YOL. X.] PHILOSOPHICAL TRANSACTIONS. 215 



water as much of the weight it had in air, as water of equal bulk with the im- 

 mersed solid would weigh in the air. Consequently, since gold is by far the 

 most ponderous metal, a piece of gold, and one of equal weight of copper, brass, 

 or any other metal, being proposed, the gold must be less in bulk, than the 

 copper or brass ; and if both of them be weighed in water, the gold must lose 

 in that liquor less of its former weight, than the brass or copper : because the 

 baser metal as well as the gold, grows lighter by the weight of a bulk of water 

 equal thereto; and the baser metal being the more voluminous, the corres- 

 ponding water must weigh more than that which in bulk is equal to the gold. 

 Whence the floating instrument above-mentioned would be made to sink deeper 

 by an ounce of gold, hanging at it under water, than by an ounce of brass, or 

 any other metal; which, on account of its greater bulk than gold, losing more 

 of its weight by the immersion, must needs retain less, and so have less power 

 to depress the instrument it was fastened to. And this will hold of other 

 metals that differ in specific gravity. 



This instrument may be of glass, copper, silver, or any other solid body, 

 that is, or may be made, fit to float on the water, with a guinea, &c. hanging 

 at it, and of a texture close enough to exclude the water. It consists of three 

 parts ; viz. the ball, the stem, and that which holds the coin. 



The ball or round part BCDE, fig. 1, plate ix, if of metal, consists of two 

 thin concave plates, exactly soldered together in the middle, and at the most 

 distant points from the juncture, there should be two opposite holes, one in 

 each plate, for the two other parts of the instrument. This middle part, though 

 for brevity sake called the ball, should not be exactly round, but of any shape 

 that shall be found fittest to make the instrument keep an erect posture steadily 

 in the water ; and it must be greater or smaller as the plates are thicker or 

 thinner ; but the general rule for its capacity is, that it should contain as much 

 air as may serve to keep the whole instrument, when loaded, from sinking be- 

 neath the top of the stem. 



The stem A B is to be soldered on to the ball at the uppermost of the two 

 mentioned holes. It may either be hollow or solid ; but it should be made .very 

 slender, that the different depressions of the instrument in the water may be 

 the more notable; and for the same reason it should not be too short, especially 

 if it be applied to other uses than the examining of guineas. 



At the undermost of the two holes in the ball, is inserted and soldered the 

 screw or stirrup F, fig. 2, which is a short piece of brass with a broad slit in it, 

 capable of receiving the edge of the guinea, to be fastened in it by a turn or two 

 of a screw. The stirrup G is made of a piece of wire bent round, and standing 

 horizontally, that the guinea may be laid on it. 



