for ascertaining Specific Gravities. 25 7 



dividing its weight in air by this difference the quotient will 

 le its specific gravity. 



The weight of a body in air is found by putting it in the 

 upper cup, and adding grains until the hydrometer sinks in 

 water to the mark on the stem. Now, as the substance and 

 the additional weights in the cup will be altogether 300 

 grains, the weight of the body will of course be so many 

 grains as the weights put in fell short of 300. Its weight 

 in water will be found by putting it into the lower cup, and 

 adding grains in the upper cup until the instrument sinks as 

 before : the complement of the weights in the top cup to 

 300 being in like manner its weight in water. 



Example. — If a body weighs in air 120 graifis, and ia 

 water 104, the difference is 16. On dividing 120 by 16, 

 we have for the quotient '75, or (taking, as before, the spe- 

 cific gravity of water at 1000) 7*500 for the specific gravity 

 of the body. 



This instrument affords us consequently a very ready way 

 of determining the purity or value of any alloy or metallic 

 ore, and is therefore particularly adapted to the mineralogist. 

 Thus, for example, the weight of a guinea, or its weight 

 in air, is 128 grains; and if the gold is of its proper standard, 

 it will weigh about 121 grains in water, or will lose one- 

 eighteenth part only of its weight in air. If it loses more, 

 therefore, it is not of its proper specific gravity, and con- 

 sequently not of standard gold. 



To find the specific gravity of any of the different species 

 of wood or other bodies lighter than water ; — after taking 

 its weight in air as before, fix it on the small screw of the 

 shank, and see how many grains it will then be necessary 

 to add in the top cup, to sink the instrument to the mark, 

 with the body on the screw ; which will in this case be more 

 than 300, on account of its buoyancy ; and dividing its 

 weight in air by the difference between the weights put in 

 the top cup in each case, the quotient will be its specific 

 gravity. 



Thus, if on putting a piece of willow in the upper cup, 

 it requires 258 grains to sink the hydrometer in water, the 



Vol. 31. No. 124. Sept. 1808. R weight 



