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INTRODUCTION TO HYDROSTATICS, 



Fig. 7. 



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weight, and will, therefore, displace some quantity of water. A body 

 lighter than water will not sink to a level with the surface of the water, 

 and therefore will not displace so much water as is equal to its bulk, but 

 a quantity equal to its weight. A ship sinks to some depth in water, and 

 the heavier it is laden the deeper it sinks, the quantity of water it dis- 

 places being always equal to its weight. This quantity cannot, however, 

 afford a convenient test of its specific gravity, from the difficulty of 

 collecting the whole quantity of water displaced, and of measuring the 

 exact bulk of the body immersed. 



In order practically to obtain the specific gravity of a body which is 

 lighter than water, a heavy one, whose specific gravity is known, must be 

 attached to it, and they must be immersed together: the specific gravity 

 of the lighter body may then be easily calculated. 



Bodies which have exactly the same specific gravity as water, will remain 

 at rest in whatever situation they are placed in water. If a piece of wood, 

 by being impregnated with a little sand, be rendered precisely of the 

 weight of an equal bulk of water, it will t *remain stationary in whatever 

 part of a vessel of water it be placed. If a few drops of water be poured 

 into the vessel (so gently as not to increase their momentum by giving 

 them velocity), they would mix with the water at the surface, and not sink 

 lower. 



The specific gravity of fluids is found by" means of an instrument 

 called an hydrometer. It consists of a thin glass ball, A 

 (Jig. 8), with a graduated tube,B, and the specific gravity of 

 the liquid is estimated by the depth to which the instrument 

 sinks in it, for the less the specific gravity of the fluid, the 

 further will the instrument sink in it. There is a smaller 

 ball, C, attached to the instrument below, which contains a 

 little mercury ; but this is merely for the purpose of equi- 

 poising the instrument, that it may remain upright in the 

 liquid under trial. 



The weight of a substance, when not compared to that of any other, is 

 perfectly arbitrary ; and when water is adopted as a standard, we may 

 denominate its weight by any number we please ; but then the weight of 

 all bodies tried by this standard must be signified by proportional num- 

 bers. If we call the weight of water, for example, 1, then that of gold 

 would be 19; or, if we call the weight of water 1000, that of gold 

 would be 19,000. In short, the specific gravity indicates how much 

 more or less a body weighs than an equal bulk of water. 



