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Updated: May 11, 2025
Torricelli, by his experiments, demonstrated the fact and invented the mercurial barometer, long known as the "Torricellian Tube." This last instrument led to another discovery; that the weight of the atmosphere varied from time to time in the same locality, and that storms and weather changes were indicated by a rising and falling of the column of mercury in the tube of the siphon-barometer.
From these three uniformities we should be able to predict another uniformity, namely, the rise of the mercury in the Torricellian tube. This, in the stricter use of the phrase, is not a law of nature. It is the result of laws of nature. It is a case of each and every one of the three laws: and is the only occurrence by which they could all be fulfilled.
The ordinary mercury barometer is essentially a Torricellian tube graduated so that the varying heights of the mercury column can be used as a measure of the varying atmospheric pressure due to change of weather or due to alteration of altitude. If we take a mercury barometer up a hill we will observe that the mercury falls.
Finding, in the case of geometrical axioms, that general names have not any talismanic virtue for conjuring new truths out of the pit of darkness, and not seeing that this is equally true in every other case of generalization, he contended that axioms are in their nature barren of consequences, and that the really fruitful truths, the real first principles of geometry, are the definitions; that the definition, for example, of the circle is to the properties of the circle, what the laws of equilibrium and of the pressure of the atmosphere are to the rise of the mercury in the Torricellian tube.
I went to the Philosophic Club, where was examined the Torricellian experiment. I presented my Circle of Mechanical Trades, and had recommended to me the publishing of what I had written upon chalcography. January 30.
A monument to Pascal, erected by the citizens, occupies the centre of the square in Clermont. It will be remembered that Pascal verified the conclusions arrived at by Torricelli regarding the pressure of the atmosphere, by carrying a Torricellian tube to the summit of the Puy de Dôme, and recording how the mercury continually fell during the ascent, and rose as he descended.
Thus in the Torricellian experiment, if the fact that air has weight had been previously known, it would have been easy, without any numerical data, to deduce from the general law of equilibrium, that the mercury would stand in the tube at such a height that the column of mercury would exactly balance a column of the atmosphere of equal diameter; because, otherwise, equilibrium would not exist.
As early as 1845 the results of these experiments were taken advantage of when Starr, a talented American who died at the early age of twenty-five, suggested, in his English patent of that year, two forms of small incandescent electric lamps, one having a burner made from platinum foil placed under a glass cover without excluding the air; and the other composed of a thin plate or pencil of carbon enclosed in a Torricellian vacuum.
If the weight of the air was really the cause which sustained the height of the mercury in the Torricellian tube, he saw at once that this height would vary at different elevations, according to the varying degree of atmospheric pressure at these elevations.
He played with the incommunicable, the inconceivable, the absolute, the antinomies, as he would have played with a bundle of jack-straws. "Brahma," the poem which so mystified the readers of the "Atlantic Monthly," was one of his spiritual divertisements. To the average Western mind it is the nearest approach to a Torricellian vacuum of intelligibility that language can pump out of itself.
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