case, the task will be found to be impracticable; but even when the overlap of the small disc is greater, the task can only be achieved by actually making new cusps out of the irradiation fringes. (Á figure would make this explanation much simpler.)

Prof. Newcomb says that he is decidedly of opinion that the irradiation of an extremely minute thread of light is not the same with that of a large disc. He does not seem to notice that if this is so, Venus just before, at, and just after internal contact, must be distorted. This even if-admitting the enlargement of the sun's disc-he denies that the disc of Venus is reduced by

irradiation.

He fails also to observe that a peculiarity such as distortion, or the formation of a ligament, may escape the notice of inferior or not very attentive observers, and so all his negative observations be explained. It is no proof of superior skill in observation to see no signs of an illusory effect. Until we have observers who recognise no traces of irradiation when looking at the solar disc, we must believe that (as Mr. Stone has, I think, already asserted) the non-recognition of distortion or ligament formation is due to inattention, or want of observing skill. That this should be more common than close and careful scrutiny is not a very surprising circumstance, and proves nothing.

RICHARD A. PROCTOR

Oceanic Circulation

IN NATURE of August 17, I have just seen the report of the discussion on Dr. Carpenter's paper on the above subject read at the late meeting of the British Association.

Dr. Carpenter, explaining the movements on thermodynamic principles, states that he has "found the primum mobile of this circulation was not in equatorial heat but polar cold," and explains that "(1) As each surface-film cools and sinks, its place will be supplied, not from below, but by a surface influx of the water around; and (2) the bottom stratum will flow away over the deepest parts of the basin, while, since the total heat of the liquid is kept up, there will be an upper stratum which will be drawn towards the cold area, to be precipitated to the bottom and repeat the action. Apply this principle to the great oceanic area that stretches between the equator and the poles, we should expect to find the upper stratum moving from the equator towards the poles, and its lower stratum from the poles towards the Equator. That such a movement really takes place is indicated, as it seems to me, by various facts."

It does not appear, however, that Dr. Carpenter has well established his claim to the theories in question, while, in a pamphlet on the same subject, published in 1869 by Dr. Adolph Mühry of Göttingen, we find such passages as the following :—“ As the cause of the latitudinal circulation we have assumed the difference of temperature in the water between the equator and the pole.' He honestly gives Arago the credit of being, perhaps, the first to put forward this view in 1836; and after remarking (p. 11) that it might be considered doubtful whether it is the upper warm current from the equator or the under cold one from the pole that ought to be considered the primary, he says (p. 12) "For us the primary arm' is the heavier, i.e., the colder polar stream, which, in obedience to gravitation, falls in a horizontal direction toward the lighter water of the hot zone; and the secondary 'arm' is the returning antipolar. It moves to replace what flows away, and is, therefore, the compensation-arm.'

Here, without following Dr. Mühry any further, we find the thermodynamic theory advanced by Dr. Carpenter, and his primum mobile as well; but by giving him credit for ignorance of Dr. Mühry's work, we may excuse him for laying claim to what is there put forward, and accepting therefore the commendation of others as unknowing as himself. J. B.

Ice Fleas

DURING a recent ramble upon the Morteratsch Glacier, I also observed a large number of the minute black creatures described by Prof. Frankland in NATURE, No. 100. My attention had been directed to them ten years ago by Lord Anson on the "snow-bones," near the summit of the Ægischorn. They are only nominal "cousins" of the flea (Pulex) of civilised life, and are not at all related to Dapnia, the "water flea," but are closely allied to the minute insects which are often seen on the surface of stagnant water, resembling grains of gunpowder, and skipping partly by help of their forked tail, folded under them so as to serve as a foot, hence their name Podura, or "skip-tail." They have been named by Agassiz Desoria saltans. Their food, I conjectured with Prof. Frankland, consists of "red snow" and

other microscopic algae. Not being myself within reach of a good library, I can only furnish your readers with a key to further information. C. A. JOHNS

IN NATURE of 28th September, Prof. Frankland, in introducing the ice flea to the readers of NATURE, uses the expression "if known at all," and concludes by asking information about it. The glacier flea, Desoria glacialis, was noticed and described by Prof. Agassiz as far back as 1845, in his Ascent of the Wetterhorn on the 29th of July of that year. Not having Agassiz's work at present beside me, I cannot refer to it, but these fleas are noticed in an extract translated from an account of the ascent, and published in Hogg's Weekly Instructor for Dec. 1845, vol. ii. P. 221. On the Aar Glacier they are described as being scattered over the "surface of the snow in millions," elsewhere, "as being collected in masses under the stones on the ice."

The New Dynameter

R. C.

THE letter from the Rev. T. W. Webb in your last number is a very tantalising letter. He tells us, and we could not wish to have a better authority, that a new dynameter has been invented by the Rev. E. Berthon, but he does not tell us how it is constructed or where it can be obtained.

I may take this opportunity of mentioning a makeshift dynameter which I have found to answer very well when extreme accuracy is not required.

I have a pocket telescope fitted with a Cavallo micrometer, i.e., a slip of finely divided mother-of-pearl screwed to the diaphragm next the eye-glass. Unscrewing the two last draws of this telescope the end of the second is applied to the eye-piece of the telescope of which the power is to be measured, and the first draw pushed in till the image of the object-glass comes sharp upon the mother-of-pearl. The diameter of the image is thus given in divisions on the mother-of-pearl, the value of which, in hundredths of an inch, has been previously ascertained.

Notaris on Mosses

W. R.

WITH reference to the notice of De Notaris' book on Mosses, I am informed by Dr. Dickie that the genus Habrodon was discovered in Great Britain several years ago by the late Mr. McKinlay, of Glasgow, and that he had received from Mr. Wilson about two years ago from his district Conomitrium julianum. Killin by Dr. Stirton. Dr. Dickie sends specimens of Habrodon Notarisii gathered at M. J. BERKELEY

In the review referred to, Prof. De Notaris was erroneously described as of Geneva, instead of Genoa.-ED. N.

"Newspaper Science "

My attention has just been called to a letter from Mr. David Forbes which appears in NATURE, Sept. 21, under the head "Newspaper Science," and in which that gentleman, writing from Boulogne, pathetically describes the emotions with which he read a recent "article" in the Globe on "Krupp's" Gun-manufactory at Essen. I need hardly say how deeply I deplore the shock which I have unwittingly been the agent of inflicting on your distinguished correspondent. It will be some small satisfaction if you will allow me to express the hope that the "desired result" of Mr. Forbes's "reluctant " compliance with the advice of his "medical man," and most wise resolve "to eschew everything scientific for the next few weeks at least, in order to recruit before the winter labours commenced," may not be utterly defeated by the perusal of "a specimen of English scientific opinion," of which I am unhappily the author. It would be a terrible reflection indeed, that a stupid error on my part had, perhaps, imperilled the accuracy and success of Mr. Forbes's "winter labours." The blunder (or rather blunders) occurred as follows:-I, too, was "knocked up with work," but being myself a "medical man" naturally only in part carried out my own prescription. I would, for the sake of Mr. Forbes, and the credit of "English scientific opinion" in the estimation of his "French acquaintance," I had exercised a little more discretion. However, unfortunately, I stumbled on the Krupp factory, and all forgetful of my dilapidated mental condition, wrote a note-paragraph (I never write "articles"), which I vainly imagined might have been innocent and interesting. It is not always possible to compress even the manuscript necessary for a paragraph on to a single sheet of paper, and I grieve to say that after my paper had passed the editorial eye three words

One fresh from the Botanical Gardens of Europe is astonished at every step taken in the Gardens by the wondrous vegetation which is shown by all the semitropical plants. Descending a few steps from the circular drive, a great palm avenue is entered. This avenue was planted in 1847, and is formed of about eighty trees of the date palm, nearly as many of the Latania Borbonica, and about 150 of the dragon's blood tree (Dracana draco). The avenue is about ten yards wide, and between every two of the date palms there are two of the dragon's blood tree and one Latania. It terminates in a clump of palm trees which are planted almost to the border of the sea. When it is borne in mind that the date palms are from twenty to fifty feet high, the Latanias averaging about twelve, and the Dracenas about eight feet in height, the general effect of this splendid avenue may be imagined. All the trees were in December last in full flower or fruit, the golden trusses of the date palm contrasting well with the more brightly-coloured clusters of Latania berries. It would require more space than is at our disposal to describe the contents of all the various small avenues that branch off from the main one. The most remarkable smaller avenues are, perhaps, the one formed of bamboo (Bambusa arundinacea), planted in 1863, and forming an immense mass of foliage, the stems supporting which are from forty to fifty feet high, and that formed of about 100 plants of Chamarops excelsa, each about ten feet in height. But remarkable as are these charming subtropical alleys, the visitor is more than surprised when on going towards the portion of the garden where the plants are grouped somewhat according to their natural orders, he finds specimens fifteen feet high of Caryota urens and C. Cumingii, growing with vigour and covered with fruit; of Oreodoxa regia, from Cuba; several plants upwards of twenty-five feet in height; and a plant of Fubaa spectabilis, which is twelve feet high; and then just a few steps more and a parterre alloted to the natural family of the Musaceæ comes to view. As both the plantain and banana are grown in large quantities for their fruit in another portion of the grounds, the family is here chiefly represented by such genera as Strelitzia and Ravenalia. Magnificent specimens of the latter genus, with stems nine to ten feet high, exhibited great combs of flowers. We are not aware if the Traveller's tree has flowered in Europe, and we were not prepared to find it in full flower in Algiers. It has not, however, matured its fruit in this garden. Near this grand parterre stood another with many fine specimens of Yucca, also a magnificent plot of Aralias, A. papyrifera, in full fruit and very handsome; the fine A. leptophylla and A. præmorsa, thickly covered with spines, and the very ornamental A. farinifera; and then one's attention is caught by a large tree (Carolinea macrocarpa) from Brazil, with a couple of dozen of its fruit, each as big as a cocoa nut; by a small forest of Anona cherimolia in full fruit, which is nearly as good as that of the closely related species which yields the custard apple. Near these is an immense tree some thirty feet in height, covered with fruit of the Avocado pear (Persea gratissima); and at its feet is a quantity of guava trees (Pisidium Cattleyanum) crowded with its perfectly ripe, large, pear shaped, golden fruit. Growing up into the trees, and forming numerous and never-ending festoons, were some specimens of Cacti, chiefly species of Cereus. Some of these were of great size, and one specimen, which had completely strangled a plantain tree some twenty-five feet, was said to have been covered in the autumn with 6c0 to 700 flowers. It must have been a sight worth a long pilgrimage to see.

Enough has been said to show what a surprising number of semi-tropical fruits luxuriate in the beds of this well-watered garden, and we might add many well-known vegetables to the list, as sweet batat, yam, papaw; but all this while we have been writing of

the great level portion of the garden. Outside of this, and on the other side of the roadway, there is a small hill, two or three hundred feet in height, which slopes towards the garden and the sea, and is traversed by several ascending walks. This is the New Holland district of the garden, and certainly not the least interesting portion of it. In one section of it are different species of Acacia, many of them large trees, twenty to twenty-five feet in height. Of the Proteaceae there were magnificent trees; of the genera Banksia, Hakea, and Grevillea, the collection of species was very large, all of them just bursting into masses of bloom. The most important of the trees growing in this corner of the hill was probably Eucalyptus globulus, of which some trees, now about forty feet in height and over four feet and a half in circumference, were planted in 1862, and were then only a few inches high. Young well-established seedlings, of about ten inches in height, are sold for 20s. a hundred, and large numbers of them have been planted from time to time throughout Algeria by the French Government. This species grows in Algeria with most surprising rapidity, under very favourable circumstances growing eighteen to nineteen inches in height each month. Its wood appears to be hard, close in the grain, and it is largely used in the construction of quays, bridges, and railways. This tree seems to do so well on the southern side of the Mediterranean that we think its culture ought to be successfully attempted in the south of Spain, in Sardinia, in Sicily, and the southern parts of Italy. In districts subject to heavy winds it requires for some years-owing to its rapid growth-some protection, but in places sufficiently warm for it, it ought to repay well for any little extra care it might be found to need.

Among the few species that we noticed that did not succeed in these gardens, we may mention the Cedrus deodara; but Casuarina equisetifolia was flourishing, and one tree of Araucaria excelsa was about sixty feet in height, and measuring a little over nine feet in circumference at its base.

The object of the Society in keeping up these Gardens is, as we said, to introduce into Algeria all useful and ornamental plants likely to grow there. In addition they grow enormous quantities of young palms and other ornamental plants for exportation to Europe, and some few plants interesting to the botanist for exchange with other establishments. In a place so favoured by nature and so easily accessible to Europe, it would be, we venture to think, well worth the while of the director of these Gardens to considerably enlarge the last portion of the Society's design. How many tropical plants are yet unknown to the large collectors of Europe, and what a vast percentage of deaths occur among the collections sent from the tropics at any season of the year to our shores! But with Gardens like these at Algeria, situated on the sunny side of the Mediterranean, to act as a halfway house, the resources of the Botanical Gardens or establishments of the North would be indefinitely increased. Another purpose for which these Gardens might be made most useful is for forming a collection of specimens of plants or fruits of economic interest. Many of the fruits, stems, &c., which ripen in these Gardens as easily as cherries or potatoes with us, are not to be seen in some botanical collections, and are not, in Europe at least, to be purchased. How gladly would some botanist buy such as we here refer to if they were on sale, say at the depôt of the Algerian Society in Paris; and the expense of putting up such in salt and water would be a mere nothing. The same remarks would apply in many cases to portions of the roots of remarkable genera, and also to flowers. In calling attention to these Gardens, we venture to suggest these hints to their well-known director, and also to that indefatigable botanist who, more than any other, now represents science in connection with the Algerian Society, Prof. Durando of Algiers.

E. P. W.

THE TEMPERATURE OF THE SUN

THE

HE increase of the volume of atmospheric air, under constant pressure, being directly proportional to the increment of temperature, while the coefficient of expansion is o'00203 for 1° of Fahrenheit, it will be seen that a

temperature of 3,272,000° Fah. communicated to the terrestrial atmosphere would reduce its density to

=

I

6643 of the existing density. Accordingly, if we assume that the height of our atmosphere is only 42 miles, the elevation of temperature mentioned would cause an expansion increasing its height to 6643 X 42 = 279,006 miles. This calculation, it should be observed, takes no cognizance of the diminution of the earth's attraction at great altitudes, which, if taken into account, would considerably increase the estimated height. Let us now suppose the atmosphere of the sun to be replaced by a medium similar to the terrestrial atmosphere raised to the temperature of 3,272,000°, and containing the same quantity of matter as the terrestrial atmosphere for corresponding area. Evidently the attraction of the sun's mass would under these conditions augment the density and weight of the supposed atmosphere nearly in the ratio of 279: 1; hence its 279,006 height would be reduced to 10,000 miles. But 27'9 if the atmosphere thus increased in density by the sun's superior attraction consisted of a compound gas principally hydrogen, say 14 times heavier than pure hydrogen, the height would be 10 x 10,000 = 100,000 miles. The pressure exerted by this supposed atmosphere at the surface of the photosphere would obviously be 14'7 X 27'9 410 pounds per square inch, nearly. Unless, therefore, the depth greatly exceeds 100,000 miles, and unless it can be shown that the mean temperature is less than 3,272,000 Fah., the important conclusion must be accepted that the solar atmosphere contains so small a quantity of matter that notwithstanding the great depth it will offer only an insignificant resistance to the passage of the solar rays. Now, the assumed mean temperature, 3,272,000°, so far from being too high, will be found to be considerably underrated. It will be recollected that the temperature at the surface of the photosphere, determined by the ascertained intensity of solar radiation at the boundary of the earth's atmosphere, somewhat exceeds 4,035,000°. Consequently, as the diminution of intensity caused by the dispersion of the rays, will be inversely as the convex areas of the photosphere and the sphere formed by the boundary of the solar envelope, viz., 152: 1, the temperature at the said boundary will be

==

4.035,000

=

2,654,600°

152 The true mean, therefore, will be 3,344,800, instead of 3,272,000° Fah., a difference which leads irresistibly to the inference that, either the solar atmosphere is more than 100,000 miles in depth, or it contains less matter than the terrestrial atmosphere, for corresponding area. It will be demonstrated hereafter that the retardation of the rays projected from the border of the photosphere consequent on the increased depth of the solar atmosphere (supposed to be the main cause of the observed diminution of energy near the sun's limb), cannot appreciably diminish the intensity of the radiant heat. The ratio of diminution of the density of the gases composing the solar atmosphere at succeeding altitudes, is represented by Fig. 5, in which the length of the ordinates of the curve a db shows the degree of tenuity at definite points above the photosphere. This curve has been constructed agreeably to the theory that the densities at different altitudes, or what amounts to the same, the weight of the masses incumbent at succeeding points, decreases in geometrical progression as the height above the base increases in arithmetrical progression. The vertical line ac has been divided into 42 equal parts, in order to facilitate comparisons with the terrestrial

atmosphere, the relative density of which, at correas correctly represponding heights, is obviously sphere.. It is true that, owing to the greater height sented by this diagram as that of the solar atmoof the latter compared with the attractive force of sphere will be relatively more powerfully attracted than the sun's mass, the upper strata of the terrestrial atmothe upper strata of the vastly deeper solar atmosphere. The ordinates of the curve a db will therefore not represent the density quite correctly in both cases. The discrepancy, however, resulting from the relatively inferior attraction of the sun's mass at the boundary of its atmosphere, will be very nearly neutralised by the increased density towards that boundary, consequent on the great reduction of temperature-fully 1,380,000° Fah.-caused by the dispersion of the solar rays before entering space. It may be well to add that, in representing the relative height and pressure of the terrestrial atmosphere, a cin our diagram indicates forty-two miles, while b c indicates a pressure of 14'7 pounds per square inch; and that in representing the solar atmosphere, a c indicates 100,000 miles and c 410 pounds per square inch. Bearing in mind the high temperature and small specific gravity, the extreme tenuity in the higher regions of the solar atmosphere will be comprehended by mere inspection of our diagram. Already midway towards the assumed boundary, the density of the solar atmosphere is so far reduced that it contains only 1300 of the quantity of matter contained in an equal volume of atmosphere at the surface of the earth.

Let us now consider the diminution of intensity occasioned by the increased depth through which the heat rays pass which are projected from the receding surface of the photosphere. Fig. 6 represents the sun and its atmosphere extending of the semi-diameter of the photosphere, mh, cg, &c., &c., being the heat rays projected towards the earth. The depth of the solar atmosphere at a distance of 18 of the radius from the centre of the luminary, will be seen to be only 2'0012 greater than the vertical depth. Now, careful actinometer observations enables us to demonstrate that when the zenith distance is under 60°, the radiant energy of the sun's rays in passing through the terrestrial atmosphere is very nearly in the inverse ratio of the cube root of the depth penetrated (see the previously published table). The increase of depth resulting from atmospheric refraction, it may be well to observe, is too small at moderate zenith distances to call for correction; nor does the atmospheric density vary sufficiently during bright sunshine to affect the radiant intensity appreciably. The table adverted to shows that an increase of the sun's zenith distance of 5' in 60° occasions a diminution of temperature hardly amounting to o'044° Fah. Adopting the same rate of retardation for the solar atmosphere as that observed in the terrestrial atmosphere, it will be found that the loss of radiant energy of the solar rays at of the radius from the border of the photosphere will be only 126 greater than at its centre. According to the researches of Secchi and others, the loss is fully three times greater than that established by the rate of diminution which we have adopted. This circumstance, in connection with the extreme tenuity of the solar atmosphere, rendering any considerable loss improbable, points to the fact that some other agency than increased depth is the true cause of the diminution of the temperature under consideration. Accordingly, the writer some time ago instituted a series of experiments with incandescent cast-iron spheres, for the purpose of ascertaining practically if the reduction of temperature could be accounted for solely on the ground that the obliquity of the rays diminishes their energy. Previous experiments had demonstrated that the accepted doctrine is quite incorrect, which teaches that heat rays emanating from the surface of incandescent radiators are projected with equal energy in all directions. It was found during those