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of the climatic features of British Columbia, may be found in an appendix written by me for the Canadian Pacific Railway Report of 1880, p. 107.

The mean temperature of Tongass at the southern extremity of Alaska, from two years' observations, is stated as 46° 5.

Observations have been maintained at Sitka with little interruption for a period of forty-five years. The latitude of this place is 57° 3', or about one degree north of Glasgow. The mean temperatures are as follows:-spring 41°2, summer 54° 6, autumn 44°9, winter 32°5, and for the year 43° 3.

According to the Pacific Pilot above quoted, that portion of the Kuro-siwo, having a temperature of 55° F. or more, approaches the coast in the vicinity of Vancouver Island. Temperatures not much lower than this however prevail much further north. The average temperature of the surface of the sea during the summer months in the vicinity of the Queen Charlotte Islands as determined by me in 1878 ("Report of Progress, Geological Survey of Canada, 1878-79") is 53°.8. Observations by the U.S. Coast Survey in 1867, in the latter part of July and early in August between Victoria and Sitka, gave a mean surface-temperature of 52°1. GEORGE M. DAWSON Geological Survey of Canada, February 1

"The New Cure for Smoke"

It was not my intention to trouble you further on this subject at present, but as Dr. Siemens has been good enough to notice the result of my trials with the coke-gas grate, and has asked a question with reference to the grate used by me, it is due to that gentleman that I should at once explain that the grate in which the trials were made is of modern construction and permanently fitted with side-cheeks and back of fire-clay lumps, and that when in use with the coke and gas the back was fitted with a copper plate, and in all other respects the grate was arranged in the manner described and illustrated in NATURE, vol. xxiii. P. 26. J. A. C. HAY

On the Space Protected by Lightning-Conductors THE very interesting article by Mr. W. H. Preece on the "Space Protected by a Lightning-Conductor" (Phil. Mag. 5th series, vol. x. P. 427 et seq., December, 1880) revives this important practical question. The old rule, first enunciated by M. Charles, which makes the radius of the protected circular area around the base of the rod equal to twice its vertical height, has never been satisfactorily verified either on theoretical or experimental grounds. This rule was adopted in the Report of the Commission of the French Academy of Sciences drawn up by M. Gay-Lussac in 1823 (Ann. de Chim. et de Phys. 2nd series, t. 26, p. 258), and also in two other reports drawn up by M. Pouillet, one in 1854 (Comptes rendus, t. 39, p. 1142), and the other in 1867 (Comptes rendus, t. 64, p. 102). But still more recently the Committee appointed by the Préfet de la Seine to superintend the construction of lightning-conductors in the City of Paris, in their Report in February, 1876, reduced the radius of the protected area to 1'45 times the height of the rod. I am ignorant on what grounds the Commission adopted this precise number.

In this state of the problem Mr. Preece's paper was both apposite and welcome. The rule which he deduces certainly has the merit of definiteness; but it seems to me that it fails to be practically satisfactory. For it is very evident that his investigation is exclusively applicable to "Blunt-Conductors," since the "Power of Points" is entirely left out of consideration. His deductions might apply to the blunt-conductors which crowned the Royal Palace of George III., but are scarcely applicable to the pointed rods now employed! His investigation assumes that the distance of the earth-connected objects from the electrified cloud is the only element which determines the direction of the discharge. It seems to me that the well established " power of points to discharge, or rather to neutralise the electricity of charged conductors, is an essential element in the problem of the protected space.

It is a well-known fact that when an electrified cloud approaches a pointed lightning-conductor which is in good conducting connection with the earth, the sharp point becomes charged by induction with o, posite electricity of high tension long before the distance between them approximates that required for a disruptive discharge; so that electricity of the opposite kind from that of the cloud escapes from the point in the form of a connective discharge or electrical glow, and

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It is very difficult, if not impossible, to estimate in a precise manner how this power of points would modify and distort the equipotential surfaces in the intervening electric field. problem is evidently one of great complexity. The following circumstances must obviously influence, to a greater or less extent, the magnitude and direction of the resultant electromotive force, which determines the path of discharge, convective or disruptive, viz.: (1) Distance of thunder-cloud from the point of the conductor; (2) variable dielectric properties of the intervening air; (3) size of the cloud; (4) the variable tension of its electric charge, especially under the neutralising action of the pointed rod; and (5) the velocity with which the thunder-cloud approaches the point of the conductor. The last consideration is very important, and at the same time most difficult to formulate; for the convective neutralisation is a gradual process requiring time. It is evident that a heavily-charged thunder-cloud rapidly driven spark, while, if slowly approaching the same, it would have been towards the point of the conductor might give rise to a disruptive silently neutralised, and the stroke averted. In fact the strength and direction of the resultant force is influenced by so many variable conditions that it would tax the resources of a powerful calculus to indicate a formula which would satisfy, even approximately, the demands of practice in the construction of lightning

conductors.

Nevertheless, it is quite certain that Mr. Preece's rule, which makes the radius of the protected circular area equal to the height of the rod for blunt conductors, is perfectly safe for pointed rods; for there can be no question as to the fact that the power of points" enlarges the protected area.

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The late Prof. Henry frequently witnessed the efficacy of convective discharges from the point of the lightning conductor attached to the high tower of the Smithsonian Institution. During violent thunder-storms at night, at every flash of lightning he observed that "a jet of light, at least five or six feet in length, issued from the point of the rod with a hissing noise."

It is proper to add that while the circumstances influencing disruptive discharges of electricity have been experimentally investigated by a number of physicists, the laws of convective discharges from points do not seem to have received attention from any experimenter. Thus I have not been able to find a satisfactory answer to the following elementary inquiry, viz.Under given conditions, at what distance will a pointed conductor connected with the earth begin to neutralise the electricity of an insulated conductor by the convective discharge of the opposite kind of electricity from the point?

In short, the whole subject of the "power of points," although one of the best-established and most conspicuous phenomena in electricity, is sadly in need of experimental investigation. This class of electrical phenomena is pretty much in the same condition in which Franklin left it more than a century ago. Berkeley, California, January I

JOHN LE CONTE

[Mr. Preece has shown by considering the area between the conductor and the charged cloud as an electric field mapped out in equipotential surfaces and lines of force, that "a lightningrod protects a conic space whose height is the length of the rod, whose base is a circle having its radius equal to the height of the rod, and whose side is the quadrant of a circle whose radius is equal to the height of the rod."-Phil. Mag., December, 1880.- ED.]

Localisation of Sound

My friend the Rev. H. J. Marston, Second Master of the School for Blind Sons of Gentlemen at Worcester, has communicated to me some very singular instances of the power of localising sound possessed by blind boys.

One of the games in which his pupils most delight is that of bowls A bell is rung over the nine-pins just as the player is ready to throw the bowl, when, totally blind as he is, he delivers it with considerable accuracy of aim. Mr. Marston vouches for the fact that it is no uncommon feat for a boy to strike down a single pin at a distance of forty feet three times in succession.

It is significant that this game cannot be played by the blind boys in windy weather. And yet the allowance for windage on a heavy bow I can be no very large quantity.

The boys also play football with great zeal and considerable skill. Bells are rung at the goals throughout the game, and the ball contains two little bells. With these guides the boys manage both to follow the ball and to direct it to the goals. Clifton College, February 15 H. B. JUPP

Migration of the Wagtail

THE inclosed extract from the New York Evening Post, a newspaper of high standing for accuracy and intelligence, contains statements which are not, I think, generally known in regard to the migration of the water-wagtail, and your insertion of the same may be the means of drawing from other correspondents some evidence in confirmation or disproof. Though riding is not quite unknown among animals other than men, yet such purposeful riding as is here described is, to say the least, very extraordinary. E. W. CLAYPOLE

Antioch College, Yellow Springs, Ohio, Dec. 12, 1880

The Singular Methods of Travel the Wagtail adopts to Cross the Mediterranean Sea.-In the autumn of 1878 I spent several weeks on the Island of Crete. On several occasions the papas-village priest-a friendly Greek with whom I spent the greater part of my time-frequently directed my attention to the twittering and singing of small birds which he distinctly heard when a flock of sand-cranes passed by on their southward journey. I told my friend that I could not see any small birds, and suggested that the noise came from the wings of the large ones. This he denied, saying, "No, no! I know it is the chirping of small birds. They are on the backs of the cranes. I have seen them frequently fly up and alight again, and are always with them when they stop to rest and feed." I was still sceptical, for with the aid of a field-glass I failed to discover the "small birds" spoken of. I inquired of several others, and found the existence of these little feathered companions to be a matter of general belief among both old and young. I suggested that possibly the small birds might go out from the shore a short distance and come in with the cranes. "No, no," was the general answer, "they come over from Europe with them." I certainly heard the chirping and twittering of birds upon several different occasions, both inland and out upon the sea. But in spite of the positive statements of the natives I could not believe their theory until con. vinced one day while fishing about fifteen miles from the shore, when a flock of cranes passed quite near the yacht. The fishermen, hearing the "small birds," drew my attention to their chirping. Presently one cried out "There's one," but I failed to catch sight of it. Whereupon one of them discharged his flintlock. Three small birds rose up from the flock and soon disappeared among the cranes.

I subsequently inquired of several scientific men, among whom were two ornithologists, as to the probability of such a state of affairs. They all agreed that it could not be, and I, too, was forced to cling to my original judgment, and let the matter go. Recently however while reading the Gartenlaube my atten tion was attracted to an article bearing directly upon the subject. The writer, Adolf Ebeling, tells the same story, and adds the statements of some ornithologists of distinction, which makes the whole matter so striking and interesting that I quote the paragraph from his book :

:

"Shortly after my arrival in Cairo I greeted various old German friends among the birds that I observed in the palm-garden of our hotel. First, naturally, was the sparrow, the impudent proletariat-I had almost said social democrat, because the whole world to-day has that bad word in the mouth. He appeared to me to be more shameless than ever in the land of the Pharaohs, for he flew without embarrassment on the breakfast table, and picked off the crumbs and bits from every unwatched place. But the mark of honour we paid to the wagtails, and in truth chiefly because we did not then know that the wagtails were birds of passage. We had thought that they passed the winter in Southern Europe, or at farthest as many of them do, in Sicily and the Grecian Islands. That they came to Africa, and especially to Nubia and Abyssinia, was then unknown to us. This appeared to us singularly strange, nay, almost incredible, particularly on account of the peculiar flight of the wagtail, which it is well known always darts intermittingly through the air in longer or shorter curves, and apparently, every few moments, interrupts

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its flight to sit again and 'wag its tail.' But there was the fact, and could not be denied. Everywhere in the gardens of Cairo you could see them under the palms that border the banks of the Nile; on the great avenues that lead to the pyramids; nay, even on the pyramids themselves in the middle of the desert. And there it was that I first heard of this singular phenomenon. "One evening we were sitting at the foot of the pyramid of Cheops, sipping our cup of fragrant Mocha and in jolly conversation, rolling up clouds of blue smoke from our Korani cigarettes. We were waiting for the sinking of the sun to make our return to Cairo. The deep silence of the surrounding desert possessed something uncommonly solemn, only now and then distur bed by the cry of the hoarse fishhawks far above us. Still higher the pelicans were grandly circling. Their flight, though heavy when seen from anear, possesses a majesty in the distance attained by no other bird. Right before us several wagtails were hopping around and tilting. They were quite tame, and flew restlessly hither and thither. On this occasion I remarked, 'I could not quite understand how these bird's could make the long passage of the Mediterranean.' Sheik Ibrahim heard this from our interpreter. The old Bedouin turned to me with a mixture of French and Arabic as follows, which the interpreter aided us to fully comprehend :

"Do you not know, Hadretch (noble sir), that these small birds are borne over the sea by the larger ones?'

"I laughed, as did our friends; for at first we thought we had misunderstood him; but no: the old man continued quite naturally :

"Every child among us knows that. These little birds are much too weak to make the long sea journey with their own strength. This they know very well, and therefore wait for the storks and cranes and other large birds, and settle themselves upon their backs. In this way they allow themselves to be borne over the sea. The large birds submit to it willingly; for they like their little guests, who by their merry twitterings help to kill the time on the long voyage.'

"It appeared incredible to us. We called to a pair of brown Pedouin boys, pointed out the wagtails to them, and inquired :— "Do you know whence come these small birds?' "Certainly,' they answered. 'The Abu Saad (the stork) carried them over the sea.'

"At supper, in the Hôtel du Nil, related the curious story to all present, but naturally enough found only unbelieving ears. "The only one who did not laugh was the Privy Councillor Heuglin, the famous African traveller, and, excepting Brehm, the most celebrated ornithologist of our time for the birds of Africa. I turned to him after the meal, and inquired of his faith. The good royal councillor smiled in his caustic way, and with a merry twinkle remarked: 'Let the others laugh: they know nothing about it. I do not laugh, for the thing is known I should have recently made mention of it in my work if I had had any strong personal proof to justify it. We must be much more careful in such things than a mere story-teller or novel-writer; we must have a proof for everything. I consider the case probable, but as yet cannot give any warrant for it.'

to me.

"My discovery, if I may so call it, I had kept to myself, even after Heuglin had thus expressed himself, and would even now maintain silence on the subject had I not recently discovered a new authority for it."

I read lately in the second edition of Petermann's great book of travels the following:

"Prof. Roth of Munich related to me in Jerusalem that the well-known Swedish traveller, Hedenborg, made the following interesting observation on the Island of Rhodes, where he stopped. In the autumn tide, when the storks come in flocks over the sea to Rhodes, he often heard the songs of birds without being able to discover them. Once he followed a flock of storks, and as they lighted he saw small birds fly up from their backs, which in this manner had been borne over the sea. distance prevented him from observing to which species of singing birds they belonged."

The

Thus wrote the famous geographer Petermann. Prof. Roth This and Hedenborg and Heuglin are entirely reliable authors. was a matter of great curiosity to me, and after I found others had made similar observations and expressed them in print, I thought they would be of no less curiosity and interest on this side of the Atlantic, and equally deserving of public notice. I hope that connoisseurs, amateurs, and experts may be excited by this to extend their observation in this line also. The instinct of animals is still, in spite of all our observations and experience,

almost a sealed bock to us. By a little attention we might hear of still more curious things in this field. PHONE

New York, November 20, 1880

Subsidence of Land caused by Natural Brine-Springs A THEORY has been put forward to account for the subsidence of land in the salt districts of Cheshire. It is said that, supposing the manufacturers of salt ceased to pump up the brine, it would run away to the sea, and subsidence would go on at as rapid a rate as now. Can any of the readers of NATURE tell me of any facts to substantiate such a theory, or refer me to any district where such rapid subsidence is going on, owing to the escape of natural brine-springs to the sea? Any reference to works giving information on this point will be thankfully

received.

Northwich, February 15

Chlorophyll

THOS. WARD

THE following experiment may be interesting in its bearing on the relation between chlorophyll-development and light.

If cress seed are grown for a few days in the dark on damp cotton-wool, and then, beneath the surface of water, introduced into an inverted glass jar filled with water, they may be exposed to daylight for an indefinite time without chlorophyll being developed. But the plants are not dead; for if, after a few days' exposure, the cotton-wool on which they have been grown is cut in two beneath the surface of the water, and one half, with its plants, is restored to the inverted jar of water, while the other is placed under an inverted glass jar containing air only, and then these two jars be exposed to full daylight, the plants beneath the jar containing air rapidly become green, while the others never do so.

Light therefore cannot always cause the development of chlorophyll in the etiolated leaves of living plan's. Liverpool, January 24 WILLIAM CARTER

[This is an interesting observation, but seems to need some further investigation. As shown by Sachs ("Text-book," pp. 665, 666) the formation of chlorophyll has a complicated dependence upon light. If the temperature be sufficiently high it is formed in the cotyledons of conifers and the leaves of ferns even in complete darkness. The seedlings of angiosperms require exposure to light for the production of chlorophyll, but it does not take place at low temperatures. All the visible parts of the spectrum possess the power of turning etiolated grains of chlorophyll green, although the yellow and adjoining rays are most effective. The failure of the seedlings immersed in water to become green can hardly therefore be attributed to the absorption of the heat rays. Is it possible that their water-bath keeps their temperature too low?]

Squirrels Crossing Water

As

IN NATURE, vol. xxiii. p. 340, I read that Mr. Godwin. Austen never had heard of a squirrel taking to the water. here are perhaps more readers of NATURE in Mr. GodwinAusten's case, I take this opportunity to transcribe what Bachman related to us about that matter in the year 1839.

The northern grey and black squirrel Sciurus leucotis, has occasionally excited the wonder of the populace by its wandering habits and its singular and long migrations. Like the lemming, Lemnus norvegicus, of the Eastern Continent, it is stimulated, either from a scarcity of food or from some other inexplicable instinct, to leave its native haunts and seek for adventures or for food in some distant and, to him, unexplored portion of our

land. The newspapers from the West contain frequent details

of these migrations; they appear to have been more frequent in former years than at the present time. The farmers in the Western wilds regard them with sensations which may be compared to the anxious apprehensions of the Eastern nations of the flight of the devouring locust. At such periods, which usually occur in autumn, the squirrels congregate in different districts of the far North-West, and in irregular troops bend their way instinctively in an eastern direction. Mountains and cleared fields, the head-waters of lakes and broad rivers, present no unconquerable impediments. Onward they come, devouring on their way everything that is suited to a squirrel's taste, laying waste the corn and wheat-fields of the farmer; and as their numbers are thinned by the gun, the dog, and the club, others are ready to fall in the rear and fill up the ranks, till they occa. sion infinite mischief and call forth no empty threats of revenge.

It is often inquired how these little creatures, that on common occasions have such an instinctive dread of water, are enabled to cross broad and rapid rivers, like the Ohio and Hudson, for instance. It is usually asserted, and believed by many, that they carry to the shore a suitable piece of bark, and seizing the opportunity of a favourable breeze, seat themselves upon this substitute for a boat, hoist their broad tails as a sail, and float safely to the opposite shore. This, together with many other traits of intelligence ascribed to this species, I suspect to be apocryphal. That they do migrate at irregular and occasionally at distant periods is a fact sufficiently established; but in the only instance in which I had an opportunity of witnessing the migrations of the squirrel, it appeared to me that he was not only an unskilful sailor, but a clumsy swimmer. It was (as far as my recollection serves me of the period of early life) in the autumn of 1808 or 1809, troops of squirrels suddenly and unexpectedly made their appearance in the neighbourhood, but among the grey ones were varieties not previously seen in those parts; some were broadly striped, with yellow on the sides, and a few with a black stripe on each side, bordered with yellow or brown, resembling the stripes of the little chipping squirrel (Tamias lysteri). They swam the Hudson in various places between Waterford and Saratoga; those which I observed crossing the river were swimming deep and awkwardly, their bodies and tails wholly submerged; several that had been drowned were carried downward by the stream, and those which were so fortunate as to reach the opposite bank were so wet and fatigued that the boys stationed there with clubs found no difficulty in securing them alive or in killing them. Their migrations on that occasion did not, as far as I could learn, extend farther eastwardly than the mountains of Vermont; many remained in the county of Rensellaer, and it was remarked that for several years afterwards the squirrels were far more numerous than before. It is doubtful whether any ever return westwardly; but finding forests and food suited to their tastes and habits, they take up their permanent residence in their newly-explored country; there they remain and propagate their species until they are gradually thinned off by the effects of improvement and the dexterity of the sportsmen around them. (The Magazine of Natural History, vol. iii., new series, 1839.) Leyden, February 16 F. A. JENTINK

Flying-Fish

WITH reference to the letter of Mr. Pascoe in NATURE, vol. xxiii. p. 312, allow me to offer a suggestion as to the mechanical means by which the flying-fish moves when out of the water. During a voyage to India and back I took a great interest in observing the movements of these beautiful creatures by means of a powerful opera-glass; and soon came to the conclusion that a slight but rapid tremor of the pectoral fins could be seen for a few moments after the fish left the water. In very calm weather I noticed a series of little ripples on each side of the fish as it skimmed along the surface before rising for its flight, evidently caused by the wing-points tipping the water. My idea is that the flying-fish springs from the sea, and by beating the surface rapidly with its pectoral fins obtains an impetus which carries it along for some distance in the air. It then descends to the surface, and in the same manner acquires a fresh accession of though the fish does sometimes resume its flight after a moment speed. This process however is never repeated more than twice,

of immersion.

R. E. TAYLOR

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"The instruments would be returned, in perfect order, as soon as possible after the transit, and, in any case, before the end of 1883.

"All communications should be addressed to the Secretary, Transit Committee, Royal Society, Barlington House."

The Committee, we are informed, is constituted as follows-The President of the Royal Society is the chairman, the other members being Prof. J. C. Adams, the Astronomer-Royal, the Earl of Crawford and Balcarres, Mr. De la Rue, Mr. Hind, Dr. Huggins, ViceAdmiral Sir G. H. Richards, Prof. H. J. S. Smith, Prof. Stokes, and Mr. E. J. Stone.

DR. 7. 7. BIGSBY

friction must have had on the motions of the earth and moon, on the supposition that time enough has elapsed for this cause to have its full effect. It then appeared that we are thus able to co-ordinate together the various elements of the motions of these two bodies in a manner too remarkable to be the product of chance.

The second part of the present paper contains a discussion of the part which the same agency may have played in the evolution of the solar system as a whole and of its several parts.

It is first proved that the rate of expansion of the planetary orbits, due to the reaction of the frictional tides raised by the planets in the sun must be very slow compared with that due to the reaction of the tides raised by the sun in the planets. Thus it would be much more nearly correct to treat the sun as a rigid body, and tides, than the converse. It did not, however, seem expedient to attempt to give any numerical solution of the problem thus suggested which should apply to the solar system as a whole.

YET another of the links that have bound the geologists to suppose the planets alone to be subject to frictional

of the present time in association with the early leaders of their science has been severed by the removal of the kindly and venerable form of Dr. Bigsby. Upwards of sixty years ago he began his geological career in North America, devoting himself mainly to the investigation of the structure of the older Palæozoic rocks of Canada and of the adjoining tracts of the States. As secretary to the Boundary Commission under the Treaty of Ghent he had opportunities of investigating the region from Quebec to Lake Superior, and published numerous descriptions, of which the exactness has been amply verified by the subsequent researches of the Geological Survey of Canada. It is chiefly as an admirable pioneer in Canadian geology that his name will be inscribed in the records of scientific progress. But he has other claims to grateful remembrance. Since he returned to spend his later years in this country he has devoted himself with the most untiring patience to the compilation of his "Thesaurus Siluricus" and "Thesaurus Devonicus "-works in which the geological and geographical range of the organisms of the earlier half of Paleozoic time is clearly shown in a series of valuable tables.

Still more recently, in 1877, he presented to the Geological Society a bronze medal which, with a sum of money derived from the interest of a fund also given by him, is to be awarded every two years as an incentive to geological study. The terms according to which he directed that the prize should be given are that the medal and interest from the fund should be awarded “as an acknowledgment of eminent services in any department of geology, irrespective of the receiver's country; but he must not be older than forty-five years at his last birthday, thus probably not too old for further work, and not too young to have done much." The founder lived to see two fitting awards of his prize go to the eminent palæontologists of the United States, Professors O. C. Marsh and E. D. Cope. He died just before the third presentation was made, last week, to Dr. Charles Barrois of Lille.

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The effect of tidal friction is to convert the rotational momentum of the tidally disturbed body into orbital momentum of the tide-raising body. Hence a numerical evaluation of the angular momentum of the various parts of the solar system will afford the means of forming some idea of the amount of change in the orbits of the several planets and satellites, which may have been produced by tidal friction. Such an evaluation is accordingly made in this paper, with as much accuracy as the data permit.

From the numerical values so found it is concluded that the orbits of the planets round the sun can hardly have undergone a sensible enlargement from the effects of tidal friction since those bodies first attained a separate existence.

Turning to the several sub-systems, it appears that, although it is possible that the orbits of the satellites of Mars, Jupiter, and Saturn about their planets may have been considerably enlarged, yet it is certainly not possible to trace the satellites back to an origin almost in contact with the present surfaces of their planets, in the same manner as was done for the case of the moon in the previous papers.

The numerical values above referred to exhibit so marked a contrast between the case of the earth with the moon, and that of the other planets with their satellites, that it might à priori be concluded as probable that the modes of evolution have differed considerably. The conclusion above stated concerning the satellites of the other planets cannot therefore be regarded as unfavourable to the acceptance of the views maintained in the previous papers. It must, however, be supposed that some important cause of change other than tidal friction has been concerned in the evolution of the solar system and the planetary sub-systems. According to the nubular hypothesis of Laplace, that cause has been the condensation of the heavenly bodies. Accepting that hypothesis, the author then proceeds to consider the manner in which contraction and tidal friction are likely to have worked together.

A numerical comparison shows that, notwithstanding the greater age which the nebular theory assigns to the exterior planets, yet the effects of solar tidal friction in reducing planetary rotation must in all probability be planets. It is, however, remarkable that the number considerably less for the remote than for the nearer expressive of the rate of retardation of the Martian rotasimilar number for the earth, notwithstanding the greater tion by solar tidal friction is nearly the same as the This result is worthy of distance of Mars from the sun. notice in connection with the fact that the inner satellite of Mars revolves with a periodic time much shorter than that of the planet's rotation; for (as suggested in a previous paper) solar tidal friction will have been com

petent to reduce the planetary rotation without directly affecting the satellite's orbital motion.

It is then shown to be probable that solar tidal friction was a more important cause of change when the planets were less condensed than it is at present. Thus we are not to accept the present rate of action of solar tidal friction as indicating that which has held true in all past time.

It is also shown that if a planetary mass generates a large satellite, the planetary rotation is reduced after the change more rapidly than before; nevertheless the genesis of such a satellite is preservative of the moment of momentum which is internal to the planetary subsystem. This conclusion is illustrated by the comparatively slow rotation of the earth, and by the large amount of angular momentum residing in the system of moon and earth.

An examination of the manner in which the difference of distances of the various planets from the sun will have affected the action of tidal friction leads to a cause for the observed distribution of satellites in the solar system. According to the nebular hypothesis a planetary mass contracts, and rotates quicker as it contracts. The rapidity of the revolution causes its form to become unstable, or perhaps, as seems more probable, an equatorial belt gradually detaches itself; it is immaterial which of these really takes place. In either case the separation of that part of the mass which before the change had the greatest angular momentum permits the central portion to resume a planetary shape. The contraction and increase of rotation proceed continually until another portion is detached, and so on. There thus recur at intervals a series of epochs of instability or of abnormal change.

Now tidal friction must diminish the rate of increase of rotation due to contraction, and therefore if tidal friction and contraction are at work together the epochs of instability must recur more rarely than if contraction acted

alone.

If the tidal retardation is sufficiently great, the increase of rotation due to contraction will be so far counteracted as never to permit an epoch of instability to occur.

Now the rate of solar tidal friction decreases rapidly as we recede from the sun, and therefore these considerations accord with what we observe in the solar system. For Mercury and Venus have no satellites, and there is a progressive increase in the number of satellites as we recede from the sun.

Whether this be the true cause of the observed distribution of satellites amongst the planets or not, it is remarkable that the same cause also affords an explanation of that difference between the earth with the moon and the other planets with their satellites, which has permitted tidal friction to be the principal agent of change with the former, but not with the latter.

In the case of the contracting terrestrial mass we may suppose that there was for a long time nearly a balance between the retardation due to solar tidal friction and the acceleration due to contraction, and that it was not until the planetary mass had contracted to nearly its present dimensions that an epoch of instability could occur.

If the contraction of the planetary mass be almost completed before the genesis of the satellite, tidal friction, due jointly to the satellite and the sun, will thereafter be the great cause of change in the system, and thus the hypothesis that it is the sole cause of change will give an approximately accurate explanation of the motion of the planet and satellite at any subsequent time. It is shown in the previous papers of this series that this condition is fulfilled with the earth and moon.

The paper ends with a short recapitulation of those facts in the solar system which are susceptible of explanation by the theory of the activity of tidal friction. This series of investigations affords no grounds for the rejection

of the nebular hypothesis, but while it presents evidence in favour of the main outlines of that theory, it introduces modifications of considerable importance.

Tidal friction is a cause of change of which Laplace's theory took no account, and although the activity of that cause is to be regarded as mainly belonging to a later period than the events described in the nebular hypothesis, yet its influence has been of great, and in one instance of even paramount, importance in determining the present condition of the planets and their satellites. G. H. D.

INDIGO

N July, 1878, an account was given in this journal of the synthesis of indigo-blue from phenylacetic acid, accomplished by Prof. Baeyer of Munich (NATURE, xviii. 251). The process there described did not permit of the successful production of indigo-blue on a manufacturing scale at reasonable cost. Since that time Prof. Baeyer has continued to work at the problem, and he has so far succeeded that he has now taken out a patent for the artificial manufacture and application of indigo-blue.

In a paper in the last number of the Berliner Berichte Baeyer gives an interesting résumé of the steps whereby progress has been slowly made, since 1865, in solving the problem of the synthesis of indigo.

Following up the work sketched in the article already referred to, Baeyer attempted to prepare orthonitrophenyl indol, which may be regarded as the parent substance of acetic aldehyde, expecting that this substance would yield the indigo group of compounds. But as the work proceeded Baeyer became more and more convinced that that which should still regulate his experiments. In 1869 the hypothesis which had guided his earlier work was he had written, "In order to prepare indol synthetically given to introduce a pair of carbon atoms and one it is necessary-in accordance with the formula already nitrogen atom into benzene, and to link these together. The necessary conditions are found in nitro-cinnamic acid, if one supposes carbon dioxide and the oxygen of the nitro-group to be removed. And indeed it has been with potash." The steps in the preparation of indigoshown that nitro-cinnamic acid yields indol by fusion blue, according to Baeyer's patent, are these :1. Cinnamic acid (or phenyl acrylic acid)— C6H5. C2H2. CO2H.

2. Orthonitrocinnamic acid

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