553 



SCIENTIFIC SIDE-LIGHTS 



Present 

 Prevision 



built, on being offered to the cook for fuel, 

 was tried and rejected as incombustible." 

 LYELL Principles of Geology, bk. iii, ch. 47, 

 p. 743. (A., 1854.) 



2722. Glass Crushed to 



Powder Implosion vs. Explosion. A most 

 beautiful experiment to illustrate the enor- 

 mous force of this pressure was made during 

 the voyage of H. M. S. " Challenger." I give 

 the description of it in the words of the 

 late Professor Moseley: 



" Mr. Buchanan hermetically sealed up at 

 Tjoth ends a thick glass tube full of air, 

 several inches in length. He wrapped this 

 sealed tube in flannel and placed it, so 

 wrapped up, in a wide copper tube, which 

 was one of those used to protect the deep- 

 sea thermometers when sent down with the 

 sounding apparatus. 



" This copper tube was closed by a lid 

 fitting loosely, and with holes in it, and the 

 copper bottom of the tube similarly had 

 holes bored through it. The water thus had 

 free access to the interior of the tube when 

 it was lowered into the sea, and the tube 

 was necessarily constructed with that object 

 in view, in order that in its ordinary use 

 the water should freely reach the contained 

 thermometer. 



" The copper case containing the sealed 

 glass tube was sent down to a depth of 2,000 

 fathoms and drawn up again. It was then 

 found that the copper wall of the case was 

 bulged and bent inwards opposite the place 

 where the glass tube lay, just as if it had 

 Tjeen crumpled inward by being violently 

 squeezed. 



" The glass tube itself, within its flannel 

 wrapper, was found when withdrawn re- 

 duced to a fine powder, like snow almost. 

 What had happened was that the sealed 

 glass tube, when sinking to gradually in- 

 creasing depths, had held out long against 

 the pressure, but this at last had become 

 too great for the glass to sustain, and the 

 tube had suddenly given way and been 

 crushed by the violence of the action to a 

 fine powder. So violent and rapid had been 

 the collapse that the water had not had 

 time to rush in by means of the holes at 

 both ends of the copper cylinder, and thus 

 fill the empty space left behind by the col- 

 lapse of the glass tube, but had instead 

 crushed in the copper wall and brought 

 equilibrium in that manner. The process 

 is exactly the reverse of an explosion, and 

 is termed by Sir Wyville Thomson an ' im- 

 plosion.' " HICKSON Fauna of the Deep Sea, 

 ch. 2, p. 19. (A., 1894.) 



2723. PRESSURE OF GLACIER Giant 

 Power of Natural Agencies. The glacier 

 does more than abrade. Rocks are not 

 homogeneous: they are intersected by joints 

 and places of weakness, which divide them 

 into virtually detached masses. A glacier 

 is undoubtedly competent to root such 

 masses bodily away. Indeed, the mere 

 a priori consideration of the subject proves 



the competence of a glacier to deepen its 

 bed. Taking the case of a glacier 1,000 feet 

 deep (and some of the older ones were 

 probably three times this depth), and al- 

 lowing 40 feet of ice to an atmosphere, we 

 find that on every square inch of its bed 

 such a glacier presses with a weight of 

 375 Ibs., and on every square yard of its bed 

 with a weight of 486,000 K>s. With a ver- 

 tical pressure of this amount the glacier 

 is urged down its valley by the pressure 

 from behind. We can hardly, I think, deny 

 to such a tool a power of excavation. TYN- 

 DALL Hours of Exercise in the Alps, ch. 20, 

 p. 239. (A., 1898.) 



2724. Its Grinding Power. 



The movement of glacial ice causes fric- 

 tion and leads to the grinding, smoothing, 

 and scratching of the rocks over which it 

 passes. The intensity of this grinding can 

 be appreciated to some extent by considering 

 the force with which a thick ice mass presses 

 on the rocks beneath. The weight of a cubic 

 foot of ice is about fifty-seven pounds, hence 

 a glacier 1,000 feet thick, which is by no 

 means the maximum, would exert a pressure 

 on its bed of twenty-eight tons to the square 

 foot. A movement of ice charged with sand 

 and stones under such a pressure cannot fail 

 to produce abrasion of the rocks beneath. 

 RUSSELL Glaciers of North America, int., p. 

 18. (G. & Co., 1897.) 



2725. PREVISION IN BIRDS AND 



INSECTS Mystery of Lower Organisms. 

 Those birds and insects whose young are 

 hatched by the heat of fermentation have 

 an intuitive impulse to select the proper 

 materials, and to gather them for the pur- 

 pose. All creatures, guided sometimes ap- 

 parently by senses of which we know noth- 

 ing, are under like impulses to provide ef- 

 fectually for the nourishing of their young. 

 It is. moreover, most curious and instructive 

 to observe that the extent of prevision which 

 is involved in this process and in the se- 

 curing of the result seems very often to be 

 greater as we descend in the scale of Na- 

 ture, and in proportion as the parents are 

 dissociated from the actual feeding or per- 

 sonal care of their young. The mammalia 

 have nothing to provide except food for 

 themselves, and have at first, and for a long 

 time, no duty to perform beyond the dis- 

 charge of a purely physical function. Milk 

 is secreted in them by a purely unconscious 

 process, and the young need no instruction 

 in the art of sucking. Birds have much 

 more to do in the building of nests, in the 

 choice of sites for these, and, after incuba- 

 tion, in the choice of food adapted to the 

 period of growth. Insects, much lower in 

 the scale of organization, have to provide 

 very often for a much more distant future, 

 and for various stages of development. AR- 

 GYLL Unity of Nature, ch. 2, p. 40. (Burt.) 



2726. PREVISION IN INSECTS Ants 

 Caring for Eggs of Aphides. Here are aph- 

 ides, not living in the ants' nests, but out- 



