462 THE POPULAR SCIENCE MONTHLY. 



in any description of the fauna of a particular region to consider 

 its physical conditions and the influence that it may be supposed 

 to have had in producing the characteristics of the fauna. 



The peculiar physical conditions of the deep seas may be 

 briefly stated to be these : It is absolutely dark so far as actual 

 sunlight is concerned, the temperature is only a few degrees above 

 freezing point, the pressure is enormous, there is little or no move- 

 ment of the water, the bottom is composed of a uniform fine soft 

 mud, and there is no plant life. All of these physical conditions 

 we can appreciate except the enormous pressure. Absolute dark- 

 ness we know, the temperature of the deep seas is not an extraor- 

 dinary one, the absence of movement in the water and the fine 

 soft mud are conditions that we can readily appreciate ; but the 

 pressure is far greater than anything we can realize. At a depth 

 of twenty-five hundred fathoms the pressure is, roughly speaking, 

 two and a half tons per square inch that is to say, several times 

 greater than the pressure exerted by the steam upon the piston of 

 our most powerful engines. Or, to put the matter in other words, 

 the pressure per square inch upon the body of every animal that 

 lives at the bottom of the Atlantic Ocean is about twenty-five 

 times greater than the pressure that will drive a railway train. 



A most beautiful experiment to illustrate the enormous force 

 of this pressure was made during the voyage of H. M. S. Chal- 

 lenger. I give the description of it in the words of the late Prof. 

 Moseley : " Mr. Buchanan hermetically sealed up at both 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 neces- 

 sarily constructed with that object in view, in order that in its 

 ordinary use the water should freely reach the contained ther- 

 mometer. 



" The copper case containing the sealed glass tube was sent 

 down to a depth of two thousand fathoms and drawn up again. 

 It was then found that the copper wall of the case was bulged 

 and bent inward opposite the place where the glass tube lay, just 

 as if it had been crumpled inward by being violently squeezed. 

 The glass tube itself, within its flannel wrapper, was found, when 

 withdrawn, reduced to a fine powder, like snow almost. What 

 had happened was that the sealed glass tube, when sinking to 

 gradually increasing depths, had held out long against the pres- 

 sure, but this at last had become too great for the glass to sustain. 



