PARIS, December 17, 1786.
DEAR SIR,--A dislocation of my right wrist has for three months past,
disabled me from writing except with my left hand, which was too slow
and awkward to be employed often. I begin to have so much use of my
wrist, as to be able to write, but it is slowly, and in pain. I take
the first moment I can, however, to acknowledge the receipt of your
letters of April the 6th, July the 8th and 30th. In one of these, you
say, you have not been able to learn, whether, in the new mills in
London, steam is the immediate mover of the machinery, or raises water
to move it? It is the immediate mover. The power of this agent, though
long known, is but now beginning to be applied to the various purposes
of which it is susceptible. You observe that Whitehurst supposes it to
have been the agent, which bursting the earth, threw it up into
mountains and valleys. You ask me what I think of his book? I find in
it many interesting facts brought together, and many ingenious
commentaries on them. But there are great chasms in his facts, and
consequently in his reasoning. These he fills up by suppositions, which
may be as reasonably denied as granted. A sceptical reader therefore,
like myself, is left in the lurch. I acknowledge, however, he makes
more use of fact, than any other writer on a theory of the earth. But I
give one answer to all these theorists. That is as follows. They all
suppose the earth a created existence. They must suppose a creator
then; and that he possessed power and wisdom to a great degree. As he
intended the earth for the habitation of animals and vegetables, is it
reasonable to suppose, he made two jobs of his creation, that he first
made a chaotic lump and set it into rotatory motion, and then waited
the millions of ages necessary to form itself? That when it had done
this, he stepped in a second time, to create the animals and plants
which were to inhabit it? As the hand of a creator is to be called in,
it may as well be called in at one stage of the process as another. We
may as well suppose he created the earth at once, nearly in the state
in which we see it, fit for the preservation of the beings he placed on
it. But it is said, we have a proof that he did not create it in its
present solid form, but in a state of fluidity; because its present
shape of an oblate spheroid is precisely that which a fluid mass
revolving on its axis would assume.
I suppose that the same equilibrium between gravity and centrifugal
force, which would determine a fluid mass into the form of an oblate
spheroid, would determine the wise creator of that mass, if he made it
in a solid state, to give it the same spheroidical form. A revolving
fluid will continue to change its shape, till it attains that in which
its principles of contrary motion are balanced. For if you suppose them
not balanced, it will change its form. Now, the same balanced form is
necessary for the preservation of a revolving solid. The creator,
therefore, of a revolving solid, would make it an oblate spheroid, that
figure alone admitting a perfect equilibrium. He would make it in that
form, for another reason; that is, to prevent a shifting of the axis of
rotation. Had he created the earth perfectly spherical, its axis might
have been perpetually shifting, by the influence of the other bodies of
the system; and by placing the inhabitants of the earth successively
under its poles, it might have been depopulated; whereas, being
spheroidical, it has but one axis on which it can revolve in
equilibrio. Suppose the axis of the earth to shift forty-five degrees;
then cut it into one hundred and eighty slices, making every section in
the plane of a circle of latitude, perpendicular to the axis: every one
of these slices, except the equatorial one, would be unbalanced, as
there would be more matter on one side of its axis than on the other.
There could be but one diameter drawn through such a slice, which would
divide it into two equal parts. On every other possible diameter, the
parts would hang unequal. This would produce an irregularity in the
diurnal rotation. We may, therefore, conclude it impossible for the
poles of the earth to shift, if it was made spheroidically; and that it
would be made spheroidical, though solid, to obtain this end. I use
this reasoning only on the supposition that the earth has had a
beginning. I am sure I shall read your conjectures on this subject with
great pleasure, though I bespeak, beforehand, a right to indulge my
natural incredulity and scepticism. The pain in which I write awakens
me here from my reverie, and obliges me to conclude with compliments to
Mrs. Thompson, and assurances to yourself of the esteem and affection
with which I am sincerely, dear Sir, your friend and servant.
P. S. Since writing the preceding, I have had a conversation on the
subject of the steam mills, with the famous Boulton, to whom those of
London belong, and who is here at this time. He compares the effect of
steam with that of horses, in the following manner: Six horses, aided
with the most advantageous combination of the mechanical powers
hitherto tried, will grind six bushels of flour in an hour; at the end
of which time they are all in a foam, and must rest. They can work
thus, six hours in the twenty-four, grinding thirty-six bushels of
flour, which is six to each horse, for the twenty-four hours. His steam
mill in London consumes one hundred and twenty bushels of coal in
twenty-four hours, turns ten pair of stones, which grind eight bushels
of flour an hour each, which is nineteen hundred and twenty bushels in
the twenty-four hours. This makes a peck and a half of coal perform
exactly as much as a horse, in one day, can perform.