NOTES. 451 



NOTE B. PROPOSITION I. CHAPTER I. 



In this proposition, and the several laws and consequences deduced from it, the 

 effect of the atmospheric pressure is entirely disregarded. It may however be 

 proper to remark, that in numerous delicate hydrostatical inquiries, the pressure 

 thus excited must be taken into the account: it is equal to the pressure of a 

 column of water 34 feet in perpendicular height, 



NOTE C. CHAPTER VI. 



Experiment 7. Page 160. Since these experiments were selected and inserted 

 in this work, a living eel has been killed in the cylinder of the hydrostatic press, 

 in which also an egg has been broken. But the eel would have been killed by 

 suffocation, if no pressure had been applied to the fluid, and the fracture of 

 the egg was due to the air it contained between the white and the shell, or 

 to the different densities of the shell, the white, the yolk, and the water. 

 Thus we can easily conceive, as Mr. Tredgold remarks, that the trial of an 

 experiment may be the means of condemning a very useful principle, merely 

 through inattention to the proportions and the mode of action. We may still 

 affirm, that fishes will endure a very high degree of fluid pressure, provided they 

 be allowed to breathe ; indeed it is recorded, that a whale in the arctic seas, being 

 struck by a harpoon, descended perpendicularly by the line about 900 fathoms, 

 before it returned to the surface to respire ; it was then under a pressure of nearly 

 164 atmospheres, or 2,460 Ibs. upon a square inch of its surface; now if the living 

 animal could sustain this natural pressure without inconvenience, we are at liberty 

 to conclude that it could sustain an equal degree of artificial pressure. It is manifest, 

 that fishes which do not come to the surface, breathe the air with which the water 

 is impregnated, at whatever depth they may be found. Moreover, if an eel were 

 killed by pressure, we suppose it would be crushed, or burst asunder. In short, we 

 require evidence of the death by pressure, to remove our belief in death by suffo- 

 cation. Air, which is invisible, by squeezing the heat out of it by strong pressure, 

 may be compressed into water ; but the contraction which water suffers at every 

 increase of pressure, exceeds not the twenty thousandth part of what air would 

 undergo in like circumstances ; and fishes are at their ease in a depth of water, 

 where the pressure around will instantly break or burst inwards almost the strongest 

 empty vessel that can be let down. 



We are perfectly aware of the experiments of Mr. Canton, in 1760, which 

 established incontestably the compression of water. Indeed the theory of com- 

 pression extends to all bodies : Dr. Young says that steel would be compressed 

 into one-fourth, and stone into one-eighth of its bulk at the earth's centre ; but a 

 density so extreme is not borne out by astronomical observation. And the late Sir 

 John Leslie, who suggests the idea that the ocean may rest upon a subaqueous 

 bed of compressed air,* says that water at the depth of 93 miles would be com- 

 pressed into half its bulk at the surface of the earth ; and at the depth of 362.5 

 miles it would acquire the ordinary density of quicksilver.t Practical men, m 

 reply to all this physical science, may justly reply, " We are seldom called upon 



* Article " Meteorology," in the Supplement to the Encyclopaedia Britannica. 

 t See Leslie's Elements of Natural Philosophy, vol. i. 



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