an instrument invented in France for measuring the volume of a body, however irregular, without plunging it in any liquid. If the volume of air contained in a vessel be measured, when the vessel contains air only, and also when it contains a body whose volume is required to be known, the volume of air ascertained by the first measurement, deducting the volume ascertained by the second, will be the volume of the body itself. Again, if the volume of any mass of air be inversely as the pressure to which it is subjected, the temperature being supposed constant, it will be easy to deduce, from the mathematical relations of quantity, the whole bulk if the difference between the two bulks under two known pressures be obtained by experiment.

Suppose that the first pressure is double the second, or the second volume of air double the first, and the difference equal to 50 cubic inches; the first volume of air will likewise be 50 cubic inches. The design of the stereometer is to ascertain this difference at two known pressures.

The instrument is a kind of funnel AB (fig. 1.) composed of a capsule A, in which the body is placed, and the tube B as uniform in the bore as can be procured. The upper edge of the capsule is ground with emery, that it may be hermetically closed with a glass cover M slightly greased. A double scale is fastened on the tube, having two sets of graduations; one to denote the length, and the other the capacities, as determined by experiment.

When this instrument is used, it must be plunged into a vessel of mercury, with the tube very upright, till the mercury rise within and without to a point C of the scale. See fig. 2.

The capsule is then closed with the cover, which being greased will prevent its communication between the external air and that contained within the capsule and tube.

In this situation of the instrument, the internal air is compressed by the weight of the atmosphere, expressed by the length of the mercury in the tube of the common barometer.

The instrument is then elevated, still keeping the tube in the vertical position. It is thus represented, fig. 2, second position. The mercury descends in the tube, but not to the level of the external surface, and a column of mercury DE remains suspended in the tube, the height of which is known by the scale. The interior air is less comprosessed.