602 



NATURE 



[October 20, 1904 



bear the yellow character and half of them the white. In 

 the last generation a similar test was applied to the male 

 germ cells with the same result. 



(4) The experiments were carried out under fully 

 " biometric " conditions, the more accurate " Mendelian " 

 method of careful pollination between individual plants being 

 deliberately avoided. Thus, in generation iv. pollination 

 was effected by the aid of the wind from some 1800 recessive 

 parents indiscriminately. 



A somewhat fuller description of the early part of this 

 experiment has already appeared in vol. ii., part ii., of the 

 Annals of the Royal Botanic Gardens, Peradeniya, and a 

 complete account will be published in a future number of 

 the same journal. R. H. Lock. 



Peradeniya, Ceylon, September 21. 



Rock Pressure at Great Depths. 



In his address to the engineering section of the British 

 Association, Mr. Parsons speaks of sinking a shaft into the 

 earth for a distance of 12 miles. 



I think, however, he overlooks a factor which sets a 

 limit to the depth to which a mine shaft can be sunk. If 

 we assume that the average specific gravity of the earth's 

 crust is 3, the superlaying rocks would exert at a distance 

 of 12 miles a pressure of about 440 tons per square inch. 



There can be little doubt that when subjected to such 

 a pressure the rock material would give way and flow 

 together like a viscous fluid, and so the walls of the shaft 

 would spontaneously close up, probably before the depth of 

 12 miles was reached. The breaking stress of steel is only 

 44 tons per square inch, and so, even were the walls encased 

 bv a steel tube, this would not avail to prevent the flowing 

 together of the walls. Geoffrey Martin. 



Kiel, Preusser-str. 191, September 17. 



I HAVE to thank you for directing my attention to Mr. 

 Martin's letter in which he gives his views as to. the prob- 

 able behaviour of rock around a very deep shaft boring, and 

 his opinion that the inward viscous flow of the rock would 

 place a limit to the possible depth. 



I have to thank Mr. Martin for directing attention to the 

 question of this possible limitation, which was considered 

 when writing my address and dismissed as unlikely to occur 

 up to depths of 12 miles, basing my conclusion on general 

 engineering knowledge of the flow of metals, of the relative 

 impressions made on hard brass and on hard rock when 

 struck by hard steel tools, and on the general behaviour of 

 metal when forged. 



I must first beg leave to point out some errors in Mr. 

 Martin's figures; he has misplaced the decimal point in 

 calculating the hydraulic pressure of the superlaying rocks 

 at 12 miles depth, which should be 40 tons and not 440 

 tons per square inch. 



Again, of the crushing stress required to make hardened 

 steel flow I have no data by me, but am aware that it lies 

 between 120 tons and 300 tons per square inch, and in the 

 case of hardened knife edges for weigh bridges, if my 

 memory is correct, the pressure per squai'e inch on the area 

 of contact reaches a still higher figure. 



Again, the pressure required to make the tough 

 brass (" cartridge metal ") flow is about 80 tons per square 

 inch. 



I think that the evidence at present available leads to 

 the conclusion that after a small amount of shrinkage of 

 the shaft sides inwards has taken place a state of equilibrium 

 would be established enabling the surrounding rock in its 

 state of great compression to withstand the so-called 

 hydraulic pressure due to a depth from the surface of at 

 least 12 miles. 



Since my address I have had the opportunity of discussing 

 the matter with Prof. G. H. Darwin, who has kindly 

 brought to my notice the article by Tresca, " Memoirs des 

 Savants Strangers sur I'^coulement des Corps solides," about 

 the year 1866, and also his own paper in the Philosophical 

 Transactions of the Royal Society, part i., 1S82, in which 

 the great shearing stresses that are thrown on the earth's 

 structure by the weight of mountain ranges on elevated 

 continents and great depths of the sea are exhaustively 

 treated. I would only point out that such stresses have 



NO. 1825, VOL. 70] 



been endured for long epochs, and that in view of the 

 established fact that rocks are viscous, it is clear that much 

 greater stresses could be sustained for the comparatively 

 short time necessary to complete a deep shaft boring. 



It would, however, be interesting to subject a cylinder 

 of granite or quartz rock, carefully fitted into a steel mould 

 and having a small hole bored through its centre, to a 

 pressure of, say, 100 tons per square inch, and see what 

 shrinkage in the hole would result, or a hole might be bored 

 into the specimen through an aperture in the mould while 

 subjected to this pressure. This pressure would correspond 

 to a depth of about 38 miles. 



Charles A. P.arsons. 



Holeyn Hall, Wylam-on-Tyne, October 7. 



The Berlin "Thinking" Horse. 



In your issue of September 22 there is a paragraph among 

 the notes (p. 510) with reference to " Clever Hans," a 

 " thinking horse " at present displaying his powers in 

 Berlin. With reference to it I wish to say that twelve or 

 thirteen years ago there was on exhibition in the Royal 

 Aquarium, London, a horse of, if I mistake not, exactly 

 the same stamp. I happened to be then attending lectures 

 at the Royal College of Science, and I went to see the 

 animal. I had, moreover, a long conversation with his 

 trainer, who eventually let me see exactly how it was all 

 accomplished. 



With all respect to the members of the " representative 

 committee " at Berlin, I am driven to hold that the per- 

 formances recorded, counting the number of the audience, 

 picking out the tallest man present, telling the hour, &c., 

 which seemed so deeply to impress them, partake of the 

 nature of a stage trick. They demonstrate what training 

 and perseverance can do with animals rather than the 

 possession on their part of any advanced mental powers. 



The Aquarium horse was named, if I remember aright, 

 Mahomel. He could work sums in addition and subtrac- 

 tion, or, for that matter, in multiplication, could count the 

 number present in the little side-show, could make a good 

 guess at the age of an individual, and so on. He had been 

 taught to begin pawing the ground when his trainer looked 

 straight at him, and to cease when the trainer turned his 

 gaze to the floor. It is easy to see the countless changes 

 that can be rung on this accomplishment. Telling the 

 time on a watch or the day of the month are readily 

 recognised to be among them. Similarly, he had been 

 drilled into bowing his head at one tone of his trainer's 

 voice, and shaking it on hearing another. Again one can 

 readily imagine how this bit of instruction will lend itself 

 to a very varied and wonderful display of cleverness. 



Mahomet's owner was an American and followed the 

 business of training horses, especially circus ones. This 

 horse, he discovered, was very easily taught — a genius 

 among his kind — and on him he then lavished years of most 

 careful labour, often, he assured me, sleeping of nights in 

 the manger at his head. The results were as shown. They 

 were in themselves sufficiently marvellous, and represent, I 

 fancy, the very utmost that a horse can be trained to do. 

 " Clever Hans " would seem to be blessed with a trainer 

 as painstaking and persevering as my American friend. 



After a seance which I had all to myself, Mahomet's 

 owner delayed with me to see the performance of a clever 

 dog on the central stage. The dog, a fair specimen of a 

 rough collie, answered questions, spelt his own name, words 

 sent up by the audience, &r. The letters of the alphabet 

 were placed in order in a wire frame towards the back of 

 the stage. The collie went along the letters, picked out 

 the one he needed, and brought and laid it before the foot- 

 lights. He then went for the next. Wonderful I thought 

 the performance until my friend the horse-trainer showed 

 me how it was done. The collie always began at A. He 

 then trotted along up the alphabet until he reached the 

 one he needed. His master carried his gloves in his hand. 

 A little twitch of the gloves as the dog passed the particular 

 letter wanted was the cue. The well trained animal took 

 in the slightest stir of the gloves with the corner of his eye. 

 This dog even played a game of cards — and won. A 

 hundred and one variations might be made on the same 

 trick. 



I have read since in an .American newspaper of a Tennessee 



