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AB (Plate III, Fig. 27) is a glass tubė, about 33 or 34 inches long, closed at top; that is, in philosophical language, hermetically sealed; D is a cup, bason, or wooden trough, partly filled with quicksilver. I fill the tube with the quicksilver, and then put my finger upon the mouth, so as to prevent any of it from running out; I now invert the tube, and plunge it in the cup D. You see the mercury subsides three or four inches; and when the tube is fixed to a graduated frame, it is called a barometer, or weather-glass, and you know it is consulted by those who study and attend to the changes of the weather.
Emma. Why does not all the quicksilver run out of the tube? Father. I will answer you, by asking another question: What is the
reason that water will stand in an exhausted tube, provided the mouth of it be plunged into a vessel of the same fluid?
Charles. In that case the water is kept in the tube by the pressure of the atmosphere on the surface of the water into which it is plunged. If
you resort to the same principle, in the present instance, why does the water stand 33 or 34 feet, but the mercury only 29 or 30 inches?
Father. Do you not recollect that mercury is 14 times heavier than water? therefore, if the pressure of the atmosphere will balance 34 feet of water, it ought, on the same principle, to balance only a 14th part of that height of mercury:
now divide 34 feet, or 408 inches, by 14.
Emma. The quotient is little more than 29 inches.
Father. By this method Torricelli was led to construct the barometer. It had been accidentally discovered that water could not be raised more than about 34 feet in the pump. Torricelli, on this, suspected that the pressure of the atmosphere was the cause of the ascent of water in the vacuum made in pumps, and that a column of water 34 feet high was an exact counterpoise to a column of air which extended to the top of the atmosphere. Experiments soon confirmed the truth of his conjectures. He then thought, that, if 34 feet of water were a counterpoise to the pressure of the atmosphere, a column of mercury, as much shorter than 34 feet as mercury is heavier than water,
would likewise sustain the pressure of the atmosphere: he obtained a glass tube for the purpose, and found his reasoning just.
Charles. Did he apply it to the purpose of a weather-glass?
Father. No: it was not till some time after this that the pressure of the air was known to vary at different times in the same place. As soon as that was discovered, the application of the Torricellian tube to predicting the changes of the weather immediately succeeded.
Charles. A barometer, then, is an instrument used for measuring the weight or pressure of the atmosphere.
Father. That is the principal use of the barometer: if the air be dense,
the mercury rises in the tube, and indicates fair weather: if it grows light, the mercury falls, and presages rain, snow, &c.*
The height of the mercury in the tube is called the standard altitude, which in this country fluctuates between 28 and 31 inches, and the difference between the greatest and least altitudes is called the scale of variation.
Emma. Is the fluctuation of the mercury different in other parts of the world?
Father. Within and near the tropics, there is little or no variation in the height of the mercury in the barometer in all weathers: this is the case at St. Helena. At Jamaica the variation very rarely exceeds three* See the rules at the end of the volume.