21 6 PHILOSOPHICAL TRANSACTIONS. . [anNO iGjQ, 



It would be convenient that the undermost stem and the screw be made by 

 itself, that it may be at pleasure thrust on the stem, and taken off again; for by 

 this means, if the ball of the instrument be made large enough, you may have 

 room to put on for ballast, as occasion shall require, one, two, or three flat and 

 round pieces of copper, lead, &c. as fig. 3, with each of them a hole in the mid- 

 dle, fitted to the size of the stem, so that they may be put on as near the 

 lower part of the ball as you think fit, and then the screw may be thrust on 

 after them ; not only to take hold of the coin, or metallic mixture, to be exa- 

 mined, but to support the thin plates. 



To adjust this instrument for examining guineas, which are by far the most 

 usual gold coins ; hang at the bottom of it, a piece of that coin you know to be 

 genuine, and having carefully stopped the orifice of the stem, immerse the in- 

 strument leisurely and perpendicularly into a vessel full of clean water, almost to 

 the top of the stem, and then letting it alone. If it continue in the same situa- 

 tion and posture, the work is done; if it emerge, you must add a little weight, 

 either by putting into the stem, if it be hollow, some dust shot, filings of lead, 

 or some other minute and heavy body ; or else by putting on the short stem, 

 that comes out beneath the ball, a flat, round, and perforated piece of lead, of 

 weight suflicient to enable the guinea to depress the weight as low as it is de- 

 sired. But if it sink quite under water; then to make it lighter, file or scrape 

 off^ a little of the ballast plate, and take out some of the weight, put into the 

 cavity: this being done, a mark H, fig. I, is to be made, just at the place 

 where the surface of the water touches the stem ; then taking out the instru- 

 ment, substitute in the place of the guinea, a little round plate of brass, of the 

 same weight, or a grain or two heavier in the air; and putting the instrument 

 into the water as before, suffer it to settle, and make another mark I, at the 

 intersection of the stem, and the horizontal surface of the water. 



It may happen, that a falsifier of money may have the skill, by washing or 

 otherwise, of taking off^ much of the quantity or substance of the guinea, with- 

 out altering or impairing either the figure or stamp ; and thus the piece of coin 

 will not be able to depress the instrument to the usual mark, and thereby be 

 judged counterfeit, when it is indeed but too light. But the balance will soon 

 resolve the doubt ; for if the suspected coin have in the air its due weight, it 

 will argue, that its great lightness in water proceeds from its not being of the 

 requisite fineness. And if it want much of its due weight in the air, it is very 

 probable that it is washed. 



A general way for finding what coins may, or may not be examined, by this 

 or that particular instrument proposed; first, weigh the piece of gold or silver 

 in the air, and afterwards in water, and subtract the latter from the former. 



