14 



HYDROSTATICS. 



sink to the bottom. So if it be lighter 

 than an equal bulk of the fluid, it will 

 rise through the fluid to the surface. 

 But if a solid, heavier than the fluid, be 

 plunged to a depth as many times 

 greater than its thickness, as the solid 

 is heavier than the fluid, and there pro- 

 tected by any means from the pressure 

 of the fluid above, it will float, notwith- 

 standing its weight, because the pres- 

 sure from below being in proportion to 

 the depth,will counterbalance the weight 

 of the body, and there will be no pres- 

 sure from above, except the weight of 

 the body. Thus, lead is somewhat above 

 eleven times heavier than water. If a 

 cube of lead be placed so as to press 

 closely against the bottom of a wooden 

 pipe one foot square, closed at the top, 

 and plunged twelve feet deep, and held 

 upright, it will there swim ; the water 

 pressing it upwards with a force greater 

 than its weight, and there being no 

 pressure from the water downwards. So 

 if a body lighter than water, as cork, be 

 placed at the bottom of a vessel, and 

 so smoothly cut that no water gets be- 

 tween its lower surface and the surface 

 of the bottom, it will not rise, but re- 

 main fixed there, because it is pressed 

 downwards by the water from above, 

 and there is no pressure from below 

 to counterbalance that from above. 



It follows from these principles, that 

 if any body be weighed in the air, and 

 then weighed in any liquid, it will seem 

 to lose as much as an equal bulk of the 

 liquid weighs. Not that the body 

 really loses its weight, but that it is 

 pressed upwards by a force equal to 

 the weight of the liquid the place of 

 which it fills. Thus, if a piece of lead 

 weigh an ounce before being plunged 

 in water, that is, requires an ounce 

 weight on the opposite scale to balance 

 it ; if you hang it by a thread from its 

 own scale, and let it be plunged so that 

 the water in a full jar covers it, a quan- 

 tity of water equal to the bulk of the 

 lead will run over the sides of the jar, 

 and a number of grains equal to the 

 weight of this quantity of water must 

 be taken out of the opposite scale, to 

 restore the balance ; for the lead is now 

 pressed downwards in the water with a 

 force not equal to its own weight, but 

 to the difference between its own weight 

 and that of an equal bulk of the water. 

 And in this manner we can determine 

 the relative weights of all bodies, or the 

 proportion which they bear to each 

 other in weight ; which is called their 



specific gravity : that is, their weight 

 in kind, and sometimes their relative 

 gravity, that is, their weight compared 

 with the weight of other bodies. By 

 weighing a known bulk, as a cubic 

 foot or a cubic inch of any two 

 substances, we can find their specific 

 gravity; or their gravity as com- 

 pared with each other : if, for in- 

 stance, we found a cubic inch of iron 

 weighed 1948 grains, and a cubic inch 

 of lead 2858, we should say, that the 

 specific gravities of the two substances 

 were in the proportion of 3 to 4^ 

 nearly ; and so we might find the spe- 

 cific gravity of a solid substance, as 

 compared with that of a liquid, by 

 weighing an equal bulk of each. But 

 this operation is extremely difficult, be- 

 cause it requires the substances com- 

 pared to be fashioned accurately into 

 the same shape and size ; and when we 

 are not allowed to change their figure, 

 the comparison cannot be made at all. 

 Thus we could not ascertain the spe- 

 cific gravity of precious stones, crys- 

 tals, metallic ores, or animal and vege- 

 table substances, without in effect de- 

 stroying them. But the Hydrostatic 

 Balance, upon the principles now ex- 

 plained, affords a perfectly easy and 

 most accurate method of comparing all 

 substances, solid and fluid. We have 

 only to weigh any substance first in air, 

 and then in water ; the difference of the 

 weights is the weight of a bulk of water 

 equal to the bulk of the substance ; and 

 by comparing any other substance with 

 water, in like manner, we ascertain its 

 specific gravity, as compared with that 

 of the first substance. And this opera- 

 tion may be performed with substances 

 lighter than water, either by fixing 

 them to a stiff pin, attached to the 

 bottom of the scale, taking care to trim 

 the balance before the pin is plunged 

 into the water ; or by loading the sub- 

 stance with a known weight of some- 

 thing heavier than water, and making 

 an allowance for the load's difference of 

 weight in air and water. 



It is evident also that the same prin- 

 ciple enables us to ascertain the spe- 

 cific gravities of different fluids. For 

 if the same substance be weighed in 

 two fluids, the weight which it loses 

 in each is as the specific gravity of that 

 fluid. Thus a cubic inch of lead loses 

 253 grains when weighed in water, and 

 only 209 grains when weighed in recti- 

 fied spirits; therefore a cubic inch of 

 rectified spirit weighs 209 grains, au 



