Pressure and vacuums

This method is unsafe, and no 
longer used.

We live submerged at the bottom of an ocean of the element of air, which by unquestioned experiments is known to have weight, and so much, indeed, that near the surface of the earth, where it is most dense it weighs about one four-hundredth of the weight of water [actually more like 1/775]. Those who have written about twilight, moreover, have observed that the vaporous and visible air rises above us to about [80 kilometres]; I do not believe its height to be so great, since if it were, I could show that the vacuum would be able to offer much greater resistance than it does….
— Evangelista Torricelli, in a letter to Michelangelo Ricci, 1644.

June 28: When the Barometer is 30 at S. Lambeth, it is 29-7 at Selborne, and 29-4 at Newton. My brother cut a good Romagna melon.
— Gilbert White (1720 – 1793), Journal, (1791), MIT Press, 1970.

Not that there is anything very mysterious … if it is remembered that a barometer is merely a weighing balance under another name. Instead of weighing a letter or a parcel against a series of standardised weights, it weighs the whole mass of air above it, right to the top of the atmosphere, against a column of mercury. An area of high pressure … is the outward and ground-level sign of a mountain of air above. The mountain of air is heavy. So the mercury has to rise higher …
— A. W. Haslett, Unsolved Problems of Science, London 1937.

I thought of filling a wine or beer cask with water and caulking it everywhere so that the outside air could not enter. In the lower part of the metal cask a metal tube was to be introduced by means of which the water could be drawn out; the water then, in consequence of its weight would sink, and leave behind it in the cask a space empty of air and therefore empty of any body.

… I arranged a brass force pump, like those used for fighting fires … worked very accurately so that the air could not go in or out … in the pump were two leather valves …

The trial, however, was not without value. After providing stronger screws, it came about at last that three strong men pulling on the piston of the pump drew out the water through the upper valve. But when this happened, a rustling noise was heard in all parts of the cask as if the water was boiling vigorously, and this lasted until the cask was filled with air in place of the water that had been drawn out.

Some way had to be found to avoid this unfortunate result. I therefore prepared a smaller cask which I placed in a larger one. A longer pipe was passed through the bases of both casks, and I then filled the smaller cask with water, sealed up its opening, filled the larger cask with water, and began again. This time we were able to draw the water out of the smaller cask, leaving, without a doubt, a vacuum behind.

However, as the day went by and the work was stopped and everything around became quiet, we perceived a varying tone, interrupted from time to time, like that of a twittering song-bird. This lasted for three whole days.

When, after that, the smaller cask was opened, it was found to be mostly filled with air and water, although it was partly empty, since as it was opened, some air entered … .

I perceived finally … that the water under pressure passed through the wood and … a little air developed itself from the water in the cask …
— Otto von Guericke (1602 – 1686) (Ottonis de Guericke) Experimenta nova (ut vocantur) magdeburgica, published 1672, translated by Magie, 80-81.

We live submerged at the bottom of an ocean of the element of air, which by unquestioned experiments is known to have weight, and so much, indeed, that near the surface of the earth, where it is most dense it weighs about one four-hundredth of the weight of water [actually more like 1/775]. Those who have written about twilight, moreover, have observed that the vaporous and visible air rises above us to about [80 kilometres]; I do not believe its height to be so great, since if it were, I could show that the vacuum would be able to offer much greater resistance than it does …
— Evangelista Torricelli (1608 – 1647).

To make it apparent how the weight of the air causes water to rise in a siphon, we will show that the weight of water makes mercury rise in a siphon which is fully open at the top and to which, therefore, air has free access. From this it will be clearly seen how the weight of the air produces this effect. And we will do so thus:

Let a siphon with one leg some twelve inches long and the other thirteen inches long be opened at the top and let a tube twenty feet in length be soldered hermetically to this opening. Then let the siphon be filled with mercury and placed with its legs dipping into separate containers also filled with mercury; and let the whole apparatus be set up in a water-filled tank at a depth of some fifteen or sixteen feet, the upper end of the open tube poking out of water. Now if one of the containers be just a little higher (say an inch) higher than the other, all of the mercury in the higher container will rise to the top of the siphon and flow through the other leg to the lower container continuously. But if the siphon is holed so that water may enter it, the mercury will drop out of each leg into its container, and water will take its place.

The mercury does not rise because of abhorrence of a vacuum, for the air has perfectly free access to the siphon. Again, if the water were removed from the tank, the mercury from each leg would fall into its container, and air would replace it in the now open tube. It is thus clear that it is the weight of water that makes the mercury rise, because it presses upon the mercury in the containers and not on that in the siphon. This weight makes the mercury rise and flow, but as soon as water is let in through a hole in the siphon, it presses inside as well as outside the siphon, and stops forcing the mercury up.
— Blaise Pascal (1623 – 1662), Physical Treatises.

… we began to pour quicksilver into the longer leg of the siphon, which by its weight pressing up that in the shorter leg, did by degrees streighten [shorten] the included air: and continuing this pouring in of quicksilver till the air in the shorter leg was by condensation reduced to take up by half the space it possessed (I say possessed, not filled) before; we cast our eyes upon the longer leg of the glass … and we observed, not without delight and satisfaction, that the quicksilver in the longer part of the tube was 29 inches higher than the other. Now that this observation does both very well agree with and confirm our hypothesis, will be easily discerned by him that takes notice what we teach … air … able to counter-balance and resist the pressure of a mercurial cylinder of about 29 inches, as we are taught by the Torricellian experiment … being brought to a degree of density about twice as great as that it had before, obtains a spring twice as strong as formerly. As may appear by its being able to sustain or resist a cylinder of 29 inches in the longer tube, together with the weight of the atmospherical cylinder, that … as we just now inferred from the Torricellian experiment, was equivalent to them.
— Robert Boyle (1627 – 1691).

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