2l6 



NA TURE 



\jfan. 8, 1885 



negative rotation in the planetary mass, but we here find 

 it used to explain positive rotation. 



The author states that the planets are "hurled" or 

 "projected " from the sun : but he does not see that even 

 if some deus ex machind were just to prevent the other- 

 wise inevitable fall back into the sun, the eccentricity of 

 the orbit must be very large instead of very small. 



On p. 14 we find that "it is moreover manifest that 

 each individual planet must from time to time have had 

 its orbit greatly extended " by the reduction of the sun's 

 mass on the birth of each planet. It is, however, the fact 

 that the orbit of Jupiter, for example, would be scarcely 

 appreciably altered were this process reversed, and were 

 all the planets interior to Jupiter either suddenly in- 

 corporated with the sun or annihilated. 



On this same page we learn " that in a system of 

 particles revolving about a fixed centre, the momentum, 

 that is the sum of the products of the mass of each into its 

 angular velocity (sic ital ), and the distance from the 

 common centre is a constant quantity." Does it not 

 follow that when a planet moves in an elliptic orbit, so 

 that its distance from the centre of force is not constant, 

 there is not constant moment of momentum ? What 

 then becomes of the generally accepted conservation of 

 areas in elliptic motion? 



The fanciful explanation of the inclinations of the 

 planetary orbits, and of the obliquity of the planetary 

 axes, need not be stated ; but we observe that " the 

 northern hemisphere, being that which contains more 

 land than the southern, was directed away from the sun 

 at the time it (the earth) was projected away from that 

 body," and this, together with the context, shows that the 

 northern hemisphere is here supposed to be heavier than 

 the southern. The fact of course is that the hemisphere 

 antipodal to Spain, by its greater density, attracts the sea 

 away from the Spanish hemisphere and leaves our half 

 of the globe drier than the other half. 



The asteroids arise from the rupture of a planet X, 

 which in cooling had been converted into a vast Rupert's 

 drop (p. 21): "What a scratch does for the Rupert's 

 drop, the pull occasioned by Jupiter's attraction effects 

 for the doomed planet : the thin crust is rent, and forth 

 in a thousand different directions fly his meteoric 

 fragments." 



On p. 23 we find that the Glacial period was a sudden 

 catastrophe, and that the fleeing mammoths were caught 

 by the intense cold and frozen to death. 



At the end of the work the author emphasises the 

 analogy, long ago pointed out, between the system of 

 Saturn and his satellites and of the sun and his planets. 



LETTERS TO THE EDITOR 



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Apospory in Ferns 



My note in Nature (p. 151) on the remarkable mode of 

 reproduction in ferns discovered by Mr. Druery has brought me 

 two friendly communications, both of which de-erve a few words. 



Dr. Vines reminds me that in an article on the pro embryo of 

 Chara, published in the Journal of Botany (1878, pp. 355-363), 

 he had suggested that this structure is homologous with the 

 spore-bearing generation (sporophore) in mosses. His arguments 

 in favour of this view are extremely ingenious, if not wholly con- 

 vincing. At any rate, if his theory is correct, the sporophore 

 in this plant remains in a rudimentary condition, " producing no 

 spores, but giving rise to the o upbore by the lateral budding 

 from one of its cells." Hence, he concludes, "we may speak 



of this plant as ' aposporous,' using a word which is symmetrical 

 with the term ' apogamous, ' applied by De Bary to those ferns 

 id whose life-history no process of sexual reproduction occurs." 

 We must give Dr. Vines, I think, the credit for first clearly 

 defining in terms the aposporous condition as the converse of 

 apogamy, though the phenomenon was first observed by Pring- 

 sheim in the Moss in 1876 (Monatsi. d. Kbnigl. Akad. tier Hiss, 

 zit Berlin, 10 Juli, 1876). At any rate, according to Dr. Vines's 

 view, what is only an occasional abnormality in the moss and 

 the fern is the normal state of things in Chara. 



The distinguished Italian botanist and traveller, Signor 

 Odoardo Beccari, points out to me that the structures now 

 known as archegonia and antheridia had been observed in 

 Salvinia nutans as early as 1834 by Savi. I had not overlooked 

 the paper in which priority for the discovery was claimed for 

 Savi by Marcucci (Nuovo Giorn. Bot. It. i. pp. 198-208). But 

 the brief chronological table which I gave in my note had refer- 

 ence only to ferns proper (Filices) and not to the heterosporous 

 group of the Filicince— the Khizocarpea. 



W. T. Thiselton Dyer 



Frost Formation on Dartmoor 



On the afternoon of Tuesday, December 30, 1884, about 

 3 p.m., we were on Yes Tor, near Okehampton, long reputed 

 the highest point of Dartmoor, though it is understood that 

 the new survey now in progress brings out the neighbouring 

 summit of High Willhayse a few feet higher. For about 200 

 feet below the Tor the ground was frozen hard. It was free 

 from snow, the weather having been fine for several days, but 

 everything was white with hoar-frost. On the rocks of the Tor 

 this frost assumed a form of singular beauty, and, we think, not 

 a common one. At least, neither of us can match it in either 

 English or Alpine experience, or remember to have teen an 

 account of anything like it. 



On the first impression the walls of the granite masses which 

 make up the Tor looked as if covered with feather-work ex- 

 quisitely wrought in congealed snow. The feathers (to call 

 them so provisionally) overlaid one another as thickly as real 

 plumage, and ranged in length from one inch or less to five or 

 six inches, being smaller on the flat and recessed surfaces of the 

 rock, and larger on the jutting and exposed ones. They lay 

 almost wholly on the eastward, that is (as the weather then was, 

 and for some days had been) the windward side of the Tor ; 

 and their tips pointed roughly in that direction, with the sort of 

 uniformity one would get by laying down a great number of 

 branches or feathers all one way. It is impossible to describe 

 the richness of this natural decoration. Only the finest Oriental 

 workmanship could come near the effect produced by the infinite 

 and minute variety which this tapestry of frost-flakes combined 

 with one dominant form and direction. Something of the same 

 type, but far less perfect, may be seen on a mussel-covered rock 

 at low water. Still more curious was the appearance of the 

 Royal Artillery flagstaff which surmounts the Tor. It was 

 loaded (on the windward side, like the rocks) with a solid fringe 

 of the same formation, but in longer and thicker flakes. We 

 judged it to be full six inches deep, and at first thought it must 

 be supported by a string attached to the staff, but there was, in 

 fact, no string at all. 



Close examination of the individual flakes revealed great 

 beauty of structure. They were mostly of an elongated lozenge 

 shape, like a squared spear-head, but sometimes more like 

 tongues of flame. Their contours and delicate surface-markings 

 showed them to be built up of laminae, into which they were 

 easily resolved by a slight blow. These lamina? again split up 

 into crystalline needles parallel to the longer diameter of the 

 flake, that is, in the line of the imaginary spear-shaft. Only 

 photography or very careful drawing (for neither of which had 

 we the means, time, or skill) would clearly convey the details 

 of the formation. 



As to the physical explanation, we conceive that the process 

 must have been set up by a thin layer of mist (probably in a 

 very finely-divided state to begin with) drifting against the rock 

 and freezing to it. Successive accretions brought in the same 

 way would gradually produce the display of giant hoar-frost 

 which we have imperfectly described. The details of form and 

 structure we leave to be considered by those who have made a 

 special study of ice-crystals. But it seems fairly obvious that 

 for such a result there must be a concurrence of many favouring 

 conditions. There must be a clear frost without snow, which of 



