Among the first in order of time--and still retaining no unworthy position in order of merit--is the "rheometer," or "flow-measurer," of M. Giroud. In this instrument a light metal bell is sealed in glycerine contained in a cylindrical case; the bottom of this latter containing the inlet-pipe, screwed for connecting to the ordinary fittings, while from the centre of its cover rises a tube leading to the burner. The bell is pierced by a small hole for the passage of the gas, and is surmounted by a cone-shaped projection, which constitutes the valve of the instrument. As the pressure of the entering gas lifts the bell, it causes this cone-valve to enter the mouth of the tube leading to the burner; reducing the area of the opening in proportion to the pressure of gas acting upon the under side of the bell, and so permitting only the required quantity of gas to pass to the burner. It might be thought that the presence of liquid would constitute an objection to the use of the instrument; but, as glycerine does not evaporate, when once the instrument is fixed and properly adjusted, it needs no further attention. With an excessive initial pressure, there is, however, a liability of the gas to bubble through the sealing liquid, and so destroy the efficiency of the instrument; but this might be obviated by increasing the depth of the bell, and so giving it a greater seal. The instrument is very reliable for the purpose which it is intended to fulfil; delivering, through a considerable range of pressure beyond that required to raise the bell, the exact quantity of gas for which it has been adjusted. It may be added that the rheometer has an advantage over many instruments of its class, in that it presents so little obstruction to the downward rays of the flame. [Illustration: FIG. 16.--GIROUD'S RHEOMETER.] [Sidenote: Sugg's Christiania governor-burner.]