Hydrostatics.

309

is heavier than Brass, we should chuse thelightest Air , i. e, when the Mercury in theBarometer stands lowest; but in buying pre-cious Stones , Pearls , &c. which are lighterthan Brass , the best Time to do it in is-when the Air is heaviest and most buoyant , viz.when the Quicksilver stands highest in theBarometer : But in felling Gold or Jewels ,the contrary Rules are to be observed inregard to the Gravity of the Air (LXVI).

Once

meter ask’d, Which is biggest, a Guinea or a Shilling,and what the Difference? He would find it vain toconsult his Art, and must borrow his Aid and Answerfrom this most useful Science, which excels equally inAccuracy as Universality.

(LXVI) 1. Since a cubic Inch of Gold weighs10,11/5 Ounces Troy, and a cubic Inch of Air *. of aGrain, when the Air is of a mean Gravity ; if we fay,

As 10,26 : f :: 12 : — 2 ^ — — the Parts of a GrainJ 72,52

which 12 Ounces or a Pound of Gold will lose when theAir is in a mean State. Now since the Air, when itsupports a Column of Mercury 28 Inches high, islighter by A Part than when it sustains a Column of 30

Inches Height; therefore ——- . ~

a 72,52 20 1450,4

the Parts of a Grain which so much Air weighs when

fieaviest, as is equal in Bulk to a Pound of Gold ; and

. _ . . 1105,04 , . , ..

twice the Quantity, viz. — - — the Weight of

1 1450,4

Air equal in Bulk to two Pounds of Gold. But this isa little less than one Grain; let us suppose it oneGrain.

2. Then two Pounds of Gold will lose of its Weightone Grain; and since Gold is twice as heavy as Brass that

X 3 it