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The Cistern Barometer in its essential and simplest form. This barometer is subject to two kinds of error, the one arising from capillarity, and the other from changes in the level of the surface of the cistern as the mercury rises and falls in the tube, the latter being technically called the error of capacity. If a glass tube of small bore be plunged into a vessel containing mercury, it will be observed that the level of the mercury in the tube is not in the line of that of the mercury in the vessel, but somewhat below it, and that the surface is convex. The capillary despression is inversely proportional to the diameter of the tube. In standard barometers, the tube is about an inch in diameter, and the error due to capillarity is less than .001 of an inch. Since capillarity despresses the height of the column, cistern barometers require an addition to be made to the observed heght, in order to give the true pressure, the amount depending, of course, on the diameter of the tube.
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