Another instrument of this class--the last which I shall notice--is Peebles's needle governor-burner. For simplicity combined with remarkable efficiency, it is undoubtedly ahead of all its compeers. Somewhat similar in principle to Giroud's rheometer, it differs from that instrument in many of the details of its construction; and while dispensing with the use of liquid, maintains equal efficiency in operation. It was described as follows by Dr. W. Wallace, in a lecture on "Gas Illumination," delivered before the Society of Arts in January, 1879:[13]--"In a little cylinder stands a so-called needle, on the point of which rests a flanged cone of exceedingly thin metal. At one side of the cylinder there is a small tube leading away the gas, and the orifice of which is influenced in area by the action of the cone. The instrument, by means of a screw leading into the side tube, can be made to deliver any desired number of cubic feet, which it does with surprising accuracy, provided that the pressure of the gas is not less than 6-10ths of an inch." As to the efficiency of the instrument, Dr. Wallace proceeded to state:--"In trials that I have made, I have not found the variations of volume at different pressures to exceed 1 per cent." For situations where this extreme nicety of operation is not absolutely essential, or where the rate of consumption is to be invariable, the instrument is constructed in a somewhat modified and simpler form. The small tube on the side of the instrument is dispensed with, and the gas permitted to pass through perforations in the lower part of the cone. With this alteration there is a nearer approach to the construction of the rheometer; but, as in that instrument, there is no provision for altering the rate of consumption to suit different circumstances. [13] See _Journal of Gas Lighting_, Vol. XXXIII., p. 162. [Illustration: FIG. 18.--PEEBLES'S NEEDLE GOVERNOR.] CHAPTER V. REGENERATIVE BURNERS. [Sidenote: Temperature of a gas flame.]