t82 



NATURE 



[7an.3, 1878 



judicially- minded person would attach much weight even to a 

 report of his own, drawn up under such circumstances, and all 

 judicially-minded persons will regret its in'roduction here. Prof. 

 Osborne Reynolds's reasoning proceeds on the hypothesis that 

 the gas is not polarised. The only real question here is. Is 

 Prof. Schuster prepared to maintain that this reasoning is 

 correct ? 



Prof. Schuster, in reporting his reminiscences, first recifes a 

 kinetic principle which is quite consistent with there being as 

 n'uch force sideways as perpendicularly to the disc, and which 

 therefore contains no explanation of the phenomenon ; and when 

 he comes to the first essential point, viz., that which requires him 

 to show that "an increased pressure on the cold side of the vanes 

 of a radiometer will not counterbalance the force acting on the 

 l>lackened sides," all that he has to say on the subject is that 

 " he does not think that such is the case ! " This is the essen- 

 tial thing to be proved before the explanation can be accepted, 

 and he recites experiments which show that it is essential. 



Prof. Schuster concludes this part of his letter with the admis- 

 sion that " he does not see how [on his theory] an increase in 

 the force can take place " as the exhaustion proceeds. So much 

 the worse for the theory, since experiment indicates that such an 

 increase in the force does take place. In proof of this I may allege, 

 in addition to Mr. Crookes's experiments, several series of experi- 

 ments made by Mr. Moss, one of the most judicially-minded, 

 patient, and dexterous manipulators I have met with. The experi- 

 ments were made with the apparatus described in a communica- 

 tion from him and myself, published last spring in the Proceedings 

 of the Royal Society, and the effect of the convection current was 

 wi!h extreme care excluded in two ways— by placing the swinging 

 disc where the influence of the convection current en it before 

 and behind wns baiairced, and by observing the motions that arose 

 before the convecion current had time to reach the disc. Both 

 methods concurred in showing that, as in Mr. Crookes's experi- 

 ments, the force on the disc uniformly increased with increasing 

 attenuation of the gas up to the limit to which we pushed the 

 exhaustion. Mr. Crookes has shown that beyond that limit it 

 begins to decrease. Prof. Schuster will do a real service to 

 science if he will devote his great skill for some months to 

 repeating these and other concurrent experiments, and either 

 confirm thern or point out why they should be set aside. 



Prof. Schuster thinks that " any theory of the radiometer 

 which makes the action depend on the comparatively large 

 [? small] ratio of the mean free path to the dimensions of the 

 vessel, must necessarily be wrong." Has not Prof. Schuster 

 here overlooked the minuteness of the phenomenon which has to 

 be accounted for ? Spheroidal drops of less than a millimetre 

 diameter are easily formed of several light liquids. The 

 Crookes's stress which supports these is an excess of vertical 

 stress over horizontal stress in the supporting layer of polarised 

 gas, amounting to about the 12- or 15-thousandth part of the 

 whole stress. This compares favourably with the minute ratio 

 to which Prof. Schuster refers, 



I will not at present enter on that part of Prof. Schuster's 

 letter in which he criticises my published views on penetration, 

 as he refers me to the researches of Messrs. Kundt and Warburg, 

 which I have not yet seen. G. Johnstone Stoney 



Glaciation of Orkney 



Last spring Prof. Geikie informed me of a correspondence 

 which was going on in the columns of Nature as to the ques- 

 tion whether the Orkney Islands bore evidence of having been 

 glaciated. It was with much surprise that I heard that there 

 could be any question on this point at all, but I refrained from 

 submitting my opinion to the public — unhesitating though that 

 opinion was — on account of my being then just about starting 

 for my native county, and thus having an opportunity of very 

 specially directing my attention anew to the matter. As the 

 observations I then made wilhout exception tended to confirm 

 me in what really required no confirmation, I think I may now 

 come forward as one who has for long known those islands, and 

 who has made a very special geognostic survey of them, during 

 many years. And I would first say, as regards the question, 

 "whether Orkney does or does not give proof of having been 

 covered by a great ice-sheet?" that I believe that no one who 

 has educated his eye — not by looking at pictures in books, 

 but among the_ rocks themselves — to the apprehension and recog- 

 nition of the hill-contours of an ice-scalped country, would hesi- 

 tate to declare Orkney to be such. Let such a one take his 

 stand, at a jufficicnt altitud'-, anywhere a^ong the north coast of 



Sutherland, with a scratched and polished boss under his feet, 

 rolling up into rounded hillocks on every side, and sweep his 

 eye from the two Ben Griams over to Hoy, and he could not 

 but exclaim, " There is a country which has suffered sore." 



In having to differ from Mr. Laing, I join issue with him on 

 two points — boulders and foreign stones, and boulder-clay, I have 

 also to corroborate Prof. Geikie as to glaciation near Stromness'; 

 for I, during last summ.er, saw to the immediate north-west of 

 Stromness a surface of gneiss, say ten feet by three, most unmis- 

 takably glaciated — both scratches and polishing being shown. 



Now as regards " boulders and foreign stones." Mr. Laing 

 will find — I wonder at his not knowing of it — about 100 yards to 

 the west of the House of Saval, in Sanday, one of the finest boul- 

 ders in Scotland. This boulder, of great size, consists of horn- 

 bler.dic gneiss ; for long I was unable to identify it with any 

 variety of the hornbkndic gneiss of Sutherland ; but this very 

 year's work enables me to say that it is very similar to that of a 

 locality near Duirness. In all probability, however, its parent 

 rock lay east, not west. 



Another boulder I have heard of, but not seen ; it was de- 

 scribed to me under the name of the " Eagle Stone " ; it lies upon 

 the side of a hill in Westray, near Pierowall, and is said to be 

 peculiar as a loose stone, both on account of its toppling posi- 

 tion, its being different from any rock in Orkney, and from there 

 being no rock near it. 



As to there being " foreign stones " in Orkney, I shall only 

 say that I have at present in my collection polished jaspers, 

 picked up in rolled masses in Orkney ; and that fragments 

 of broken agates are found not unfrequently, on the hill tops and 

 sides, in Hoy. These are quotations, ex gre«e. 



Mr. Laing's very precise observations on the clay beds — let us 

 call them — do call for special investigation. 



If the conclusions arrived at by other observers are found to 

 coincide with his — while they could not affect the ultimate 

 decision as regards the ice-clad country— they certainly would 

 strike these clay beds out of the category of boulder-clays. 

 But, ST, I have seen these clays, and I did not see what Mr. 

 Laing saw ; and what I did observe leads me to doubt the 

 correctness of his conclusions. For I found it to be a notable 

 circumstance as regards these Orkney clay-beds that they are 

 very markedly r/^j'-beds ; i.e., that the amount of clay relatively 

 to that of the stoney matter therein is very much greater than 

 that of most boulder-clays. 



Now this is a fact which saps the very foundation of Mr, 

 Laing's observation — an abnormally argillaceous clay bed being 

 the result of the disintegration of a normally siUceous sandstone 

 is difficult to conceive. Nay more, although the cement of certain 

 of the Orkney beds is silicate of alumina, forming the blue argil- 

 laceous flag, it is an unque-tionable fact, that these flags do not 

 disintegrate by the action of the weather. Even the Picts knew 

 that when they built their Broughs thereof. Silicate of alumina 

 is not affected by carbonated waters. 



Upon — nearly all along — the west shore of Shapinsha there 

 are cliffs — sea-cliffs of these clay beds, which lie bet'vixtthe rocks, 

 or the last visible rock, and the sea ; that last rock is a red 

 ferruginous loose-grained sandstone, with little or no cement, 

 what there is being micaceous ; the clay beds are ochre yellow. 

 The disintegration of this rock never could have yielded these 

 clay beds. 



But Mr, Laing may argue that they resulted from the decay 

 of an overlying argillaceous bed. The argument will not stand. 

 Firstly, because the dip is the wrong way ; the rock dips at a 

 high angle to the east ; the clay slightly caps it, and stands as a 

 bank between its escarpment and the sea. Secondly, because a 

 friable yellow freestone, destitute of argillaceous cement, should 

 overlie the red beds. Thirdly, because on the other side of the 

 bay where the argillaceous flags do appear they are quite per- 

 manent. Ice might grind them up — the "weather" does not rot 

 them down. But here no clay beds are seen. 



Finally, sir, I would request my talented countryman — whom 

 I have great pleasure in breaking a lance with in this field — to 

 consider how or why it is that these clay beds are four.d only on 

 ^«^ «rtV of the long depression which runs up the centie of the 

 islands? M, Fokster Heddle 



University, St, Andrews, December 19, 1877 



Northern Afifinities of Chilian Insects 



I THINK I may be allowed to express my surprise at Mr. 

 McLachbn's statement that this subject has never yet been 



