Here follows his plan: Let the

Philosophical pound consist of 10 ounces, or 10,000 grains.
the ounce " " 10 drachms or 1,000 "
the drachm " " 100 grains.

Let all elastic fluids be measured by the ounce measure of
water, by which the valuation of different cubic inches will be
avoided, and the common decimal tables of specific gravities
will immediately give the weights of those elastic fluids.

If all philosophers cannot agree on one pound or one grain, let
every one take his own pound or his own grain; it will affect
nothing but doses of medicines, which must be corrected as is
now done; but as it would be much better that the identical
pound was used by all. I would propose that the Amsterdam or
Paris pound be assumed as the standard, being now the most
universal in Europe: it is to our avoirdupois pound as 109 is to
100. Our avoirdupois pound contains 7,000 of our grains, and the
Paris pound 7,630 of our grains, but it contains 9,376 Paris
grains, so that the division into 10,000 would very little
affect the Paris grain. I prefer dividing the pound afresh to
beginning with the Paris grain, because I believe the pound is
very general, but the grain local.

Dr. Priestley has agreed to this proposal, and has referred it
to you to fix upon the pound if you otherwise approve of it. I
shall be happy to have your opinion of it as soon as convenient,
and to concert with you the means of making it universal.... I
have some hopes that the foot may be fixed by the pendulum and a
measure of water, and a pound derived from that; but in the
interim let us at least assume a proper division, which from the
nature of it must be intelligible as long as decimal arithmetic
is used.

He afterward wrote, in a letter to Magellan:

As to the precise foot or pound, I do not look upon it to be
very material, in chemistry at least. Either the common English
foot may be adopted according to your proposal, which has the
advantage that a cubic foot is exactly 1,000 ounces,
consequently the present foot and ounce would be retained; or a
pendulum which vibrates 100 times a minute may be adopted for
the standard, which would make the foot 14.2 of our present
inches, and the cubic foot would be very exactly a bushel, and
would weigh 101 of the present