches deep there, apparently oozing through a tiny hole level only reduces the sum of the squares of the differences between with the floor on the outside wall, into which the point of a calculation and observation in the case of the caresul comparisons pencil could only penetrate for an inch. Even had it been possible from 1163 to 1111.
for these little creatures to come in that way they must have The angle of position of the upper arc seems to make no burrowed down six feet from the outside level. Only one or difference in the results.
T. W. BACKHOUSE. Two were found in the cellar adjacent, which is lighted by a West Hendon House, Sunderland, October 24.
grating into the garden, whereas in the wine cellar two or three
dozen were caught, many of them drowned by the food. Proper Motions of the Stars.
Is it not unusual for bats to Ay in the day-time? Here one Miss CLERKE, in her very interesting article (NATURE, vol.
has been doing so for two afternoons, coming out about 2.30, and xliv. p. 572) on the motion of th: sun in space, seems to
Aying backwards and forwards after insects in most brilliant think that we have only the two alternatives of supposing sunshine. The gardener tells ine he has never observed them do that the brightness of a star is independeot of its distance,
so besore ; and having sometimes caught them in the day-time, or that the motions of the stars increase with their distance.
always noticed that when thrown into the air they would drop I suspect that, when the proper motions of all stars down at once, and run instead of flying. to the gth magnitude have been tabulated, the necessity of
R. HAIG THOMAS. 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, when used in contection with a powerful telescope, ought to be able to show
BOTANY OF THE EMIN RELIEF whether the fainter stars as a rule move more rapidly in the line
EXPEDITION. of sight than the brighter ones; for if the average motion in the
THE line of sight is the same in both cases, astronomers will be slow
HE botanical exploration of Tropical Africa leaves
so much to desire that it was somewhat disappointto accept an explanation of phenomena which supposes a different | ing to find that Mr. Stanley brought nothing back which average velocity on the whole. But even instruments incapable would give any idea of the nature of the dense forests of deciding this question may throw light on the subject. It
which he traversed. The conditions under which such 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
an expedition is necessarily executed make naturalwould appear more than one magnitude brighter. Hence, before history-collecting extremely difficult. Travellers, howwe can use magnitudes as in any sense a test of distance, we must ever, often suppose that because they cannot make ascertain the relative proportion of Sirian and solar stars in the extensive collections they can do nothing to add to our gruups which we are comparing. It would also be very desirable knowledge. Yet to fill a small portfolio with well-selected i hat ihe magnitudes of ihe stars employed by Profs. Eastman, and significant specimens is not a very difficult matter. Boss, and Stumpe, should be photometrically determined. The | And these may often furnish the basis of useful and imphotometer has at all events the advantage over the eye that portant conclusions as to the general nature of the flora. its results are in all cases (allowing for errors of observation) | Sir Joseph Hooker was able to give the first account of comparable.
W. II. S. MOCK.
the vegetation of Kilimanjaro from a small parcel of Dublin, October 17.
plants collected by a missionary, the Rev. Mr. New, who California Foxes.
was supplied for the purpose by Sir John Kirk, with “a
bundle of old Guardians.” An officer of the Ashanti IN Nature of September 10 (p. 452), there is a paragraph in Expedition brought from Comassi the fruit of what proved praise of the intelligence of the (English) fox, with examples in proof. Permit me to say that his California cousin is next door
to be a new species of Duboscia. And quite lately Lord to a fool. My young son has amused himself for the past three Lamington sent to Kew a small parcel of plants collected summers in trapping (in large box-traps) the small California by himself in an expedition through the Shan States, foxes which infest these mountains, and which live on a mixed | which contained good specimens of an interesting plant diet of Manzanita berries and astronomer's chickens. I pass only known previously from imperfect material collected over the fact that each trap has painted over its door “Danger by Griffith. It has now been worked out and figured in to all who enter here !”, and I proceed to show that our California the Kew “Icones Plantarum." foxes are barely one remove from idiots. When they are caught, Nor is it so difficult as it might be supposed to do even my boy is in the habit of fastening a small leather collar about more than this. And I am not sure that a little careful their necks, and of chaining them with light chains to stakes near and intelligent plant-collecting would not be a healthy the Observatory buildings. Many of them have escaped by part and useful distraction to the tedium and strain of an ing the chains (by dint of strength, not of intelligence), and have arduous journey. Nothing could probably exceed the been again caught within two or three days in the same traps ! One of them was caught three times in quick succession! I pre
difficulties under which Joseph Thomson travelled in sume an English fox, once caught, would emigrate to North
Masailand; yet he managed, notwithstanding, to get Britain, or at least to the next county. My own ideas of the together a tolerably extensive and most valuable botanical intelligence of the fox, until I came here, were derived from collection. Upon this Sir Joseph Hooker was able to Goldsmith's "Animated Nature," and, generally, from English base the first attempt at a rational theory of the geowritings.
graphical relations of the high-level flora of Eastern Equatorial Africa. Nothing, again, could be more ad- trees, lilies,' and Begonias. There is, however, a great mirable than the collections made by Brigade-Surgeon i wealth of different kinds of ferns, such as I have often Aitchison when attached to the Kuram Field Force seen cultivated in hot-houses in England. In many under Sir Frederick Roberts in Afghanistan. And the places the damp ground was covered by a thick growth Government of India has now arranged -and it is an of filmy ferns and Lycopodium of the most beautiful indication of the sympathy for science which animates description. its members-that, as part of the organization of the “ Here is a short extract from my journal, which will Botanical Survey of India, a botanist shall for the future give some idea of the every-day sights we saw on the be attached to all frontier expeditions.
banks of the Lower Congo, 1700 feet above the sea, and Major Jephson, who accompanied Mr. Stanley, seems, 250 miles distant from it :however, to have had his eyes about him. A corre- * * At the bottom of a piece of swampy ground I came spondent has sent me a copy of the October number to a small stream, on the banks of which were growing of the Mayflower, a small monthly horticultural perio- Osmunda regalis, or Royal fern. It was slightly ducal published in New York, which contains (pp. 155, stunted in growth, being not more than 2 feet in height. 155 a short paper by him on the “ Plants of the It is the first I ever have yet seen in the tropics. Close Iark African Wilderness.” This seems to me worth by the stream was growing a group of beautiful ground putting on record in the pages of NATURE, where it will orchids, in form like a Hyacinthus candicans. There be at least more accessible for future reference. At my were clusters of great pink flowers with yellow centres ; request, Mr. Baker, the Keeper of the Kew Herbarium, the whole had a very gorgeous effect. Here, also, was a has had the paper annotated with such critical comments profusion of Lycopodium. It is of a kind I have not yet a> were possible.
seen ; it creeps up and over everything in great blueTo Major Jephson's paper Mr. Stanley has prefixed a green masses; its long tendrils creep up the tree trunks brief introduction, which adds nothing of importance. like ivy, to a height, in some cases, of 4 feet. There He remarks :
were quantities, also, of the ribbon fern, exactly like ** In this branch of science I fancy we were all but the Davallia pentaphylla,” which has been introduced amateurs, and considering what very little time any of us into English hot-houses from the Malayan Archipelago could devote from the engrossing business of marching, What would not florists at home have given for an acre and seeking for food to sustain life, Mr. Jephson shows of this ground ?' what might have been done by him had circumstances “In the forest there were two kinds of lilies which were been more favourable.”
common. One, which grew in swampy ground, was in This is, however, erring a little on the side of modesty. form like an Amaryllis. It was white, with a deep crimAs I have already shown, amateurs can do very useful son centre, and had a delicious but heavy scent. The turk without much difficulty, if they are content to do other was a lily, which grew everywhere through the only a little, but to do that little carefully. Some further whole length of the forest. It was of a brilliant scarlet observations are open to more serious criticism :
colour, and was formed of several hundreds of small " Africa is yet too young and too crude for the scientific Aowers, forming a round ball like a huge Guelder rose, botanist. We have only been pioneers to stake the high-four inches in diameter. It was of such a brilliant scarlet way to make ready for those who shall come after us. that it looked almost metallic, growing in the darkest reWhen the rails have been laid in pairs of iron lines across cesses of the forest. One of the commonest and most the swamp and desert, and the engined boat cleaves the striking of all the ferns we saw was the Platycerium alci. se i bosoms of the great rivers, and furrows the dead green corne. It is an extremely interesting fern, one of a face of the fresh-water seas, then the tender-nurtured singular genus of epiphytal plants, growing on the botanist, conveyed from point to point without danger to branches of trees. Cur Zanzibaris called it elephant hıs valuable life, may be trusted, with his enthusiasm ear,' from its curious shape. There was another of the and devotion, to bring to us results worthy of science and same family, Platycerium Stemmaria, which we found the age of those who have given us an insight into the growing upon rocks in the open country. Both these botanic treasures of the African world, Schivernfuth (sic) ferns grew at altitudes from iooo to 5000 feet. Tree. is by far the best, but he has also laboured under such dis- ferns of the ordinary kind we found growing in all the advantages and discomforts that he was not able to do gullies and streams on the slopes of the mountains above for Equatorial Africa a tenth part of what Bates did for the Albert Nyanza. The altitude was from 5000 to 6000 the Amazon."
feet above the level of the sea, and I noticed especially One cannot but wonder a little at the ignorance of the that the flora here was remarkably like that in the Central literature of African travel which this paragraph displays. Province of Ceylon, which is an altitude of 2500 to 4000 Men like Grant, Speke, Kirk, Welwitsch, Mann, Vogel, feet above the sea. Barter, and Thomson, tó mention only a few of those to
“ By far the most common plant which we saw in the whom we owe our knowledge of the African flora, would jungle was the Amomum, or wild cardamom.10 It was have thought it comical to be described as "tender- almost precisely the same in form as the cardamom nurtured' botanists. The work of Schweinfurth was ad- which is cultivated in Ceylon. It grew almost throughmirable ; yet no one would, I think, be more surprised out the whole of Central Africa. li has a large purple than that distinguished naturalist, Mr. Bates, to learn flower, which grows in clusters on the ground at the root that the botanical collections which he never even pro- of the plant, and from it a bright scarlet fruit forms, of a fessed to make, were ten times better.
pear shape, and about the size of a small fig. "It is W. T. THISELTOY-DYER.
divided into four quarters, and contains some white, Royal Gardens, Kew.
fleshy pulp, very juicy and acid. This pulp is full of 1 Crinum.
Osmunda regalis is cosmopolitan, but in tropical zone is high up only. * It is difficult to give an accurate idea of the flowers 3 Mr. Rolfe cannot suggest anything better than Lissochilus. we saw in our march through Africa in a short magazine
4 Selaginella scandens, no doubt.
5"Ribbon fern anicle, but I here give a short sketch, mentioning some but they are not like Davallia pentaphylla.
would suggest Ophioglossum pendulum or Vittaria, few things which I think may be interesting to my readers.
6 Crinum seylanicum.
7 Bruxstigia toricaria. ** The great forest of Central Africa through which we
8 Platycerium alcicorne is not African, but P. Stemmaria is widely spread.
9 No doubt Cyathea Thomsoni, Baker, which is very near C. Dregei of passed is not so rich in variety of flowers and orchids as the Cape. the forests of Mexico and Brazil, or even the jungles of fruits are 3- not 4-celled. See A. Daniellii
, &c., in Oliver and Hanbury's
to There are a large number of Amomums in West Tropical Africa. The India and Ceylon. It is chiefly rich in flowering vines, / raper in Journ. I inn Soc , vii. 109.*
small black aromatic tasting seeds like those of the culti- Lieutenant Stairs who made the ascent of the mountains, vated cardamom. If ever planters go into Africa, the gives the following facts in his report :cardamom will be an important product of the soil for ““The baromeier stood at 21'10, thermometer 70° F. commerce, for there are vast tracts of forest with the į Ahead of us and rising in one even slope stood a peak, climate, soil, and checkered shade which are necessary in altitude 1200 feet higher than we were. This we now for the cultivation of the cardamon. Orchilla weed started to climb, and after going up a short distance should also become a valuable article of commerce ; it came upon three heaths. Some of these must have been grows in many parts of the forest. I consider, however, 20 feet high, and as we had to cut our way foot by foot ihat when the great forest of Central Africa is opened through them our progress was necessarily slow. Here lip to civilization, by far the most valuable article of and there were patches of inferior bamboos, almost every commerce will be india-rubber, the want of which is stem having holes in it made by some boring insect, and increasingly felt in the civilized world. Now that electri- quite destroying its usefulness. Under foot was a thick city is so much used for various purposes, the demand spongy carpet of wet moss, and the heaths on all sides of for india-rubber grows larger and larger : the supply us we noticed were covered with “ Old Man's Beard” which is shut up in the African forest is practically un- (Usnea). We found great numbers of blue violets which limited. There are various trees of the fig tribe which had no smell, and from this spot I brought away some yield this product, but by far the greatest amount is specimens of plants for Emin Pasha to classify. The contained in the india-rubber vines 1 which abound in altitude was 8500 feet. We found blueberries and blackthe forest and hang from almost every tree. In cutting berries 1 at an altitude of 10,000 feet. The following? are our way through the forest in some places, we got covered the generic names of the plants collected as named by with the milky glutinous sap, which dropped upon us Emin Pasha :from the vines we cut through.
Moschosma. “The natives know its value, and use it largely for
Lissochilus. smearing the inside of their buckets in order to make
Luzula. them hold water. They use it largely also for covering Impatiens.
Carex. the ends of their drum-sticks. The india-rubber obtained Tephrosia.
Anthistiria. is of a clear, yellowish colour, like glue, and is of the Glycine.
Adiantum. most elastic description.
Pellæa. "In the forest region I saw no water-lilies, but in Vaccinium.
Pteris aquilina. Emin Pasha's Province, in the Bari country, I saw two
Asplenium. kinds. They were both about the size of an ordinary
Aspidium. white water-lily, and the leaves and flowers floated on the
Polypodium. surface of the water, but the stalks and formation of the
Lycopodium. leaves and flowers was finer and more slender. One was
Marchantia. of a pink coral-like colour, not white like the Zanzibar
Parmelia. lily, and the other of a pale bluish lavender. They were
Dracæna. growing in small clear pools only a few miles apart in the Heliotropium.
Usnea. valley of the Nile, at an altitude of about 3000 feet above
Tree Fern.' the sea.
“One of the most interesting botanical discoveries I “These were just a few specimens Lieutenant Stairs made in the forest was the discovery of a wild orange brought down with him. But the slopes of Ruwenzori tree. During our march through the forest I had con- will, when properly explored, yield numbers of unknown tinually come upon trees varying from 8 to 15 feet high. treasures to be added to the Botanical Encyclopædia. They had double leaves of a peculiar shape, which had a
“For many weeks we drank coffee which we made delicious smell like orange leaves ; the branches were
from the berries of the wild coffee-trees which abound covered with long sharp thorns, and I at once pronounced on the highlands round the great lakes of Central Africa. them to be orange-trees. My fellow officers smiled The Arabian coffee was originally supposed to have come incredulously, and exclaimed : Orange-trees 3 in the from Kaffa, in Abyssinia. That which we found in Karagwe, middle of the forest !' But I held to my opinion, and Ankori, and Uganda is equal in favour to the finest Arajust before reaching the open country, I came upon a tree
bian coffee, and will, when Central Africa is opened up, with both flowers and fruit upon it. The flowers were
be another of the chief articles of commerce. exactly the same as the flowers of a cultivated orange
“ I. A. M. JEPHSON.” tree. The fruit, which was green, was about the size of a marble. On cutting through it with a knife I found it had the same divisions as an ordinary orange, but each TOWN FOGS AND THEIR EFFECTS.3 division was full of small seeds, which were very bitter and aromatic. On reaching Emin's Province I told him UNTIL 1880 the formation of fog was looked upon as about it, and he regretted very much that I had not brought a specimen with me, for he was a good botanist
, probably in the form of hollow vesicles, from an atmoand wished to add it to his collection of dried plants. He sphere saturated with aqueous vapour; but in that year told me my discovery was doubly interesting, as many
Aitken showed that really the determining cause of the years before a German had penetrated the forest on the separating out of liquid water, and consequent formation west coast of Africa, and reported that he had found wild
of fog, was dust present in the air. He pointed out that orange-trees. His story was discredited, and now our
a change of state, a gas passing to a liquid, or a liquid to discovering the orange-tree in the forest pointed that his
a solid, really always occurred at what he terms a' " free report was after all true.
" It would be very interesting to have these identified. The two highest“I have not space to speak much about the flowers we known species of Rubus are pinnatus and rigidus, at 5000-6000 feet. saw in the open country, but will say a few words about
2 This list is in Stanley's bouk. The l'iola is no doubt abyssinica, com
mon to the mountains of Madagascar, Abyssinia, the Cameroons, and Ferthose flowers which we found at a high altitude on the nando Po. There are three heaths known on the high mountains of Central slopes of Ruwenzori, or the Mountains of the Moon. Africa, viz. Erica arborea, Ericinella Vannii, and Blaeria spicata.
There is no l'accinium known before in Tropical Africa; though ihree or four
are plentiful in Madagascar, and there is one on the Drakensberg, so that its ? Landolphia.
occurrence is most probable. The ferns of Tropical Africa are nearly all Nymphaa stellata and N. Lotus are both plentiful in Upper Nile-land. species widely spread in other continents. 3 This reads like a tree Citrus, and if so is an interesting discovery, as 3 The paper by Dr. W. J. Russell, F.R.S., introducing the discussion on no species is hitherto known there.
Town Fogs at the Hygienic Congress.
surface"; that as long as a molecule of liquid water is these results deal with the clearness of air in relation to surrounded by like molecules, and the same with gaseous the number of dust-particles present, and other results water, we do not know at what temperature, or whether at show how little effect rain has in diminishing the amount any temperature, they would change their state ; but if in of the finer dust in air. Evidently towns will supply dust contact with a solid then at the surface, where they meet, of all kinds, and therefore offer every inducement for the change will occur ; if the solid be ice it may become rogs to form there, and that at least some of the particles liquid or the liquid may become solid, and the same kind will be capable of causing the condensation of moisture of action occurs when the liquid is in contact with its even from an atmosphere which is not saturated with own vapour ; in fact, that what we call the freezing and aqueous vapour. This condensation of moisture is a very boiling-points of a body are the temperatures at which complete process for removing all kinds of iinpurities from these changes take place at such free surfaces. The dust the air. Floating particles are free surfaces, and become always present in the atmosphere offers this free sursace weighted by the moisture they condense and tend to sink, to the gaseous water, and thus induces its condensation. and even the gaseous impurities in the air will be disThis specific action of dust varies very considerably, first solved and carried down by the moisture present. with regard to its composition, and second with regard to Experiment confirms this, for it has been proved how the size and abundance of the particles present. Sulphur correctly the impurities of an air can be ascertained by burnt in the air is a most active fog-producer, so is salt. determining the composition of dew, even if it be artifiMany hygroscopic bodies form nuclei having so great an cially and locally formed. It is of importance to note affinity for water that they can cause its condensation from that even the purely gaseous emanations from our towns an unsaturated atmosphere. At the same time non- cannot pass away when a fog exists, as is shown by the hygroscopic bodies, such as magnesia and many others, accumulation of carbonic acid which takes place during are powerful fog-producers ; no doubt their activity may a fog. Taking 4 volumes in 10,000 volumes as the normal in part be attributed to their being good radiators of heat, amount of carbonic acid in London air, some years ago I and thus becoming cooled, induce condensation. Mr. found that it increased in the case of a dense fog to as Aitken also shows that the products of combustion, even much as 14'1 volumes, and often to double the normal when the combustion is perfect, are powerful fog-pro- amount, which must represent a very serious accumulation ducers, a fact which has important bearing on the pro- of the general impurities in the air. duction of town fogs. One other point in these experi- A fog in this way becomes a useful indicator of the ments I cannot omit mentioning, it is the exceedingly relative purity of the atmosphere in wbich it forms. If minute amount of matter capable of inducing fog. In pure aqueous vapour be condensed it gives a white misthis first series of experiments Mr. Aitken showed that a country fog, a sea fog-and a white light seen through ido of a grain of iron wire, however often it was heated, it is not converted into a red light; but in town fogs the evolved on each heating sufficient dust to cause a visible whiteness of pure mist disappears and becomes dark, in fog, and afterwards with still more delicate apparatus, that some cases almost black in colour, the change being proTuon of a grain of either iron or copper, when treated in duced by the foreign matters floating in the air, and by the same way, gave a similar result, and from these last far the most abundant colouring matters of our town fogs experiments he infers that even jooboo grain of either are the products generated by the imperfect combustion wire, if only slightly beated, would yield sufficient nuclei of coal; but in addition to these bodies, many others must to cause a visible amount of fog. It is of much impor- obviously find their way into the air over a town. tance and interest, seeing how small a quantity of dust is Especially will there be dust from the universal grinding required to produce fog, to know that even this small and pounding going on in street traffic and many amount may be filtered out of the air by passing it mechanical operations, from the general disintegration through cotton wool, and thus an air be obtained in of substances and the decomposition of perishable which a fog cannot be produced. Mr. Aitken's description materials-all will add something to the air, and it will of such an atmosphere is at first most alluring, for he become an integral part of the fog. However, although says, if there was no dust in the air there would be no it is often said that a town fog is so dense that it may be fogs, no mists, and probably no rain ; but he goes on to cut with a knife, still it is difficult to condense so much of state that when the atmosphere became burdened with it that it can be subjected to a searching chemical supersaturated vapour, it would convert everything on the analysis. In 1885, by washing foggy air, I was able to surface of the earth into a condenser ; every blade of grass determine the amount of sulphates and chlorides present, and every branch of a tree would drip with moisture de- and as indicators of organic matter the quantity of carbon posited by the passing air ; our dresses would become and nitrogen in the fog. The results showed strikingly wet and dripping, and umbrellas useless ; but our miseries how largely the amounts of organic matter and ammonia would not end here, for the inside of our houses would salts in the air varied with the weather ; no case of dense become wet, the walls and every object in the room would fog occurred when the experiments were being made ; but run down with moisture. I think, if we picture to our- the mean of several experiments clearly showed that in selves this state of things, we may be thankful that there foggy weather the amount of organic matter was double is dust and fog. Dust in its finer forms is invisible to as much as existed in the air in merely dull weather, and us; but as its delicate particles become loaded with mois- that the amount of sulphates and chlorides increased ture, it becomes a fine mist, dense if the number of under like conditions, but not to the same extent. Fog particles are many ; if, however, the dust-particles are may, however, be made to give its own account of its fewer, and the amount of aqueous vapour the same, constituents, for we have only to collect and analyze the each particle will have a larger amount of condensed deposit which it leaves to learn what its more stable conmoisture to carry, and it will give rise to a more coarse- stituents are, and we have to thank the air-analysis comgrained fog ; the moisture, or some of it, will be more mittee of the Manchester Field Naturalists' Society for feebly attached to its nuclei, producing then what is the most complete analysis of such a deposit which has known as a wet fog, whereas at least a most important yet been made. The deposit analysed occurred during fact in the production of a dry fog is the strong affi- the last fortnight in February of this year (1891), and was nity between the nuclei and the condensed vapour. As obtained from the previously washed glass roofs of the most of you are no doubt a vare, Mr. Aitken has invented plant-houses at Kew, and Messrs. Veitch's orchid-houses a most ingenious method for counting the number of dust- at Chelsea. At Kew 20 square yards of roof yielded 30 particles in air, and has obtained most interesting and grammes of deposit. At Chelsea the same area gave 40 valuable results. I can only mention here that some of grammes, which represents 22 lbs. to the acre or 6 tons
to the square mile, and the composition of these deposits logical Society, I learn that the number of fogs thus is as follows:
registered which have occurred each winter since 1870 Chelsea,
is as follows, winter being represented by the months Carbon
December, January, and February. I have divided these Hydrocarbons
20 years into four groups of 5 years each :Organic bases (pyridines, &c.)
4:8 Sulphuric acid (S03)...
Between 1870 and 1875, 93 fogs occurred. 43
4'0 Hydrochloric acid (HCI)
1875 and 1880, 119 ,,
1880 and 1885, 131 ,
1885 and 1890, 156 , of iron...
It appears, then, that during the last twenty years there Mineral matter (chiefly silica and
415 ferric oxide)
has been a steady increase in the number of winter fogs.
I am not aware of any data to prove whether the density Water, not determined (say disference)
of these fogs has increased, but it is probable that the 53
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 colourThese analyses give, I believe, for the first time, a defi- ing matter of fogs arises from the combustion of coal, nite account of the composition of fog-deposit. Soct and I have drawn up the following table from information dust are by far its principal constituents, rendered sticky which has been kindly furnished to me by Mr. G. and coherent by hydrocarbons, but I should like to give Livesey and Mr. J. B. Scott, of the Coal Exchange. It you the striking description which Prof. Thiselton Dyer gives the amount of coal really consumed annually in has sent me of this deposit, collected at Kew. He says : London ; it does not include the coal used by the differ“It was like a brown paint, it would not wash off with ent gas companies. For the first five years, the amount water, and could only be scraped off with a knife. It given in the table is rather too high, as the quantity used thickly coated all the leaves of the evergreens, and upon by the suburban gas companies could not be ascertained what have not yet been shed it still remains.” In the and deducted. The quantities apply to what is known as above analysis it is curious to note the large amount of the London district-an area, on an average, of 15 miles metallic iron and magnetic oxide of iron.
round London. The table shows an absolute increase, The details with regard to these very interesting during the last fifteen years, of 2,000,000 tons of coalanalyses we shall hear from a member of the Manchester that is, half as much again is now burnt as was burnt in Committee, and I will only ask you to note how large a 1875. proportion of these deposits arises from the imperfect combustion of coal. We also learn from the Manchester
Coal consumed in London (that used by Gas Compames Committee some interesting facts with regard to fog
deducted). deposits 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
1875 state of combination, but the deposit was, they say,
5,872,310 1876 4,988,280
1884 5,669,281 “actually acid to the taste.” Also, that three days' fog
1885 6,026,063 deposited per square mile of surface, in by no means the 1878 4,973,147
6,096,732 worst part of Manchester, i} cwt. of sulphuric acid, and 1879 5,833,891
1887 6,231,956 even as far out of the city as the Owens College, on the 1880 5,334,823
6,463,498 same area, over i cwt. of acid and 13 cwt. of blacks.
1889 6,390,850 There is still one other point characteristic of town
1882 5,343,974 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 Supposing only 1 per cent. of sulphur in this last yearly apparently does not do so. There seem to me to be two amount is converted into sulphuric acid (H,SO.) and reasons for this: one is that the moisture is protected, passes into the air ; this would give 195,720 tons of this and its evaporation to a large extent hindered, by tbe acid. presence of oily matter ; and secondly, when the moisture The five years' averages of winter sogs, we have seen, has really gone, the soot and dust remain, and produce a give a steady increase, but obviously the number each haze.
winter will vary much with the atmospheric conditions: The great distance to which fogs will travel is also sor instance, last winter was remarkably favourable for remarkable, for they have on many occasions been traced the development of fog; for, again taking the last twenty to a distance of at least 25 to 35 miles from London, and years, the average number of days of fog during the I believe I might say to 50 miles.
winter is 25, but last winter the actual number was 50. I have so far discussed the production and compo- The general atmospheric conditions which induce fogs sition of town fogs, and before considering their effects, are a still and moist air and a high barometer- a state of would say a word on the question of whether in Lon- the air most usual under anticyclonic conditions. The don they are increasing in frequency and density. A immediate determining cause, however, of a fog is usually complete and accurate record of fogs in London is a sudden and considerable fall of temperature. Mr. not kept ; several stations are required, and a cor- Brodie also points out that last winter was a time of rect method of registering the density and distinguish calms ; the percentage of such days on the average for ing the difference between haze and fog is necessary; the last twenty years is 97, but last winter the number but fortunately there is a fair approximation to this was 22. Emphatically, he says, it was an anticyclonic complete registration of London fogs published by the winter. Meteorological Office in their daily reports. The obser- A form of fog, well termed a “high fog," now frequently vations are made every morning at Brixton, and every occurs in London. The lights in a street during this form afternoon at Victoria Street, and from a paper by Mr. of fog are often as visible as on clear nights, but above Brodie, on “Some Remarkable Features in the Winter of hangs a fog so dense that the darkness of night may pre1890-91," published in the Journal of the Royal Meteoro- vail during the day. This particular form of fog appears