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Evangelista Torricelli (1608-1647)Letter to Michelangelo Ricci concerning the Barometer(1644) Collected Works Vol. III (1919) [from William Francis Magie, A Source Book in Physics (New York: McGraw-Hill, 1935) (translator?)]To Michelangelo Ricci in Rome. Florence, June 11, 1644
Most Illustrious Sir and Most Learned Patron:
Several weeks ago I sent to Sig. Antonio Nardi several of my demonstrations
of the areas of cycloids, and asked him that after he had examined them
he would send them on at once to yourself or to Sig. Magiotti. I have already
called attention to the fact that there are in progress certain philosophical
experiments, I do not know just what, relating to vacuum, designed not
simply to make a vacuum but to make an instrument which will show the changes
in the atmosphere, as it is now heavier and more gross and now lighter
and more subtle. Many have said that a vacuum does not exist, others that
it does exist in spite of the repugnance of nature and with difficulty;
I know of no one who has said that it exists without difficulty and without
a resistance from nature. I argued thus: If there can be found a manifest
cause from which the resistance can be derived which is felt if we try
to make a vacuum, it seems to me foolish to try to attribute to vacuum
those operations which follow evidently from some other cause; and so by
making some very easy calculations, I found that the cause assigned by
me (that is, the weight of the atmosphere) ought by itself alone to offer
a greater resistance than it does when we try to produce a vacuum. I say
this because a certain philosopher, seeing that he cannot escape the admission
that the weight of the atmosphere causes the resistance which is felt in
making a vacuum, does not say that he admits the operation of the heavy
air, but persists in asserting that nature also concurs in resisting the
vacuum. We live immersed at the bottom of a sea of elemental air, which
by experiment undoubtedly has weight, and so much weight that the densest
air in the neighborhood of the surface of the earth weighs about one four-hundredth
part of the weight of water. Certain authors have observed after twilight
that the vaporous and visible air rises above us to a height of fifty or
fifty-four miles, but I do not think it is so much, because I can show
that the vacuum ought to offer a much greater resistance than it does,
unless we use the argument that the weight which Galileo assigned applies
to the lowest atmosphere, where men and animals live, but that on the peaks
of high mountains the air begins to be more pure and to weigh much less
than the four-hundredth part of the weight of water.
We have made many vessels of glass like those shown as A and B
and with tubes two cubits long. These were filled with quicksilver,
the open end was closed with the finger, and they were then inverted in
a vessel where there was quicksilver C; then we saw that an empty
space was formed and that nothing happened in the vessel where this space
was formed; the tube between A and D remained always full
to the height of a cubit and a quarter and an inch over. To show that the
vessel was entirely empty, we filled the bowl with pure water up to D
and then, raising the tube little by little, we saw that, when the opening
of the tube reached the water, the quicksilver fell out of the tube and
the water rushed with great violence up to the mark E. It is often
said in explanation of the fact that the vessel AE stands empty
and the quicksilver, although heavy, is sustained in the tube AC,
that, as has been believed hitherto, the force which prevents the quicksilver
from falling down, as it would naturally do, is internal to the vessel
AE, arising either from the vacuum or from some exceedingly rarefied substance;
but I assert that it is external and that the force comes from without.
On the surface of the liquid which is in the bowl there rests the weight
of a height of fifty miles of air; then what wonder is it if into the vessel
CE,
in which the quicksilver has no inclination and no repugnance, not even
the slightest, to being there, it should enter and should rise in a column
high enough to make equilibrium with the weight of the external air which
forces it up? Water also in a similar tube, though a much longer one, will
rise to about 18 cubits, that is, as much more than quicksilver does as
quicksilver is heavier than water, so as to be in equilibrium with the
same cause which acts on the one and the other. This argument is strengthened
by an experiment made at the same time with the vessel A and with
the tube B in which the quicksilver always stood at the same horizontal
line AB. This makes it almost certain that the action does not come
from within; because the vessel AE, where there was more rarefied
substance, should have had a greater force, attracting much more actively
because of the greater rarefaction than that of the much smaller space
B.
I have endeavored to explain by this principle all sorts of repugnances
which are felt in the various effects attributed to vacuum, and I have
not yet found any with which I cannot deal successfully. I know that your
highness will perceive many objections, but I hope that if you think them
over they will be resolved. My principal intention I was not able to carry
out, that is, to recognize when the atmosphere is grosser and heavier and
when it is more subtle and lighter, because the level AB in the
instrument EC changes for some other reason (which I would not have
believed) especially as it is sensible to cold or heat, exactly as if the
vessel AE were full of air.
Your devoted and obliged Servant,
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