452 NOTES. 



to execute undertakings much below the level of low water, and those investigations 

 suit us best, which are confined to the depth of a few fathoms, where we know that 

 water is, to all intents and purposes in our business, wholly unaltered by com- 

 pression." 



NOTE D. CHAPTER X. 



The principle of fluid support, and the doctrine of specific gravity, which we are 

 now considering, explain many curious facts that daily pass unobserved. Thus, a 

 stone which two men on land can h ardly lift, may be borne along by one man in water ; 

 and in diving, a dog will bring to the surface a human body, which the strongest of 

 his species could not lift on land : hence the ease also with which a bucket is lifted 

 from the bottom of a well to the surface of the water. And as the human body in 

 an ordinary healthy state, with the chest full of air, is lighter than its equal bulk 

 of water, a man naturally floats with about half the head extant ; " having," 

 as Dr. Arnott says, " then no more tendency to sink than a log of fir." When a 

 swimmer floats on his back, with merely his face above water, in which position 

 he can breathe freely, he exhibits the true position of floatation, in which the human 

 body is lighter than water, for its specific gravity is one ninth less than that of water, 

 being about 0.891. In some cases however, the bodies of men are heavier: thus, a 

 person who weighs 135 Ibs. would be 12 Ibs. heavier than two cubic feet of river 

 water, and would require a float of cork equal to 4 Ibs. to keep him from sinking ; 

 for 123 + 4# 135 -\-x, where x represents the weight of cork; consequently, 

 123 + 3a;=135; therefore, 3x= 135 123=12; whence x =4=. 



When a solid specifically heavier than a fluid, is immersed to a depth which is to 

 its thickness, as the specific gravity of the solid to that of the fluid, and the pressure 

 of the fluid from above is removed, the body will be sustained in the fluid ; for the 

 pressure from above being removed, the body is in the same state with respect to 

 the contrary pressure, as if the same weight filled the whole space to the surface of 

 the fluid; which means, as if its specific gravity and that of the fluid icere equal. 



The principle here enunciated helps the philosophers in their explanation of the 

 common experiment of making lead to swim, in consequence of being fitted to the 

 bottom of a glass tube. 



In the case cited above, of solid bodies being lighter in water than in air that 

 is to say, being more easily moved in the water than on dry land the meaning of 

 the proposition is, that all bodies, when immersed in a fluid, lose the weight of an 

 equal volume of that fluid. Thus, in raising a bucket of water from the bottom of 

 a well, so long as the bucket is under the water, we do not perceive it to have any 

 additional weight beyond the wood it is made of; but the moment we raise the 

 bucket to the surface, and suspend it in air, then we feel the additional weight of 

 the water, which if equal to 6 gallons, or to one cubic foot, will add nearly 62 

 pounds, or 1000 ounces avoirdupois weight to the bucket. Now all this weight 

 existed in the bucket when under the surface of the water, being supported by an 

 equal bulk, or 62J pounds. The weights thus gained or lost by immersing the 

 same body in different fluids, are as the specific gravities of the fluids ; hence 

 we affirm that all bodies of equal weight, but of different volume, lose in the 

 same fluid, weights which are reciprocally as the specific gravities of the bodies, 

 or directly as their volumes. In the salt sea it will be one thirty-fifth lighter than 

 i n fresh water. 



