The
avoirdupois ounce contains 18 pennyweights 5 1/2 grains troy weight.
Divide it, then, into 18 pennyweights, and the pennyweight, as
heretofore, into 24 grains, and the new pennyweight will contain
between a third and a quarter of a grain more than the present troy
penny-weight; or, more accurately, it will be to that as 875 to 864
-- a difference not to be noticed, either in money or medicine, below
the denomination of an ounce.
But it will be necessary to refer these weights to a
determinate mass of some substance, the specific gravity of which is
invariable. Rain water is such a substance, and may be referred to
everywhere, and through all time. It has been found by accurate
experiments that a cubic foot of rain water weighs 1000 ounces
avoirdupois, standard weights of the exchequer. It is true that
among these standard weights the committee report small variations;
but this experiment must decide in favor of those particular weights,
between which, and an integral mass of water, so remarkable a
coincidence has been found. To render this standard more exact, the
water should be weighed always in the same temperature of air; as
heat, by increasing its volume, lessens its specific gravity. The
cellar of uniform temperature is best for this also.
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