INTRODUCTION TO HYDROSTATICS. lix 



The specific gravity of a body means simply its weight compared with 

 that of another body of the same size. When we say that substances, 

 such as lead and stones, are heavy, and that others, such as paper and 

 feathers, are light, we speak comparatively : that is to say, that the first are 

 heavy, and the latter light, in comparison with the generality of the sub- 

 stances in nature. Mahogany is a heavy body when compared to most other 

 kinds of wood, but light when compared to stone. Chalk is a heavy body 

 compared to coal, but light if compared to metal. You perceive therefore 

 that our notions of light and heavy are vague and undefined, and that 

 some standard of comparison is required, to which the weight of all other 

 bodies may be referred. The body which has been adopted as a standard 

 of reference is distilled water. It may perhaps appear surprising that a 

 fluid should have been chosen for this purpose, as it must necessarily be 

 contained in some vessel, and the weight of this vessel must be deducted. 

 This is true, when the specific gravity of fluids is to be estimated ; but 

 with regard to solids, it is necessary simply to weigh the body under trial 

 in water. If a piece of gold be weighed in a glass of water, the gold will 

 displace just as much water as is equal to its own bulk : a cubic inch of 

 water must make way for a cubic inch of gold. The bulk alone is to be 

 considered, the weight has nothing to do with the quantity of water dis- 

 placed ; for a cubic inch of gold does not occupy more space, and there- 

 fore will not displace more water, than a cubic inch of ivory, or any other 

 substance that will sink in water. 



The gold will weigh less in water than it did out of it, on account of 

 the upward pressure of the particles of water, which in some measure 

 supports the gold, and by so doing, diminishes its weight. If the body 

 under trial be of the same weight as the water in which it is immersed, it 

 will be wholly supported by it, as was the water, the place of which it 

 occupies ; if it be heavier, the water will offer some resistance to its 

 descent ; and this resistance will in all cases be the same to bodies of 

 equal bulk, whatever be their weight. All bodies of the same size, there- 

 fore, lose the same quantity of their weight when completely immersed 

 in water. A body weighed in water loses as much of its weight as is 

 equal to that of the water it displaces ; so that were this water put into 

 the scale to which the body is suspended, it would restore the balance. 



These observations, however, require some modification when applied 

 to the case of bodies lighter than an equal bulk of water, and which, 

 therefore, do not sink entirely in water. The method of ascertaining 

 their specific gravity will be presently pointed out. At present we may 

 observe, that the rule given above has an application even to them, if 

 forcibly immersed in water ; but the resistance, or upper pressure of the 

 water, being greater than the weight of the body, that weight is not 

 merely diminished, but the body has a tendency upwards equal to the 

 difference between the resistance and its weight. 



When a body is weighed in water, in order to ascertain its specific 

 gravity, it may either be suspended to a hook at the bottom of the basin 

 of the balance, or, taking off the basin, suspended to the arm of the 

 balance (fig. 7). Now, supposing that a cubic inch of gold weighed 

 nineteen ounces out of water, and lost one ounce by being weighed in 

 water, the cubic inch of water it displaces must weigh that one ounce : 

 consequently gold would be nineteen times as heavy as water. 



The specific gravity of a body lighter than water cannot be ascertained 

 in the same manner. If a body were absolutely light, it would float on 

 the surface without displacing a drop of water ; but bodies have all some 



