An instrument, to which this name has been given by the inventor, Dr. M. Hall, for making the necessary corrections in pneumatic experiments to ascertain the mean bulk of the gases. It consists of a bulb of glass, of 4 1/2 cubic inches capacity, blown at the end of a long tube, whose capacity is 1 cubic inch. This tube is inserted into another tube of nearly equal length, which is supported on a sole; and the first tube is sustained at any required height within the second by the pressure of a spring. Five cubic inches of atmospheric air, at a medium pressure and temperature, are to be introduced into the bulb and tube, of the latter of which it will occupy one-half. The other half of this tube, and part of the tube in which it is inserted, are to be occupied by the fluid of the pneumatic trough, whether water or mercury. The point of the tube at which the air and fluid meet, is to be marked by the figure 5, to denote 5 cubic inches. The upper and lower halves of the tubes are each divided into five parts, representing tenths of a cubic inch.

The external tube has a scale of inches attached.