June 24, 1897] 



NA rURE 



^11 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he umiertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



The Great Submergence. 

 Ti{E glaciation of the hills and glens all over the west of 

 Argyllshire present phenomena which have for many years con- 

 vinced me that the chief glaciating agent cannot possibly have 

 been what is called an "ice-sheet." Ordinary glaciers, de- 

 scending from the hills, have probably been concerned more or 

 less. But the main agency has been, in my opinion, heavy 

 " floe, " or floating ice, driven by tides and currents during a sub- 

 mergence and a re-emtrgence of the land to the e.xtent of at least 

 1 500 feet. The main objection to this view has always been the 

 absence of marine deposits, and especially of marine organisms. 

 It is true that we have very few remaining beds of gravel or 

 sand, and in those, as yet, no mollusca have been discovered. But 

 I have never thought this objection a very strong one, because 

 the submergence may not have been under conditions favourable 

 to molluscan life, and dead shells, carried where they never 

 lived, may easily have long ago been dissolved out. Quite recently, 

 however, shells in great variety have been found at high elevations 

 in Ayrshire, by Mr, Smith, of Kilmarnock. Moreover, these are 

 embedded in the true boulder clay. Ard where these are want- 

 ing, marine microzoa have been detected in abundance by the 

 most skilled and distinguished microscopist of the day. Stimu- 

 lated by these discoveries, I have lately made a closer search 

 for any remains of loose sands or gravels among our hills. That 

 search has been rewarded by finding a well-marked fragment of 

 an elevated sea-beach at the height of 350 feet above the sea, 

 and in this deposit foraminifera have been found in fair 

 abundance. Another place, quite 500 feet above the sea, has 

 yielded similar evidence ; and I have now very little doubt that 

 such evidence will accumulate as the result of further search. 

 This, however, is enough to prove a very recent submergence 

 to a depth which would profoundly alter the distribution of sea 

 and land along all our shores, and would divide the county of 

 Argyll into a group of islands. No reasonable limit can be 

 placed to the possible depth of this submergence. I have no 

 theory as to the causes of it. But it may quite as well have been 

 1500 as 500 feet ; and we must yield to any clear evidence of 

 effects which cannot otherwise be explained. I need not say that 

 the evidence, which in the latest years of his life carried con- 

 viction to the mind of the late Sir Joseph Prestwich, that there 

 has been very lately in geological history a great submergence, 

 which was comparatively sudden and comparatively transient, 

 is evidence which no geologist can put aside without the fullest 

 and most candid investigation. Argyll. 



The Visibility of a Sound Shadow. 



Two months ago I received the following letter from Mr. 

 E. J. Ryves. 



' ' On Tuesday, April 6, I had occasion, while carrying out 

 some experiments with explosives, to detonate 100 lbs. of 

 a nitro-compound. The explosive was placed on the ground 

 in the centre of a slight depression, and, in order to view 

 the effect, I stationed myself at a distance of about 300 yards 

 on the side of a neighbouring hill. The detonation was 

 complete, and a hole was made in the ground 5 feet deep and 

 7 feet in diameter. A most interesting observation was made 

 during the experiment. 



" The sun was shining brightly, and at the moment of 

 detonation the shadow of the sound wave was most distinctly 

 seen leaving the area of disturbance. I heard the explosion as 

 the shadow passed me, and I could follow it distinctly in its 

 course down the valley for at least half a mile : it was so plainly 

 visible, that I believe it would have photographed well with a 

 suitable shutter." 



On receiving this note, I asked and obtained permission from 

 Mr. Maxim to be present if a further experiment of the kind 

 were contemplated. I designed and had prepared a special 

 shutter which, on the breakage of an electrical circuit, would 

 in the 40th second make an exposure of about 1/300 second. 

 Unfortunately the battery that I brought with me proved in- 

 sufficient, so that I had to discard this, and use an ordinary 



NO. 1443, VOL. 56] 



Thornton-Pickard shutter actuated by hand. Neither the exact 

 moment nor duration of the exposure were under such perfect 

 control, and, as events proved, I could not get the exposure 

 until the shadow had passed me. I had, however, fortunately 

 asked Mr. Paul if he would be so good as to take a picture of 

 the explosion with his animatograph. Mr. Hunt, his manager, 

 came himself, and secured a good series. 



Returning now to the eye observation : Mr. Ryves informed 

 me he saw the expanding shadow on an intermediate occasion 

 when only about 10 lbs. were detonated. On the day on which 

 I was present (May 19), about 120 lbs. of nitro-compound were 

 detonated, and 10 lbs. of black powder was added to make 

 sufficient smoke to show on the plate. As the growth of the " 

 smoke cloud' is far less rapid than the expansion of the sound 

 shadow, no confusion could result from this. 



At the time of the explosion my whole attention was con- 

 centrated upon the camera, and for the moment I had forgotten 

 to look for the " Ryves ring," as I think it might be called ; but 

 it was so conspicuous that it forced itself upon my attention. I 

 felt rather than heard the explosion at the moment that it passed. 

 We stationed ourselves as near as prudence would allow at a 

 distance of 120 yards, so that only about one-third of a second 

 elapsed between the detonation and the passage of the shadow ; 

 but the precision of observation of coincidence when very rapid 

 movement occurs is so great, that I am quite satisfied that the 

 observation was correct. The actual appearance of the ring \\&s 

 that of a strong black circular line, opening out with terrific 

 speed from the point of explosion as a centre. It is impossible 

 to judge of the thickness of the black shadow ; it may have been 

 3 feet, or it may have been more at first, and have gradually 

 become less in thickness or, possibly, in depth of shade. 



I have some difficulty in understanding why the whole ring 

 should be visible if the phenomenon is the same as that which 

 Prof. Mach and I have photographed. In our cases a bullet 

 travelling at a speed greater than that of sound, forms a hyper- 

 boloidal shell of compressed air round about it. Light waves 

 passing nearly tangentially, but just entering the shell, are re- 

 fracted inwards, and thus leave a black line when they fail to 

 strike the photographic plate. As they strike the plate within 

 this black line on a part to which other rays have come nearly 

 direct, an extra bright line is formed within the dark one. That 

 which is essential, however, for the formation of the dark line 

 is the tangency of the incident light. 



Now, in the case of a hemispherical explosive wave it is clear 

 that the sunlight can only be tangential over a semicircle, and 

 that the shadow of such a wave should be a semi-ellipse, the 

 eccentricity of which would depend upon the altitude of the 

 sun. There could be no true tangential shadow on the sun- 

 ward side of the explosion. Observation, however, showed a 

 complete ring. If, as has been suggested to me as possible, the 

 explosive wave does not travel at equal speeds in all directions, 

 but is retarded near the gnnmd, the wave front near the ground 

 might be sufficiently inclined for the sun's rays to be tangential 

 over much more than a semicircle. On May 19, when I 

 made the observation, the sun had an altitude of about 58°, but 

 on April 6, when Mr. Ryves made his first observation, the 

 altitude was only 45". 



Mr. Maxim, with his usual ingenuity, suggested that perhaps 

 what was observed was not a sound shadow at all, but merely a 

 progressive bending down of every blade of grass as the ex- 

 plosive wave passed by. This would no doubt occur, and might 

 be visible, but it is difficult to see how so black a ring should 

 be produced in this way. 



The animatograph fails to show any black ring ; and this is 

 not surprising, as with the exposure of about l/ioo second 

 the shadow would have to be at least il feet thick, in order 

 that some part .should remain obscured during the whole ex- 

 po.sure. If the black line is just without an equal bright line, 

 as in the bullet photographs, and both are taken in by an 

 exposure, it is hardly to be expected that any definite result 

 should be obtained. As a fact, there is clearly seen a circular 

 light shading, which does— so far as one can judge from the 

 supposed rate of working, and the known distances — expand at 

 about the same rate as the observed shadow, but it is lighter 

 than the ground and shaded, instead of being dark and sharp, 

 as seen by the eye. 



I feel that the imperfect account which I am able to give is of 

 sufficient interest to appear in the columns of Nature, but I 

 hope that it may induce those who have occasion to detonate 

 heavy charges to make any observations that opportunity allows. 



