Florence, June 11, 1644Most 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.