PASCAL

Experiments With the Barometer

September 22nd, 1648.

We therefore met on that day at eight o’clock in the morning in the garden of the Pères Minimes, which is in almost the lowest part of the town, where the experiment was begun in the following way:

First, I poured into a vessel sixteen pounds of quicksilver, which I had purified during the three preceding days; and taking two tubes of glass of equal size, each about four feet long, hermetically sealed at one end and open at the other, I made with each of them the ordinary experiment of the vacuum in the same vessel, and when I brought the two tubes near each other without lifting them out of the vessel, it was found that the quicksilver which remained in each of them was at the same level, and that it stood in each of them above the quicksilver in the vessel twenty-six inches three lines and a half. I repeated this experiment twice in the same place, with the same tubes, with the same quicksilver and in the same vessel; and found always that the quicksilver in the tubes was still at the same level and the same height as I found it the first time.

When this had been done, I left one of the two tubes in the vessel, for continual observation: I marked on the glass the height of the quicksilver, and leaving the tube in its place, I begged the Rev. Father Chastin, one of the inmates of the house, a man as pious as he is capable, who thinks very clearly in matters of this sort, to take the trouble to observe it from time to time during the day, so as to see if any change occurred. And with the other tube and a part of the same quicksilver, I ascended with all these gentlemen to the top of the Puy-de-Dôme, which is higher than the Minimes by about five hundred toises, where, when we made the same experiments in the same way as I had at the Minimes, it was found that there remained in the tube no more than twenty-three inches two lines of quicksilver, whereas at the Minimes there was found in the same tube a height of twenty-six inches, three lines and a half; and so there was between the heights of the quicksilver in these experiments a difference of three inches one line and a half: this result so filled us with admiration and astonishment, and so much surprised us, that for our own satisfaction we wished to repeat it. I therefore tried the same thing five times more, with great accuracy, at different places on the top of the mountain, once under cover in the little chapel which is there, once exposed, once in a shelter, once in the wind, once in good weather, and once during the rain and the mists which came over us sometimes, having taken care to get rid of the air in the tube every time; and in all these trials there was found the same height of the quicksilver, twenty-three inches two lines, which makes a difference of three inches one line and a half from the twenty-six inches three lines and a half which were found at the Minimes; this result fully satisfied us.

This account cleared up all my difficulties and I do not conceal the fact that I was greatly delighted with it; and since I noticed that the distance of twenty toises in height made a difference of two lines in the height of the quicksilver, and that six or seven toises made one of about half a line, a fact which it was easy to test in this city, I made the ordinary experiment of the vacuum at the top and at the bottom of the tower of Saint-Jacques de-la-Boucherie, which is from twenty-four to twenty-five toises high: I found a difference of more than two lines in the height of the quicksilver; and then I made the same experiment in a private house, with ninety-six steps in the stairs, where I found very plainly a difference of half a line; which agrees perfectly with the account of Périer.