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waters of the Tamina." The only points in Prof. Tyndall's description to which we a little demur are when he speaks of the traveller "passing along the chasm midway between top and bottom," the fact being that the well-known gallery is only a few yards above the Tamina; and where he quotes the gorge as an illustration of water-action upon limestone rock. It is true that the strata here are not crystalline, and they may be occasionally calcareous, but we should hardly venture to apply the name of limestone to the hard black shales or slates out of which the gorge itself is cut. Professor Tyndall also omits to call attention to the close connection between the direction of the principal joints and the form of the gorge. This is especially noteworthy at Pfäffers, where the chasm is not vertical, but inclined to the horizon at an angle of some 70°, the water having followed, as is its wont, the direction of least resistance, viz., one of the sets of joint planes. The gorges of the Pantenbrücke, the Aar above Im-Hof, with many others, might be quoted as instances of the same. We think, indeed, that in arguing against those who ascribe alpine sculpture mainly to fracture, the professor does not quite do justice to the influence which fissures, faults, and joints (which last may, in many cases, be connected with the others) exercise in directing the meteoric agents. These have not, indeed, fashioned the mountains, but they have obliged the sculpturing forces to work in certain directions, have been like the rails or the points which cause a locomotive to follow a particular course instead of wasting its power in wandering over the fields.

Further on in the chapter, Prof. Tyndall refers to his own favourite theory of glacier sculpture, with regard to which he expresses himself more guardedly than in the paper originally published in the Philosophical Magazine (vol. xxiv. p. 169). Still we cannot say that we are convinced by his arguments even in their modified form. No one, of course, would deny that a glacier can deepen its bed; the question is simply one of degree. With regard to this our space will allow us to do little more than express dissent, and indicate one or two points where, while not disputing Prof. Tyndall's facts, we cannot accept his inferences.

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is devoted to a résumé of the “viscous " and "regelation" theories of glacier motion; a controversy which can hardly yet be regarded as concluded, seeing that the experiments of Mr. Mathews and Mr. Froude, to which Prof. Tyndall briefly alludes, appear likely to have a very important bearing upon the question of whether or not ice under any circumstances is a flexible or plastic substance to an appreciable extent.

Among the very miscellaneous scraps with which the volume terminates, is an account of the voyage to Algeria to observe the Eclipse. This, so far as its main purpose went, was a dismal failure, but remarks are introduced on the colour of the sea and sky, a subject already treated by the author in his "Glaciers of the Alps." During the voyage home a number of bottles of sea-water were secured from various stations, which were afterwards examined in London by passing through them a beam of electric light-the purity or impurity of the water is then shown by the less or greater amount of light which it scatters. Briefly, the result was that the dark blue water was very pure, the cobalt-blue rather less so, while the green tints denoted the presence of much suspended matter, and the yellowish green was very thick. A remarkable instance of this variety of colour which, if our memory serve us, he has not quoted, is in the Lakes of Thun and Brienz; the waters of the latter, which receives the silty streams of the Aar and the Lutschine, are distinctly green, while those of the former, into which no important glacier torrent directly enters, are of a beautiful

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The silt which is brought down by a glacier stream HAVING noticed, on its publication, the first volume of cannot, we think, be taken as a measure of the abrasion Prof. Baillon's "Histoire des Plantes" (see NATURE, exercised by the glacier; surely the greater part is de- vol. i., p. 52) we need scarcely do more than call attenrived from the stones crushed between the ice and rock; tion to the English edition which now lies before us. The it is the grist from the glacier mill, rather than the detritus translation, we may say at the outset, appears to us to be of the nether stone. We fail also to see how, unless well done; the meaning of the original is, as far as we have observed, carefully preserved; and a better knowunder exceptional circumstances, a glacier can do more ledge of his subject is shown by the translator than is than abrade." Granted that "rocks are not homo- always the case in English renderings of foreign scientific geneous, they are intersected by joints and places of works. The co-ordination of the natural orders followed weakness which divide them into virtually detached in the work is, as was mentioned in our notice of the masses," we doubt if it follows that "a glacier is un- original, novel; whether it will stand is a question on which we ought not, perhaps, to express an opinion until doubtedly competent to root such masses bodily away." the plan is more fully developed. We could have wished A heavy body sliding over such masses and in close conthat the author had given in this first volume some general tact with them, would, we think, be more likely to keep sketch of his new system, with a defence of its peculiarithem in their place, and certainly rocks from which ties. So competent an authority as Prof. Baillon cannot glaciers have retreated do not exhibit evidence of this have departed from the ordinary arrangement without kind of erosive action. We confess, therefore, to still cogent reasons, which we should have liked to have known. regarding the effects of glaciers as comparatively super-know the views of their fellow-workers on the Continent. It is always a great advantage to English systematists to ficial, and classing the ice ploughs of past ages as among the efforts of scientific imagination.

A considerable portion of the latter part of the volume

We miss also the great assistance that is afforded to the systematist by a tabulated clavis of the genera belonging to each natural order. The amount of information con

tained in the volume as to the various relationships of the natural orders described in it, the morphology of their genera, the distribution of the different types, and the economic products obtained from the species, is immense. It possesses, however, the defect so common in foreign scientific works, of the absence of any table of contents or index to the subjects treated of. Had the publishers of the English edition supplemented the index of genera and subgenera with one referring to the various topics discussed, they would have rendered the English edition a practically more useful contribution to botanical literature

CALYCANTHUS FLORIDUS: Floriferous shoot.

than the French original. The illustrations are profuse, and of that excellence which we look for in vain in works originally published in this country. We append one of the well-known "Allspice Tree," the Calycanthus floridus. The small order Calycanthaceæ, including only the American Calycanthus and the Japanese Chimonanthus, is one the true position of which has been much disputed by systematists. Baillon makes it a series" of Monimiaceæ, with which he also unites the Australian Atherospermeæ, bringing this order forward from its usual position among the Incomplete to close alliance with Magnoliacea and Anonaceæ. A. W. B.

LETTERS TO THE EDITOR [The Editor does not hold himself responsible for opinions expressed by his Correspondents. No notice is taken of anonymous communications.]

A New View of Darwinism

I HAVE only just seen the two letters in answer to one from me on Darwinism which you were good enough to insert in NATURE, and to which I ask the favour of being allowed to reply. I have to thank Mr. Darwin for his references and for the tone of his letter, which is in such marked contrast to the angry dogmatism of Mr. Wallace.

Mr. Wallace commences by ridiculing the phrase the Persistence of the Stronger. The phrase was not mine, it has been used by a better man than I, namely, by Prof. Jowett, and it has the advantage of not involving an identical expression, which the Survival of the Fittest does. That those forms of life survive which are best adapted or best fitted to survive," is not a very profound discovery; it might have suggested itself even to a child, and if Mr. Wallace means nothing more than this when

he speaks of the theory of Natural Selection, he cannot claim to have added much to the world's philosophical opinions. He then complains that I have only touched one of the many facts relied upon by Darwinians; I refer him to my letter, in which I distinctly say that it contained only one of my objections, and that I have many more which will follow if the Editor have patience with the discussion. The reply to Mr. Wallace will confine me, however, in this letter to the ground covered by the former one. Having disposed of the formal and personal matters, I now approach the matters of fact about which we are at

issue.

Here, I am sorry to say, I am met in a very different spirit by Mr. Wallace to that in which Mr. Darwin meets objections. Dogmatism, bold and unwavering, was the privilege of the philosophy of the Schools, but in the 19th century it is puerile. Mr. Wallace states boldly, without any authorities, merely as an imperial ipse dixit, that the most vigorous plants and animals are the most fertile. I had, at least, the decency to quote the book of Mr. Doubleday, containing a magazine of facts and examples in support of my view, and which tells exactly the other way.

This view has not been correctly stated by Mr. Wallace. The position I maintain is this, that, as a general law, those individuals which are underfed and lead precarious lives, are more fertile than those whose advantages make them vigorous and healthy. The ringing of the bark and the pruning of the roots of barren fruit trees and the starving of domestic animals to make them fruitful were examples to this end.

Mr. Wallace quotes only one example in his own support, and I will accept it as a crucial test of my position, which he will acknowledge to be fair; the case of the Red Indian and the Backwoodsman. The Red Indian lives entirely on flesh, the Backwoodsman almost entirely on vegetable food. Like meat livers in every part of the world, in Mexico, on the River Plate, in Siberia, in Turkestan, and in some parts of Russia, the Red Indian is not a fertile creature. The Backwoodsman, like vegctable feeders everywhere who are not luxurious, in India, China, Poland, and the Russian provinces bordering on it, Ireland, &c., is comparatively fertile, but only comparatively. It is a mistake to suppose that the Backwoodsman is specially fertile, and in a few years he becomes, as the inhabitants of Kentucky and Tennes-ee have been long known to be, diminishing in numbers, the population of the States being kept up by immigration.

Mr. Chadwick, in his "Sanitary State of the Labouring Classes," observes that where mortality is the greatest there is much the greatest fecundity; thus, in Manchester, where the deaths are one to twenty-eight, the births are one to twenty-six, while in Rutlandshire, where deaths are but one to fifty-two, births are one to thirty-three, showing that a state of debility of the population induces fertility. This only supports the common dicta of doctors that consumptive patients are generally very fertile. The pastoral tribes of Eastern Russia which have recently taken to agriculture, such as the Tchuvashes, &c., have begun to increase most rapidly. The Hottentots at the Cape, who were formerly a numerous race living very hard lives, are almost extinct now that they are carefully tended and well fed. The Yeniseians, the Yukahiri, and other Siberian tribes, have disappeared like smoke before the advance of Russian culture; they have suffered little if at all from the Russian arms.

Let me quote a curious example in answer to Mr. Wallace from the very race to which he has referred. Captain Musters, in describing his recent journey through Patagonia at the Anthropological Institute, told us that it was the custom for the Patagonian women to be bled at certain times referred to, as they believed it made them fertile. Among the Patagonians, therefore, we meet with empirical witnesses, unsophisticated by our philosophy, to the truth of the position I maintain. But those who live in large cities need not travel to Patagonia. The classes among us who teem with children are not the well-to-do and the comfortable, but the poor and half-fed Irish that crowd the lowest parts of our towns. I am not contrasting now the fat with the lean, but the comfortable classes with those who lead precarious lives--the vigorous in health with the sickly, the half-fed, and the weak. It will be asked, why rely so much upon man? The answer is that I quite agree with Mr. Darwin that man is subject to the same natural laws as the animals, and further I believe that since we have studied man more closely and under a greater variety of conditions, facts derived from our experience of man are of greater value than those deduced from our examination of the other animals.

But let us turn to these latter for a space; and here I tread with much greater diffidence, for I am aware of the vast ex

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perience and fund of illustration possessed by Mr. Darwin, and I have to say that I am unconvinced by the arguments he has adduced. With the transparent frankness of all his writings, Mr. Darwin, in one of the references to which he has commended me, has collected a very large number of examples that tell very strongly against him, and which I again commend to Mr. Wallace. I refer to the 18th chapter of Mr. Darwin's book on the "Variation of Plants and Animals under Domestication." and especially to that portion beginning on page 149. In speaking of animals, he says:" The most remarkable cases, however, are afforded by animals kept in their native country, which, though perfectly tamed, quite healthy, and allowed some freedom, are absolutely incapable of breeding Rengger, who in Paraguay particularly attended to this subject, specifies six quadrupeds in this condition, and he mentions two or three others which most rarely breed. Mr. Bates, in his admirable work on the Amazons, strongly insists on similar cases, and he remarks that the fact of thoroughly tamed wild animals and birds not breeding when kept by the Indians, cannot be wholly accounted for by their negligence or indifference, for the turkey is valued by them, and the fowl has been adopted by the remotest tribes. In almost every part of the world, for instance, in the interior of Africa, and in several of the Polynesian islands, the natives are extremely fond of taming the indigenous quadrupeds and birds, but they rarely or never succeed in getting them to breed," and so on, through sixty pages of closely-packed examples. And what is Mr. Darwin's commentary on these facts? I again quote page 158:-"We feel at first naturally inclined to attribute the result to loss of health, or at least to loss of vigour, but this view can hardly be admitted when we reflect how healthy, long-lived, and vigorous many animals are under captivity, such as parrots and hawks when used for hawking, chetahs when used for hunting, and elephants. The reproductive organs themselves are not diseased, and the diseases from which animals in menageries usually perish are not those which in any way affect their fertility. No domestic animal is more subject to disease than the sheep, yet it is remarkably fertile." Mr. Darwin, with equal clearness and conclusiveness, decides that this sterility cannot be due to a failure of sexual instincts, change of climate or of food, or want of food or exercise; and be concludes that certain changes of habits and of life affect in an inexplicable manner the powers of reproduction. But what is true of man it is reasonable to suppose is true of all these instances-namely, that it is a more luxurious habit, a more vigorous health, a less precarious existence, induced by the care and attention of domesticators, that have caused the sterility; that these animals are too well off, and not that they are ill off in any way; and this theory explains the whole most conclusively. On the other hand, and in opposition to this vast and uniform collection of examples, Mr. Darwin adduces a few instances which tell the other way, but they are very few in number, and seem to me explicable on other grounds. Ferrets, it is notorious, are always kept in a state of extreme depletion and as thin as possible. Domestic poultry are fed almost entirely on poor vegetable food, while their wild and semi-wild relatives feed much more on worms, insects, and on animal diet generally. In regard to sheep, it is notorious that very weak ewes generally bear twins, that Somersets and Dorsets are more fertile than Southdowns and Leicesters. We have, I may add, no facts to guide us in regard to wild dogs, and few in regard to wild cats; but we do know that in tame ones the half-fed lantern-ribbed curs are more prolific than their sleek relations. In regard to domestic fowls, and especially pigeons, we must remember that their condition is materially altered by the disuse or only very partial and irregular use of their powers of flight, this must reduce their circulation and vigour very considerably, and make them pro tanto so much weaker. But these instances, upon which Mr. Darwin relies to answer Doubleday and others, are very partial indeed. In his own pages, as I have already said, they form a very small element compared with the overwhelming cases he quotes on the other side. So much so, indeed, that these cases may be taken as exceptions which prove the rule that domestication and improved conditions of life induce sterility in animals.

It savours of scholastic philosophy to speak of Nature as exercising any influence on the regeneration of races, and yet there may be sound philosophy in the old notion that when an individual or a class is in danger of being extinguished from want, Nature puts forward a special effort to preserve it. The sickly mother, the half-starved plant, is more likely to breed than the healthy and the vigorous. If we remove the peasant's family to the drawing room, it will cease to be composed of ten and twelve children. If we remove our daisies and

cowslips to the greenhouse, their flowers grow double, and they ripen no seeds. The vine that has felt the frost is the one to pay the rent. Wherever we turn, in fact, we meet with examples of the universal law; and this law seems to be at issue with an important portion of Mr. Darwin's theory, namely, that in the struggle for existence, the vigorous, the hearty, and the well-to-do, elbow the weak and decrepid until they elbow them out of existence, and supplant them. If I have said anything above which can be construed into an impertinence, I unconditionally withdraw it. The only excuse for soreness, is an impatience at what seems to the writer to be indefensible dogmatism. The days will not be ripe for scientific dogmatism until the Infallibility of Positive Philosophers has been generally accepted, and it does not do to forestal that millennium. H. HOWORTH

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I HAVE been long accustomed to register the first appearance of new words and phrases. Of course the vast majority of these take no root, perishing where they fall. Here is a sample of the latest and impolicy, which were brought in by the Franco-Prussian War, issue: Survival, introduced, I think, by Darwin; indiscipline and also the vulgarism to telegram. The greatest atrocities in this line are committed by "physicists," if the shade of Faraday wil pardon me the use of that word; and far away the worst coinage I ever encountered is due to Mr. Alfred R. Wallace. As it is "meet and right and our bounden duty" to stigmatise such intruders, and if possible prevent their adoption, I take the liberty of making my feeble protest against Mr. Wallace's "prolificness," which he introduces to our notice in his letter on Mr. Howorth (NATURE, July 6, 1871, p. 181). In this case the hideousness of the coinage is some guarantee against its reception. Malvern Wells, July 8 C. M. INGLEBY

Affinities of the Sponges

I HAVE just read with much interest the paper in NATURE by Mr. W. Saville Kent, criticising my friend Carter's article in the "Annals of Natural History" for this month, in which I fully concur. How Mr. Carter can have fallen into such an error, for such I must call it, I cannot imagine, as comparing a group of animals in Botryllus to those sponge cells, even in so highly a developed form as Grantia. For, taking this as the highest known form of sponge animal, it is at most only a monociliated sac, as shown both by Prof. Clark and by Mr. Carter. Now, it is well known to all investigators, and Mr. Carter has shown it himself, that the animals of Botryllus have distinct oral and fæcal apertures, whereas the sponge cell, so far as has yet been seen, has only an oral aperture. Again, the Ascidian Botryllus is shown to be far higher in the scale when we come to compare its internal organisation, and not merely to confine ourselves to the sac-like tunic. The discharge of the fecal matter into a common cloacal canal is to me not a sufficient reason for comparing these groups of animals to the sponge animals in Grantia.

But what I wish to draw attention to more particularly is this, that in the hurry and bustle of our investigators of the present day, all old associations are mostly, if not entirely, forgotten. I can scarcely think that they are ignored, but are forgotten. Thus, Prof. Grant was, I believe, the first to determine the character and the full importance of the seed-like body in Halichondria by placing watch-glasses in the vessel in which living specimens of the above sponge was placed; the bodies were thus discharged from the fæcal canal of the parent sponge, and attached themselves to the watch-glasses, and he then carefully watched their development. Mr. Carter, being a pupil of Dr. Grant, no doubt followed his teacher's plan of investigation, which has led to the brilliant results of this gentleman's in

vestigations of the fresh-water species in the tanks at Bombay. The clear and lucid manner of investigation detailed by Prof. Grant in the Edinburgh New Philosophical Journal (1826-27) might be held as a pattern for investigators, but he appears to be almost entirely lost sight of.

Again, as regards the animals of Grantia compressa, Prof. Reay Greene certainly preceded both Prof. Clark and Mr. Carter in his investigations, and has figured these monociliated animals in his handbook, published in 1859, p. 31, fig. 6. The figures are on the same scale as those given by Mr. Carter, and indeed some of the groups figured are so much like those given by Mr. Carter in the " Annals," pl. I, sqr. 13, a, g, h, that it would be difficult to separate them, and the same may be said of Fig. 41 of Prof. Clark's in "Ann. Nat. Hist." pl. 6, 1868. The only difference being the want of the funnel-shaped mouth, which seems to have escaped the observation of Prof. Greene, probably owing to want of definition in the instrument used in the investigation. Now there is an amount of credit due to the first demonstrator of these animals, which, so far as I have seen, does not appear to have been accorded to him; and I therefore take the liberty of directing attention to this fact. I do not know Prof. Greene, and therefore do not take up this matter on personal grounds, but only in fairness due from one scientific man to another, and I hope my friend Carter will take this in the spirit it is intended. EDWARD PARFITT Exeter, July 8

Cramming for Examinations

I ENCLOSE one or two bona fide extracts from "Middle Class" examination papers which have during the past few weeks come under my notice officially.

I do not wish thereby to reflect so much on the candidates as upon the mode of teaching in Middle Class schools, which produces such results.

As might be expected, where evidence of "cramming" from a text-book and want of practical knowledge are equally manifest, some of the answers in the papers from which these are selected are pretty good-but what can be the real value of knowledge of this sort?

The questions are sufficiently indicated by the answers.

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"We have now the most excellent season you can imagine in these latitudes, the average temperature for this month (June) being as high as 59°, which is 12° higher than the mean temperature of the past four Junes. I was yes erday near the Hengil Mountain, just at that place where we pitched our tent last time you were here, and the heat was quite unsupportable in the valleys. The wind has been continually blowing from the southwest. Some Englishmen setting out for the Geyser will have something to tell of the extraordinary heat we have at present." ALEXANDER BUCHAN

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The Late Thunderstorm

AN ash tree in the garden attached to the farmhouse of Wester Cringate, near Fintry, struck by lightning on the 20th of June, presents a singular appearance.

About 20ft. from the ground a large branch has been torn from the trunk. The bark has been neatly peeled off for a few feet above and below the place from which the branch shot out. The wood has been first struck a little above the branch, and shows a clean cut, such as might have been made by a sharp-edged tool, as if a chisel three inches broad had been driven into the wood for about four inches. The branch itself has been torn, not cut, and a stripe of the trunk about two feet long below the branch has also been torn out.

For the next four or five feet the tree has suffered no damage of any kind, but after that space the trunk bears six parallel downward scars, varying in length from two to five feet. The scars do not all begin or end at the same height, although each might be cut in some point by a horizontal plane passing through the tree. They spread over about half the circumference of the trunk, and can all, or nearly all, be seen from one standpoint. The most striking circumstance, however, is the almost perfect parallelism of the scars, which are not vertical, but a little twisted round the trunk like the rifling of an Armstrong gun, the rifling in this case being on the outside of the barrel. Six chisels of about half an inch in breadth seem to have ploughed into the wood, tearing off at the same time rather broader stripes of the bark. Towards their lower ends the three right-hand scars cease to be quite parallel, and tend to converge; but all three die out before the convergence takes place, and the tree for the next two feet or so is unscathed. Five feet from the ground (at about the point at which the three scars would converge, if produced) a single rut cutting deeply into the wood commences, which continues down to the soil.

The garden wall (which is a "dry-stone dyke," i.e. of loose uncemented stones) passes some three feet behind the tree, on the side directly opposite to that on which the markings above described occur. Outside of this garden the lightning has ploughed two pretty deep parallel ruts through the grassy soil some four feet apart, and stretching from the foot of the wall to the edge of a ditch, a distance of three feet. These ruts are the last observable traces of the passage of the lightning, and were probably made by the currents which engraved the three left-hand scars on the tree. Of course it is impossible to decide whether the currents passed through the open wall, or down the outside of it.

Three sheep on the neighbouring farm of Spittalhill were killed in the same thunderstorm. Their carcases were found lying in a line and were very much swollen, but bore no external marks of injury. A small patch of wool had been stripped from the flank of one of them, but probably this had no connection with the cause of death. R. L. JACK Geological Survey, Fintry by Glasgow, July 5

Saturn's Rings

As you have favoured my work on "Saturn's Rings and the Sun with criticism, I feel sure that as that criticism is adverse to my views, you will in fairness allow me to reply to it.

I will do so in detail. Your reviewer commences very much under the impression that Prof. Clerk Maxwell having investigated the "Stability of Saturn's Rings," no one else is to venture into any discussion touching on their nature or origin. In fact he issues a caveat-Prof. Clerk Maxwell has concluded the subject! Next he asserts that I have not seen the Professor's work, because I ascribe to the perusal of Mr. Proctor's "Saturn and its System," the enlistment of my "interest in favour of the Satellite Theory." This is surely beyond his province, as I am free to choose my own point of starting. Mr. Proctor's work interested me, and so did Mr. Clerk Maxwell's, but the former elicited my work, the latter did not.

He next accuses me of placing too great faith in figures, and shows surprise at my giving the hourly rate of the solar motion to a mile, and the solar parallax to four places of decimals. The solar motion is that given by the Herschels, and the solar parallax by several observers. He is hard to please. But my reviewer has unfortunately missed the point of my arguments. The actual velocity of this solar motion is perfectly immaterial; indeed, had he followed the reasoning, he would have seen how pointless are his objections.

As regards my arguments in favour of the meteoric theory of the sun, the reviewer is equally inaccurate. As to my being

blindly enraptured with that theory, as he is pleased to state, I only reply that very clever men have held it, as he is perhaps aware; and certainly none of the modern theories, cumbrous vagaries of the brain, can compare with it. I have never said that the meteoric theory is the real explanation, but I doubt if we shall ever arrive at a more truthful representation of the solar phenomena.

Lastly, he culminates by saying I am "either innocently or wilfully ignorant of the palpably cyclonic appearance which spots frequently present." All I can say in answer to this is that, having observed sun-spots myself for many years, probably as often as the reviewer, I have never observed one single appearance of a cyclonic nature. As I possess Mr. Carrington's valuable work, I have again referred to it, and find it in agreement with the assertion in my book and my own observation. I must apologise for my lengthy letter. A. M. DAVIES

2, Gloucester Terrace, Sandgate, July 4

On an Error in Regnault's Calculation of the Heat Converted into Work in the Steam Engine

IN Watts's "Dictionary of Chemistry" (vol. iii. p. 125), in the article on Heat by Prof. G. C. Foster, it appears to me that an important error has crept into the discussion of the above calculation.

The nature of the calculation is as follows:-A unit weight of saturated steam at the temperature of 152°C. contains 653 units of heat. Suppose we allow the steam to expand and to do work until the temperature falls to 503°C. the steam then contains 621 units or 32 units less than before, hence starting with water at o°C., we give it 653 units of heat, and of this 32 only are converted into work, giving us the fraction as the amount of heat converted into work; but the real work produced by an engine is more than twice this. This difference in theory and practice is accounted for by the fact that saturated steam, in expanding and doing work, is partly condensed, hence the body with which we have to deal at the lower temperature is not all steam, but partly condensed water, therefore, does not contain so much heat as was allowed it.

This explanation is so intelligible as to be at first sight sufficient to account for the whole difference; there is, however, another cause, quite as important, and which is this; every time steam passes from the boiler to the cylinder it does work before it is cut off, and allowed to expand; this work is not done at the expense of the steam that passes into the cylinder, but of the whole mass of steam in the cylinder and boiler, which expands and is thereby cooled. The mass of water and steam in the boiler is, however, so large compared to that which passes into the cylinder, that a thermometer could scarcely detect the cooling effect upon it, and before the next stroke this loss of temperature is made up by the fire. Though thus inappreciable, it is nevertheless very important, and in most engines would amount to one-third the work done; in fact all the work done by the steam before it is cut off and allowed to expand is entirely neglected in this calculation, and a source of error introduced.

To correct it there should be added to the heat in the steam at the initial temperature, as many units of heat as the work done before the steam is cut off, would, if converted into heat, raise the amount of water which passes at every stroke in the form of steam into the cylinder. A. W. BICKERTON

Hartley Institution, Southampton, June 26

THE CAUSES OF THE COLOURS OF THE SEA*

PROF. TYNDALL, in his article in the Fortnightly

Review for the 1st of March, attributes the greenness of the sea to the matter which it holds in solution. Perhaps the following may corroborate his theory. About the Andaman Islands, where the sea is of the deepest blue, there are most startling and sharply-defined changes of colour, from bright blue to green, where a bed of coral exists. This coral is white out of the water, what its colour when growing may be I know not, but the change I mention appears to corroborate the remarks in the article in question, which are appended below, about the green hues observed upon the plate, the screw blades, and

* Communicated by Prof. Tyndall.

the white bellies of the porpoises. One looks down from a hill into a bay of the brightest blue; you see it broken up here and there like a child's puzzle map by irregular patches of as bright green, often crossing several acres as sharply defined as it is possible to imagine, and indicating the existence of coral beds or reefs just below. Bellary, Madras Presidency W. M'MASTER

[We give the passages referred to from Prof. Tyndall's lecture.-ED.]

"Let us clear our way by a tew experiments towards an explanation of the dark hue of the deep ocean. * Colour, you know, resides in white light, appearing generally when any constituent of the white light is withdrawn. Here is a liquid which colours a beam sent through it purple, and this colour is immediately accounted for by the action of the solution on a spectrum. It cuts out the yellow and green, and allows red and blue to pass through. The blending of these two colours produces the purple. Does the liquid allow absolutely free passage to the red and blue? No. It enfeebles the whole spectrum, but attacks with special energy the yellow and green colours. By increasing the thickness of the stratum traversed by the beam, we cut off the whole of the spectrum. Through the deeper layer, which I now place in the path of the beam, no colour can pass. Here, again, is a blue liquid. Why is it blue? Its action on the spectrum answers the question. It first extinguishes the red; then as the thickness augments it attacks the orange, yellow, and green in succession; the blue alone finally remains, but everything might be extinguished by a sufficient depth of the liquid.

"And now we are prepared for a concentrated but tolerably complete statement of the action of sea water upon light, to which it owes its blackness. Here is our spectrum. This embraces three classes of rays-the thermal, the visual, and the chemical. These divisions overlap each other; the thermal rays are in part visual, the visual rays in part chemical, and vice versa. The vast body of thermal rays is here beyond the red and invisible. They are attacked with exceeding energy by water. They are absorbed close to the surface of the sea, and are the great agents in evaporation. At the same time the whole spectrum suffers enfeeblement; water attacks all its rays, but with different degrees of energy. Of the visual rays the red are attacked first, and first extinguished. While the red is extinguishel, the remaining colours are enfeebled. As the solar beam plunges deeper into the sea, orange follows red, yellow follows orange, green follows yellow, and the various shades of blue, where the water is deep enough, follow green. Absolute extinction of the solar beam would be the consequence if the water were deep and uniform, and contained no suspended matter. Such water would be as black as ink. A reflected glimmer of ordinary light would reach us from its surface, as it would from the surface of actual ink; but no light, hence no colour, would reach us from the body of the water. In very clear and very deep sea water this condition is approximately fulfilled, and hence the extraordinary darkness of such water. The indigo, to which I have already referred, is, I believe, to be ascribed in part to the suspended matter, which is never absent, even in the purest natural water, and in part to the slight reflection of the light from the limiting surfaces of strata of different densities. A modicum of light is thus thrown back to the eye, before the depth necessary to absolute extinction has been attained. An effect precisely similar occurs under the moraines of the Swiss glaciers. The ice here is exceptionally compact, and owing to the absence of the internal scattering common in bubbled ice, the light plunges into the mass, is extinguished, and the perfectly clear ice presents an appearance of pitchy blackness.

"The green colour of the sea when it contains matter in a state of mechanical suspension has now to be accounted for; and here, again, let us fall back upon the sure basis of experiment. This white plate was once a complete dinner-plate, very thick and strong. It is, you see, surrounded securely by cord, and to it a lead weight is fastened. Forty or fifty yards of strong hempen line were attached to the plate. With it in his hand, my assistant, Thorogood, occupied a boat fastened as usual to the davits of *A note written to me the 22nd of October, by my friend Canon Kingsley, contains the following reference to this point:-"I have never seen the Lake of Geneva, but I thought of the brilliant, dazzling dark blue of the mid Atlantic under the sunlight, and its black blue under-cloud, both so solid that one might leap off the sponson on to it without fear; this was to me the most wonderful thing which I saw on my voyage to and from the West Indies."J. T.

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