Rule III.: As the Weight of fine Silver in the Shilling, is to its Value in England, so is the Weight of pure Silver in

The Quantity of fine Silver in any Piece is had, by knowing the Weight of the Piece and the Weight of Allay in a given Weight of Silver, out of which that Piece is made.

For Instance. In the Standard Silver of England a Pound Weight, or 240 Penny-weights, contains 222 Penny-weights of fine Silver, and the same Weight of Spanish Silver out of which the Mexico Piece of Eight is formed, contains 221 Penny-weights. Then as 240 is to 222, so is 92.9 Grains, the Weight of the Shilling, to its Weight of fine Silver; and as 240 is to 221, so is 418.5 Grains the Weight of the Mexico Piece of Eight, to its Weight of pure Silver. The Weights of fine Silver in the Shillings and Mexico Piece will therefore be and and rejecting the common Denominator, will be expressed by 222 × 92.9 and 221 × 418.5.

Then as 222 × 92.9 is to 12 Pence, the Value of the Shilling, so is 221 × 418.5 to the Value of the Mexico Piece of Eight in England; which Value is therefore equal to that is, equal to 53.82 Pence, which is near 54 Pence.

When the Weights and Allays of Coins are known, it is easy to determine by these Rules, at what Value they ought to pass in any other Country.