(22.)

If such an apparatus be observed from time to time, it will be found that the column of mercury sustained in the tube will be subject to variation between certain limits, never falling below twenty-eight inches, and never rising above thirty-one inches. This variation of the mercurial column is produced by a corresponding variation in the weight of the atmosphere.

If the apparatus be transported to any height above its ordinary position, it will have a less quantity of atmosphere above it, and therefore the surface of the mercury in the cistern will be pressed by a less weight, and consequently the [Pg041] column of mercury will fall proportionally. In virtue of this effect, such an instrument has been rendered a means of measuring heights, such as the heights of mountains, the ascents of balloons, &c. &c.