closes the fact that Yuang Chuang, the pilgrim monk, who, in the seventh century A.D., returned after sixteen years' wanderings in India, brought cats with him to protect his collection of Sanskrit Buddhist books from rats. That account, however, is somewhat invalidated by an anecdote of Confucius, who is related to have one day seen a cat chasing a rat. These conflicting statements are from authoritative sources, and it is impossible to offer a satisfactory explanation. Possibly the cat of Confucian times was only a partially domesticated wild cat. There must have been some ground for the statement of the cat having been brought from India, as it is hardly likely that in all the long period of Chinese history it should be named but twice as a domestic animal. He quotes from Chinese folk-lore on the subject of cats. As cruelty to cats and other animals is followed by retribution, so services rendered to them meet with supernal recognition. As anciently the tiger was sacrificed to because it destroyed wild boars, so the wild cat was worshipped because it was the natural foe of rats; boars and rats being the natural enemies of husbandry. At the commencement of the Sui dynasty (A.D. 581), the cat spirit inspired greater terror than the fox did subsequently. The hallucinations of cat spirit mania prevailed, forming a remarkable episode in Chinese history, only to be likened to the fanatical delusion of witchcraft that frenzied Europe a thousand years later. It was believed that the spirit of a cat possessed the power of conjuring away property from one person to another, and inflicted through incantations bodily harm. The popular belief was intensified and spread like an epidemic, until every disastrous affair that took place was ascribed to cat spirit agency set in motion by some mischievous enemy. Accusations were lodged against suspected persons, and, the slightest evidence sufficing for conviction, the malicious were encouraged to trump up charges against the innocent, until the country became a pandemonium. No one was safe, from the Imperial family down to the humble clodhopper. Even a magnate of the reigning house, who enjoyed the titular distinction of Prince or King of Szechuan, was executed for nefariously employing the agency of cat spirits. In this manner several thousands were immolated before the delusion was dispelled. Happily the period appears to have been of brief duration : incentives such as kept up the witch mania for centuries were wanting in China. Coming down to our own times we find a cat-craft delusion prevailed over a great portion of Chêkiang. "In the summer and autumn of 1847 frightful wraiths appeared throughout the departments of Hangchow, Shaohsing, Ningpo, and Taichow. They were demons and three-legged cats. On the approach of night a foetid odour was perceptible in the air, when dwellings were entered by something by which people were bewitched, causing alarm everywhere. On detecting the effluvium in the air, householders commenced gong-beating, and the sprites, frightened by the sonorous noise, quickly retreated. This lasted for sveral months, when the weird phenomena ceased." Well did he remember, said Dr. Macgowan, the commotion that prevailed in Ningpo throughout those months of terror. Every gong that could be procured or manufactured for the occasion was subject to vigorous thumping through the livelong night, maintained with vociferations by relays of zealous beaters. This deafening din was but a recrudescence of what had occurred a few generations before-a panic which was only exceeded by that which subsequently prevailed over the entire empire.

With regard to sheep, Dr. Macgowan said the ancient mode of writing the character for yang, goat, was ideographic-four strokes on the top to represent horns, two horizontal strokes representing legs, and a perpendicular one to represent body and tail. The modern form gives an additional parallel stroke, like the word for horse; it is a simple not a compound character, and when sheep came to be known, instead of making a new character, the sheep was called the "Hun-goat," thus indicating its origin and affinity. Yang, goat, is often translated sheep, the earliest instances being found in one of the Odes, wherein the Court habiliments of Wen Wang are called "lamb-skins and sheep-skins." This was about 1160 B.C., but it is doubtful if these robes are really the skins of sheep. It is not certain that such was the case, for the skins of goats were used then, as now, for clothes. Hun-goats are not named before the period of the Tang dynasty, say the seventh century A.D. The goat was one of the sacrificial animals, as at present, and was at the first selected for sacrifice when sheep were unknown.

In the discussion which followed, the conclusions of the pap were not accepted by all the speakers; and it was agre that the subject was one worthy of scholarly investigation.

HAINAN,

THE great island of Hainan, off the south-eastern coast of Chin

is but little known to Europeans, although since 1877 the has been a treaty port there. Mr. Parker, the Consulat Kiungcho the port in question, lately made a short journey in the interior the island, of which he gives some account in a recent report. H travelled about sixty miles up the Poh-Chung River, to withi a mile or two of Pah-hi, which is, at most seasons of the yea considered the limit of navigation for all but the smallest craf He walked round the walls of Ting-an city, one of the disturbe districts during the recent rebellions, on New Year's Da (February 9); they are just one mile in circuit, and differ litt from those of other Chinese cities. Wherever he had an opp tunity of walking diametrically across lengthy curves of the river he found the inclosed area to be extremely well cultivated though not so flat, its general appearance recalled many feature of the Tonquin delta, especially in its great wealth of bamboos The productions of the soil are much the same, the papaw areca-palm, sweet potato, turnip, ground-nut, orange-tree, & but a peculiar Hainan feature is the cocoa-nut palm. Anothe peculiarity of this region is the ubiquitousness of the dwi Pandanus, probably the same as the P. odoratissima of Fy the fibre of which is used in the manufacture of grass-cloth and is usually known to foreign trade here as hemp. Muchthe land was under sweet potato cultivation, and every house hold seemed to possess a few pigs, of the very superior ac stereotyped Hainan variety, black as to the upper and white : to the lower part of the body, with a dividing line of gr running along the side from the snout to the tail. These who some-looking pigs are fattened on the sweet potato, and do n rely for sustenance upon precarious scavengering, as is the ca with the repulsive and uncleanly animals of North China Land contiguous to the river is irrigated by enormous wheel forty feet in diameter, of very ingenious construction, moved by the current, needing no attention, and discharging perhaps or hundred gallons of water in a minute into the trough above day and night without intermission. He passed several large pottery establishments; but as at the New Year all business and cultivation are suspended for a few days, the opportunity was not a very good one for gathering precise information The temperature during the week ranged between 50 ar 60° F. Game seemed plentiful everywhere, and he me tions that a German resident has recently made a very fin collection of about 400 Hainan birds, embracing 154 species which will shortly be on their way to a Berlin Museum. One of the commonest birds in the river is a spotted white and black kingfisher of large size. Amongst the trees which attracted hi attention was one locally called the "great-leafed banyan, which looks remarkably like the gutta-percha tree; the native seem to use its gum mixed with gambier, in order to make tha dye "fast"; but there is some doubt whether it is not the sa of the real banyan-tree which is used for the purpose. A ver strong silk is made from the grub called the "celestial silk worm," or, locally, "paddy-insect." This grub is found on sort of maple. When full-grown it is thrown into boiling vinegar, on which the "head" of the gut, or "silk," appears; this is sharply torn out with both hands drawn apart, and is as long as the space between them, say five feet; it is so strong that one single thread of it is sufficient to make a line with which to catch the smaller kinds of fish.

UNIVERSITY AND EDUCATIONAL
INTELLIGENCE.

OXFORD.-The Chancellor of the University, acting a Visitor of Pembroke College, has appointed the Rev. Bartholo mew Price, M.A., F.R.S., Senior Fellow, and Vicegerent the College, Sedleian Professor of Natural Philosophy, to be Master of the College in the place of the late Dr. Evans. Prof Price, whose contributions to mathematics are well known, has long taken a leading part in University business, and his appoint ment to the Mastership of the College, of which he has been Fellow since 1843, will be warmly approved.

ཨ་

trement of Fall Term.-A meeting of the heads of leges and Halls was held under the authority of the hancellor at the residence of the Regius Professor of ne, Sir Henry Aclani. A report having been presented e me lical officer of health as to the great prevalence of enza in Oxford, and the difficulty of procuring nursing and cal attendance for the patients, it was unanimously red to recommend the Colleges and Halls, and the delegates he non-collegiate students, to postpone the attendance of ndergraduates to the end of the first week in February, eg a fortnight later than the time originally fixed.

AMBRIDGE. -The University Reporter of January 19 contains cal notification by Prof. Liveing, F.R.S., Chairman of heuncil of Cavendish College, that the College ceased on ry 15 to be a recognized Public Hostel of the University. M- Bachanan, University Lecturer in Geography, announces *e present term a course of lectures on the development of surfaces under climatic and other agencies.

The Special Board for Physics and Chemistry propose to this, two new special examinations for the ordinary B. A. tree, one in chemistry, including certain papers in heat, elecy, and magnetism, and another in physics, including papers sams, elementary chemistry, and more advanced electricity magnetism. The examinations will include practical work me of these subjects.

M W N. Shaw, F.R.S., has been elected a member of

Ear1 and Mr. S. F. Harmer a member of the Board for at. Geology.

SCIENTIFIC SERIALS.

the Turnal of Botany for December 1891, Mr. W. West es a collection of Freshwater Algæ from Maine, including Dew species and varieties; and we have also Mr. W. *terss Report of the Department of Botany in the British em for 1890, recording important additions to the heran' collections, by purchase, exchange, and gift; the more interesting being the late Mr. J. Ralfs's typeers for his "British Desmidieæ.”—Dr. D. H. Scott gives - account of the life and writings of the late Prof. Carl

Se botanical Gazette for November 1891, Mr. E. J. escribes the remarkable propulsive power possessed by ing fruit" of Cryptotenia canadensis, belonging to The feræ, by which the seeds are thrown out to a disfat least 5 feet; and Prof. Byron D. Halsted, a bacterial ce which is exceedingly destructive to the melon crops and carpaceae in America.The most important article in either for December is by Prof. Douglas H. Campbell, on tships of the Archegoniata, under which term he the Gymnosperms, as well as the Muscine and ar Cryptogams. As in previous essays, Prof. Campbell be på vlogeny of all the higher forms of vegetable life to Herance; both Gymnosperms and Angiosperms having been derived through the Ophioglossaceæ, Marattiaceæ, New-Prof. C. V. Riley describes the new insect-pest committing great ravages on dried plants in herbariaof Carphoxera ptelearia, belonging to the Geometrida.

SOCIETIES AND ACADEMIES.
LONDON.

Zoological Society, January 5.-Prof. A. Newton, F.R.S., lent, in the chair.-The Secretary read a report on s that had been made to the Society's Menagerie the months of November and December 1891. Amongst **atention was called to four Spotted-billed Pelicans (Pelemilena ), received from Calcutta, and to a second men of the Formosan Fruit-Bat-a species originally *mel from an example received alive by the Society in *-D, E. C. Surling exhibited some specimens of the new ran Marsupial (Notoryctes typhlof.), and gave a short ant of the habits of this remarkable animal, as observed in esmen recently kept in captivity by one of his correAn extract was read from a letter received from Dr. Annk, calling attention to the recent acquisition by se this correspondents in Java of additional specimens of the

rare Bush-Rat (Pithechir melanurus).—Mr. Ernst Hartert exhibited a series of eggs of the Common and other Cuckoos, mostly collected by himself and trustworthy friends, and made remarks on the question of the similarity of the eggs of the Cuckoos to those of the owners of the nest in which they are deposited.—A communication was read from Dr. J. Anderson, F.R. S., containing notes on a small collection of Mammals, Reptiles, and Batrachians made during a recent visit to Algeria and Tunisia.-Mr. F. E. Beddard read a paper upon the Earthworms collected by Dr. Anderson during the same expedition. Amongst them were examples of a new species of the genus A second new species of the same genus, based Microscolex. on examples collected by Mr. E. B. Poulton, F.R.S., in Madeira, and proposed to be called M. poultoni, was also described. A communication was read from Mr. R. I. Pocock on some Myriopoda and Arachnida collected by Dr. Anderson during the same expedition.-Mr. M. F. Woodward read a paper on the milk dentition of Procavia (Hyrax) capensis. The author showed that Lataste's canine has a counterpart in the lower or mandibular series, and he described for the first time two small vestigial upper incisors. He concluded that the teeth named belong collectively to the first or milk set, and that the formulation of the incisors of this genus as is probably due to the occasional persistence of the second upper milk-incisor. -Mr. Oldfield Thomas gave an account of the species of the Hyracoidea, of which order he had lately examined a large series of specimens. The author recognized fourteen species of this group of Mammals, all of which he proposed to refer to one genus (Procavia). Besides these, four geographical sub-species were recognized. A new species was described as P. latastei, from Senegal.

Geological Society, January 6.-Mr. W. H. Hudleston, F.R.S., Vice-President, in the chair.-The following communications were read :-On a new form of Agelacrinites (Lepidodiscus Milleri, n. sp.) from the Lower Carboniferous Limestone of Cumberland, by G. Sharman and E. T. Newton.-The geology of Barbados; Part II. The oceanic deposits, by A. J. Jukes-Browne and Prof. J. B. Harrison.—Arch@opneustes abruptus, a new genus and species of Echinoid from the oceanic series in Barbados, by J. W. Gregory. This genus belongs to a group of Echinoidea which has given some trouble to systematists, owing to the union of the characters of the orders Cassiduloidea and Spatangoidea; the other genera belonging to the group are Asterostoma, Pseudasterostoma, and Palcopneustes. The evidence of the new Echinoid throws light upon the affinities of these genera. The main points suggested by a study of the new species are: (1) the abandonment of the name Pseudasterostoma as a synonym of Palæopneustes; and (2) the inclusion of the true Asterostoma, Palæopneustes, and Archaopneustes in the Adete Spatangoidea, whereby the Plesio spatangide are left as a more homogeneous family, though bereft of the chief interest assigned to it. A tabular summary fossil species of Asterostoma and Palaopneustes occur in Cuba, of the nomenclature of the group is given. The best-known in deposits referred to the Cretaceous owing to the resemblance of these Echinoids to the common Chalk Echinocorys scutatus. The new genus includes a species from the same deposit, which is probably of the same age as the Bissex Hill rock from which the new species was obtained; this is at the top of the oceanic series, and belongs to the close of the great subsidence. After the reading of this paper, there was a discussion in which the Chairman, Dr. Blanford, Prof. Sollas, Prof. Harrison, Mr. J. W. Gregory, and Mr. W. Hill took part.

DUBLIN.

Royal Society, December 16, 1891.-Prof. A. C. Haddon, President of the Scientific Section, in the chair.-Mr. E. W. L. Holt read a paper on the eggs and larval and post-larval stages of Teleosteans, obtained during the Society's survey of fishing grounds on the west coast of Ireland. Thirty-three species, chiefly food-fish, are dealt with. The eggs of Galus smarkii, G. follachius, and Rhombus megastoma, are described for the first time; those of Hippogossa" "platessoides (the long rough dab), Scomber scomber the mackerel), and Caranx trachuru, (the scad), are also described. The development of the long rough dab, turbot, brill, and several other species of flatfish, is traced upwards, to the assumption of the adult characters, with more or less continuity. The paper concludes with a series of tables containing an artificial classification of the pelagic eggs of British marine Teleosteans for purposes of easy identification.

-A second paper by the same author deals with the shore and deep-sea fishes obtained during the survey, and contains descriptions and figures of some of the more interesting forms. The vertical and horizontal distribution is also discussed, and it is pointed out that the west coast of Ireland is to a great extent the meeting ground of the Scandinavian and Lusitanian or Mediterranean fish faunas. An attempt is made to give a complete list, with references, of all deep-sea fishes which have been taken in Irish waters.-A paper was then read by Dr. E. J. McWeeney, on a method of preparing Hyphomycetes, Saccharomycetes, and Schizomycetes, as museum specimens, with a demonstration of illustrative cultivations. After pointing out that our natural history collections did not, as a rule, contain specimens illustrative of these minute organisms, the author showed that the appearances presented by the aggregate masses formed by their cells could in many cases be made perfectly perceptible to the unaided eye. A collection contained in suitable vessels, comprising nearly ninety specimens, and including Actinomyces grown on agar, potato, and turnip, Trichophyton, Acharion, various species of Saccharomyces, Bacillus tetami, and many others, was then demonstrated. The specimens were prepared by Král, of Prague, and the writer hoped soon to be able to add species from Irish sources.

PARIS.

Academy of Sciences, January 11.-M. Duchartre in the chair. On the resistance of coiled elastic springs to small deformations, by M. H. Resal.-On the spontaneous oxidation of humic acid and of vegetable soil, by MM. Berthelot and André. If humic acid is allowed to stand in diffused light for a short time, a change of colour occurs, and an appreciable quantity of carbon dioxide is developed. It appears from this that the brown-coloured constituents of vegetable soil furnish carbon dioxide, and tend to become discoloured under the influence of air and sunlight, by oxidizing. The action is said to be purely chemical, and not the result of the growth of microbes. Some new observations on the estimation of sulphur in vegetable soil, and on the nature of the compounds containing it, by the same authors.-New contribution to the chemical history of the truffle; analogy between the Terfaz or Kama of Africa and Asia and the truffles of Europe, with regard to the relation between the chemical composition of soils and tubercles, by M. A. Chatin. On the Ecureuil of Barbary, by M. A. Pomel.-On the hypergeometric series, by M. André Markoff.-On plane réseaux with equal invariants and asymptotic lines, by M. G. Konigs. On series with positive terms, by M. V. Jamet.-On the use of orthochromatic plates in astronomical photography, by MM. Fabre and Andoyer. (See Our Astronomical Column.) On the thecry of regelation, by M. H. Le Chatelier. The consequence which follows from the formula developed is that compressed pulverulent ice, in contact with a liquid or vapour less compressed, experiences an increase of solubility, fusion, or vaporization that brings about an unstable condition of supersaturation, which disappears by the crystallization of the ice in the interspaces: this solution, followed by crystallization, continues until the spaces have completely disappeared and the névé has become transformed into a block of ice. The mode of hardening is thus comparable to that of cements. On a new model of a reversible thermometer for the measurement of deep-sea temperatures, by M. V. Chabaud.-New condensation hygrometer, by M. Henri Gilbault. In order to determine absolute or relative humidity with a condensation hygrometer, the moment at which dew is deposited must be observed, and the temperature of the surface upon which it is formed. Many methods have been proposed to perfect the observation of the moment when the dew appears, but only a few have taken into account the equally important second condition. The author has endeavoured to improve existing methods by causing condensation to take place on a thin sheet of platinized glass, and measuring the variations of electrical resistance of the metal. He finds it possible to determine the dew-point within of a degree by his method.-Loss of the two kinds of electricity brought about by light of high refrangibility, by M. E. Branly.-On metallic borates, M. A. Ditte.-On manganates of potash, by M. G. Rousseau. It appears from the experiments that manganate of potash, heated in the presence of a flux, gives rise to two hydrated manganates. About 600° C., the hydrate obtained is K,O,16MnO,6H,O; between 700° and 800", KO,8MnO2, 3H,O is produced; and the former compound reappears between 800 and 1000°.-On the reduction of benzine

hexachloride; condensation of benzine, by M. J. Meunier.On the formation of dextrines, by M. P. Petit.-On a new unsaturated fatty acid of the series CH-40, by M. A. Arnaud. -Influence, in bare soils, of the proportions of clay and organic nitrogen on the fixation of atmospheric nitrogen, on the preservation of nitrogen, and on nitrification, by M. P. Pichard.On the whistling language of the Canary Islands, by M. J. Lajard. On the pelagic flora of Naalsoëfjörd (Faroe Isles), by M. Georges Pouchet.-On the Upper Cretaceous of the Aspe valley, its age and its relations, by M. J. Seunes.

BOOKS, PAMPHLETS, and SERIALS RECEIVED.

Books. Anthropological Religion: F. Max Müller (Longmans)Adventures amidst the Equatorial Forests and Rivers of South America V. Stuart (Murray).-Power and Force: J. B. Keene (Unwin).-British Flies, vol. i. Part 3: F. V. Theobald (Stock.).-Methods of Gas Analysis: Dr. W. Hempel; translated by L. M. Dennis (Macmillan).-Egypt under the Pharaohs; new edition: H. Brugsch-Bey (Murray).-Life in Ancient Egypt and Algeria: G. Maspéro: translated by A. Morton (Chapman and Hall).-List of Snakes in the Indian Museum: W. L. Sclater (Calcutta.Manipulation of the Microscope: E. Bausch (Collins).-Magnetic Induction in Iron and other Metals: Prof. Ewing (Electrician Office).The Realm of Nature: Dr. H. R. Mill (Murray).-Annuaire de l'Académie Royale des Sciences, Belgique, 1892 (Bruxelles, Hayez).-The Optics of Photography and Photographic Lenses: J. T. Taylor (Whittaker) - New Fragments: J. Tyndall (Longmans).-The Art and Craft of Cabinetmaking: D. Denning (Whittaker). -Electric-Light Cables: S. A. Russell (Whittaker).-Bergens Museums Aarsberetning for 1890 (Bergen, Griegs).Istituto Chimico Kicerche, 1890-91 (Regia Università degli Studi di Roma) (Rome). Catalogue of Scientific Papers (1874-83). compiled by the Roya Society of London, vol. ix. (C. J. Clay).-Dynamics of Rotation: A. M. Worthington (Longmans).-Christian Doctrines and Modern Thought: Dr T. G. Bonney (Longmans).

PAMPHLETS.-Aids to Natural Philosophy: R. S. Trivedy (Calcutta, Auddy). The Science of Homopathy: W. B. Picken (London).-The Glory of the Imperfect: Prof. G. H. Palmer (Boston, Heath).

SERIALS. Journal of the Royal Agricultural Society of England, and series, vol. ii., Part 4, No. 8 (Murray).-Quarterly Journal of Microscopical Science, No. 129 (Churchill).-Journal of the Royal Statistical Society, December (Stanford).-Mind, January (Williams and Norgate)Geological Magazine, January (Kegan Paul).-Physical Society of London Proceedings, vol. xi., Part 2 (Taylor and Francis)-Ann. des k.k. Naturhistorischen Hofmuseums, Band 6, Nos. 3 and 4 (Wien, Hölder).-Natura ed Arte, No. 2 (Milano). Journal of Anatomy and Physiology, January (Williams and Norgate).-L'Anthropologie, Tome ii., No. 6 (Paris, Masson). -Brain, Part 56 (Macmillan).—Mineralogical Magazine, December (Simpkin). Journal of the Chemical Society, January (Gurney and Jackson).Veröffentlichungen aus dem Königlichen Museum für Völkerkunde, ii. Band, 1-2 Heft (Spemann) -Himmel und Erde, January (Berlin, Paetel)

THURSDAY, JANUARY 28, 1892.

THE ASTRONOMICAL THEORY OF THE GLACIAL PERIOD.

The Cause of an Ice Age. By Sir Robert Ball, Astronomer-Royal for Ireland. Pp. 180. (London: Kegan Paul and Co., 1891.)

THIS

HIS book gives a popular account of the theory of Adhemar and Croll as to the causes of glacial periods in geological history.

The author's power as a popular expositor is well known, and this little book shows him at his best. He knows when to drive a point home, and yet is never tedious in his reiteration But he has given here something more than a lucid explanation, for he makes a valuable contribution to the subject, and the book may be read with advantage by those who are already acquainted with the literature bearing on the theory. The theory itself may be sketched in outline as follows:

It is known that, under the perturbations of Venus and Jupiter, the eccentricity of the earth's orbit varies within certain limits. When the eccentricity is large, and when the precession of the equinoxes brings the perihelion to near the middle of, say, the northern winter, the annual supply of solar heat is so distributed that there will be a glacial period in the northern and a mild climate in the Southern hemisphere. Two or three maxima of glaciation and mildness will usually succeed one another at intervals of 10,500 years, because the eccentricity varies with extreme slowness. When the eccentricity is small, as at present, a moderate climate will prevail in both hemispheres, whatever be the position of the perihelion. The keynote of Sir Robert Ball's presentation of this theory is given in a short mathematical appendix. I am disposed to dissent to some extent from the manner in which this view is set forth, but the general argument will, I think, do much to convince the scientific world of the truth of the theory, even where Croll's more elaborate discussions failed to do so.

I will now give a paraphrase of the argument, and will point out where it appears to me open to objection. The time taken by the earth to describe a degree of longitude round the sun varies as the square of its distance from the sun, and the intensity of solar radiation varies inversely as the square of the same distance. Hence the amount of heat received by the whole earth during the description of a degree of longitude is

constant.

of summer added to that incident on it during the similar portion of winter be denoted by 2, and suppose that the excess of the heat incident in the portion of summer above that incident in the similar portion of winter be denoted by za; then it is clear that I + a is proportional to the amount of heat received by the specified area during the specified portion of its summer, and 1 a is proportional to the amount of heat received by the area during the similar portion of winter. Thus we may say that the contrast between the summer and winter supplies of heat (for given area and given portions of summer and winter) is represented by the fraction (1+ a) ÷ (1 − a).

This is, of course, equally true when the whole hemisphere, and the whole of summer and winter, are considered, and Sir Robert Ball shows that a is then equal to 2 sin 23 27 ÷′′; (1+a) ÷ (1 − a) is found to be almost exactly as 5 to 3. Using percentages he gives the ratio as 63 to 37, but the simple numbers 5 to 3 afford a closer approximation to accuracy.

It is clear that if the specified portions of summer and winter embrace the solstices, and if the specified area is tropical, a will be small, and if it is polar it will be large. The fraction (1 + a) ÷ (1 − a) continually increases as we go northward, and it may be taken as a measure of the severity of a climate. It is quite uncertain how far the climate of any one place depends on the heat supplies of the whole hemisphere on which it lies, and therefore it is uncertain how large an area and how long a season we ought to take into consideration in the present investigation. But I should have thought it legitimate, in treating of the causes of glaciation, only to consider the semi-annual heat supply of a polar cap, comprising, say, all the area north of latitude 30°; thus would have made (1 + a) ÷ (1 − a) much greater than 5 to 3. It does not seem to me, however, that we are bound to find an answer to this almost insoluble problem.

So far we have considered the supply of heat whilst the earth describes so many degrees of longitude round the sun, but climate depends on the supply of heat during a given time.

When the earth's orbit is circular, summer and winter are of equal length, and so also are similar portions of summer and winter; thus the two ways of estimating the heat supply coalesce, and the contrast between the summer and winter daily supplies of heat is also represented by the fraction (1 + a) ÷ (1 − a). The present condition of affairs differs but little from this standard case, and we know that the contrast between the summer

and winter daily supplies of heat is such as to produce certain known climates, differing according to latitude.

1

summer when the sun is south of the line. Also let similar days in summer and winter be defined as days on which the sun sets (say at Greenwich) as much after 6 p.m. as before 6 p.m.; similar parts of summer and winter will mean parts limited by similar days.

Now consider the solar heat incident on any specified area of one hemisphere, during any specified portion of the summer and during the similar portion of the winter. Suppose that the heat incident on the area in the portion

If be the sun's hour angle at sunset on any day in summer, and on the corresponding day in winter, and if the sun's parallax on those days be received by unit area in the course of the day is proportional to

The expression for the heat supply on unit area during any portion of summer or winter involves elliptic integrals, which might be given if it were worth while.

area during any specified portion of the year. A triple integral is required to express the heat supply of any specified

The question we have to ask is, If the orbit becomes eccentric, how will the contrast of daily supplies be affected?

In order to answer this, let us go at once to the extreme, when the eccentricity of orbit is a maximum. We learn that if aphelion is at midsummer, summer will be 199 days, and winter 166 days; and the converse is true when the perihelion is at midsummer.

Since 199 is to 166 nearly as 6 to 5, we see that with midsummer perihelion there are 5 days of summer to 6 of winter, and with midsummer aphelion there are 6 of summer to 5 of winter.

Hence, with midsummer perihelion, the short summer daily supply of heat may be taken as proportional to (1 + a), and the long winter daily supply as proportional to (1 a). Hence the contrast between the short summer and long winter daily supplies is repre6(1+a). sented by ; that is to say, the standard contrast 5(1 a) is augmented in the ratio of 6 to 5. Next, with midsummer aphelion, the long summer daily supply of heat may be taken as proportional to (1 + a), and the short winter daily supply as proportional to (a). Hence the contrast between the long summer and short winter 5(1+a) daily supplies is represented by ; that is to say, 6(1 − a) the standard contrast is diminished in the ratio of 5 to 6. In the first case, the heat supply is less evenly distributed through the year than at present, and we have a much more severe climate; in the second, it is more evenly distributed, and we have a much milder one. It follows also that, if we compare the two extreme cases together instead of both with the mean case, the change of contrast is represented by the ratio of 62 to 52, or of 36 to 25.

I must refer the reader to the able discussion in the book of the effects which we have reason to suppose would flow from a change of contrast represented by the numbers 36 to 25; and it must suffice to say here that it seems enough to explain on the one hand the occurrence of the glaciation of England, and on the other hand the occurrence of sub-tropical plants in Greenland.

Now, the above seems to me to be substantially the argument in the book, but I dissent from the stress laid on the numerical determination of the quantity a. p. 90 Sir Robert says:

On

"This theory will be entirely misunderstood unless the facts signified by these numbers (the evaluation of (1 + a) (a)) are borne in mind. No one can discuss the astronomical theory of the Ice Age unless the figures 63 and 37 (5 and 3 are more accurate) form a portion of his consciousness, and the refrain of his every argument."

It may be admitted that it might have been more difficult to present the argument in a popular form without assigning a numerical value to a, but Sir Robert Ball is fully equal to such a task; and I contend that the numerical value of a is beside the mark, even if a valué, appropriate to the investigation in hand, were attainable.

After presenting his own view of the question, Sir Robert Ball says (p. 134) that Croll does not seem to have been really aware of the full strength of the astronomical theory, and in this I entirely agree. Croll, in fact, rather weakens than strengthens his position when

he tries to trace in detail the action and reaction of the astronomical cause, for in doing so he is led to maintain various theses which are not susceptible of proof, and are even highly doubtful. He thus takes as the central point of his position one at which it appears to me to be weakest. In 1886 I wrote :

Adequate criticism of Mr. Croll's views is a matter of great difficulty, on account of the diversity of causes which are said to co-operate in the glaciation. In the case of an effect arising from a number of causes, each of which contributes its share, it is obvious that if the amount of each cause and of each effect is largely conjectural, the uncertainty of the total result is by no means to be measured by the uncertainty of each item, but is enormously augmented. Without going far into details, it may be said that these various concurrent causes result in one fundamental proposition with regard to climate, which must be regarded as the keystone of the whole argument. That proposition amounts to thisthat climate is unstable.

"Mr. Croll holds that the various causes of change of climate operate inter se in such a way as to augment their several efficiencies. Thus, the trade-winds are driven by the difference of temperature between the frigid and torrid zones, and if from the astronomical cause the northern hemisphere becomes cooler, the trade-winds on that hemisphere encroach on those of the other, and the part of the warm oceanic current, which formerly flowed into the cold north zone, will be diverted into the southern hemisphere.1 Thus the cold of the northern hemisphere is augmented, and this in its turn displaces the tradewinds further, and this again acts on the ocean currents, and so on; and this is neither more nor less than instability.

"But, if climate be unstable, and if from some of those temporary causes, for which no reasons can as yet be assigned, there occurs a short period of cold, then surely some even infinitesimal portion of the second link in the chain of causation must exist; and this should proceed, as in the first case, to augment the departure from the original condition, and the climate must change."

I see no reason to depart from what I said five years ago, but I now learn from this book how it is that Croll

mistook the strong points of his own theory, and that a more forcible proof of it may be contained in a short work than in an elaborate volume. After expressing this opinion, it is but fair to quote and indorse the following passage (p. 112) on Croll's famous work on "Climate and Time":

"I was greatly struck," says the author, "by this work when I first read it many years ago. Subsequent acquaintance with this volume.. has only increased my respect for the author's scientific sagacity, and my admiration for the patience and the skill with which he has collected and marshalled the evidence for the theory that he has urged so forcibly."

There are a few other points in the "Ice Age," not involved in the main line of argument, on which I should like to comment.

The method adopted of stating the disturbing forces of the planets on the earth appears to me unduly sensational. We learn (p. 74) that the disturbing force of Venus is 130 million million tons, and it is impossible not to be impressed with the magnitude of the force. But if we had been told that the disturbing force on each pound of the earth's mass was only 1/7000 of a grain, 1 Ball (p. 134) fails to see the force of this argument. 2 Brit. Assoc. Keport, 1886, Address to Section A.