March 15, 1877] 



NATURE 



431 



I do not wonder at these omissions, since unfortunately the 

 Italian language is very little understood out of our country, 

 Rome, February 24 V. R. Secchi 



P.S. — On the 15th of this month I obtained a sight of the 

 spectrum of Borelly's Comet It was composed of a bright line 

 very large in the green, another more refrangible in the blue, 

 and another less refrangible in the yellow (?) but this was narrow 

 and faint. Their figure was approximately as follows : — 



nn 



I could not detarmine them better, 

 central one, were pretty brilliant. 



The lines, especially the 



"Stone Rivers" 

 The interesting account of the mode of formation of "Stone 

 Rivers," given in a late number by Sir C, Wyville Thomson, 

 recalls to my mind some apparent moraines which I observed, 

 and somewhat similarly explained, some years ago in the Hartz 

 Mountains. In a paper read before the Geological Society of 

 Dublin in 1872,^ I thus speak of them: " The first thing that 

 one notices on entering this valley [the * Ockerthal '] from the 

 north is that the bed of the stream is crowded with granite 

 pebbles and boulders, which become of greater size as we pro- 

 ceed up the valley. The boulders are soon so large — many of 

 them some tons in weight — and are situated so far up the slopes 

 on each side, that the first idea is, we have here the morass of a 

 former great glacier. I looked diligently for ice marks, but 

 could see none ; and I soon found that the causes which have 

 the effect of scattering huge blocks of rock on the slopes and on 

 the bed of the river are now at work and are slowly, but surely, 



altering the contour of the adjoining granitic mountains 



All over the sides and upon the summits of these mountains are 

 scattered the most fantastic piles of immense boulders. Some 

 of them are over thirty feet in height and form conspicuous 

 objects in the landscape ; others, again, are deep in the forest, 

 away from pathways, and are not to be seen until one climbs 

 quite up to them It is quite clear that the contiguous 



A pile of t»ranit« rocks on a mountain overlookinp; Ockerthal. 



surfaces of the blocks in these piles are undergoing a slow de- 

 composition, that the joints are becoming gradually looser, and 

 in consequence the cohesion of the component pieces less and 

 less. Sooner or later the upper portions must either slip off or 

 topple over, and sofl down the mountain side. And this is not 

 mere theory, for I hear that every now and then a boulder does 

 fall and comes crushing down the hill until quietly deposited near 

 the bottom. It would appear that while the surrounding rock 

 has been decomposed and has fallen down in the manner indi- 

 cated, these heaps have the longer resisted. But they are yet 

 to follow in their turn ; when the atmospheric agencies have 



' " Notes on tlie Geology of the Harlz," by P. S. Abraham, M.A., B Sc, 

 &c. Plates Vlir. to XI. Jcurn. ,K. Geo!. Soc, of Ireland, vol xiii. Pt. 3, 

 p 92, 1873. 



sufficiently done their work, gravity will come in and lower the 

 whole." 



Instances of the turning over of the edges 'of slaty strata from 

 the weight of the superincumbent mould and vegetation are 

 common in the Hartz. I find in an old note-book the following 

 entry : — " I was interested at seeing the upper slates on the left 

 wall [of a quarry near Goslar] bent so much over that [their dip 

 has become 75° to the north [the regular cleavage dip of the dis- 

 trict is about 40° south']. Whether this is due to the weight of 

 the ground above, to a landslip, or to the action of a glacier, I 

 am not quite sure. I incline, however, to the first theory, for, 

 although the slope of the hill is not high, the constant weight of 

 the superincumbent earth and rubbish, bearing downwards for 

 ages, would, it seems to me, be enough to cause such a result." 



Scientific Club Phin. S. Abraham 



The Measurement of the Height of Clouds 



Among the various parallactic methods for determining cloud 

 heights, one of which Mr. Malloch has put in practice (Nature, 

 vol. XV., p. 313), the use of the cloud shadow as a second sta- 

 tion seems worthy of notice, as it requires very simple apparatus 

 and observations. 



On any partially cloudy day at the sea-side, an observer with 

 a sextant may, from a cliff, easily determine cloud heights by 

 the following elements : — A. Altitude of a given point of cloud 

 above the horizon, allowing for dip. B. Depression of the 

 shadow of the same point on the surface of the sea. C. Sun's 

 altitude. D. Lineal elevation of observer above sea-level. 

 The measurements should be taken when the cloud, the sun, 

 and the observer all lie in a perpendicular plane ; i.e., when the 

 cloud shadow is seen on the sheen of light reflected from the 

 wavelets ; otherwise azimuth observations, and less simple calcu- 

 lation?, must be applied. Full moonlight might also be used at 

 night. 



On practically trying this method in September, 1875, the 

 time of day was selected when the sun was in the direction to or 

 from which the wind was blowing ; thus the cloud shadows 

 slowly sailed along the sheen on the sea, and could be followed 

 by successive tiired observations for half an-hour or more, so 

 that their velocity, and any variation in their height, could be 

 ascertained. 



The results are of course most accurate when both clouds and 

 sun are at considerable altitudes, and I believe that this method 

 will give results quite as accurate as the photographic process. 

 The rounded forms are the greatest trouble, and measurements 

 of the centres of little isolated masses of cloud are the best. 

 The height of the observer above the sea is of course easily 

 obtained by the angular width of a base measured on the beach. 



The same method might be employed with shadows on land, 

 by using a theodolite and a map ; and though it is only applic- 

 able to one or two classes of clouds, yet its simplicity may 

 induce some of your sea-side readers to make such observations. 



Bromley, Kent W. M. Flinders Petrie 



The " Hog- Wallows " of California 

 My friend, Mr. Thomas Belt, F.G.S., has kindly sent me the 

 following extract from a paper by Prof. Joseph I.e Conte, in the 

 American Journal of Science iox 1874 (p. 366), in which an ex- 

 planation is given of the above-named formation (Nature, 

 vol. XV. p. 274) and of similar mounds farther north. It will 

 be seen that Prof. Le Conte refers them ^wholly to " surface- 

 erosion," but it is not clear whether he means "pluvial" or 

 "aerial" erosion, or the two combined. More explanation 

 seems required to account for the removal of the eroded mattei 

 over a surface thirty miles wide without producing any con 

 tinuous ravines or other water channels : — 



" Prairie Mounds.' — The irregularly ramifying grassy glades or 

 prairies already described as existing at the southern extremity ot 

 Puget Sound are studded over as thickly as possible with mounds 

 about three to four feet high and thirty or forty feet in diameter 

 at base." . . . 



" The whole country between the Dalles and the upper bridge 

 of Des Chutes River, a distance of about thirty miles, is literally 

 covered with these mounds." . . . " The true key to their forma- 

 tion is given here, as it was not at Mound Prairie, by the great 

 variety of forms, sizes, and degrees of regularity which they as- 

 sume. They vary in size from scarcely detectable pimples to 

 mounds five feet high and forty feet in diameter at base, and in 

 form from circular through elliptic and long-elliptic to ordinary 

 hill-side erosion-furrows and ridges. "... 



