=Pneumatic Pyrometers.=—Attempts have been made to deduce furnace temperatures by blowing air at uniform pressure through a pipe located in the hot space, and noticing the increase in the temperature of the air. In the Uehling pyrometer, air from the hot space is drawn through an opening of fixed size by means of a steam-jet, which acts as an aspirator. The opening is placed at one end of a chamber, and the steam-jet aspirator at the other end; and a diaphragm with a central hole divides the chamber into two parts. The pressures existing in the two portions of the chamber vary according to the temperature of the air drawn in, and are measured by water-gauges, the readings of which may be translated into temperatures by calibration against a thermo-electric or other pyrometer. The method is ingenious, but is elaborate and costly; and is therefore little used. =Conduction Pyrometers.=—If one end of a rod of metal be inserted in a furnace, heat will be conducted along it to the portion external to the furnace, and a steady condition will be obtained when the heat escaping from the external part of the rod, by convection and radiation, is equal to the quantity conducted along the rod. The hotter the portion in the furnace, the higher will be the temperature of all parts of the external length. A series of thermometers placed at intervals in the exterior portion would show a progressive fall in temperature along the rod; and the hotter the furnace the higher would be the reading on each thermometer. In applying this principle to the measurement of high temperatures, a bar of copper or iron is passed through the wall of the furnace, so that a length of 2 feet or more protrudes on the outside. Near the end of the external portion a hole is drilled to a sufficient depth to cover the bulb of a thermometer, which is inserted in the hole, into which a quantity of mercury is poured to make a metallic contact between the bulb and the bar. The reading of the thermometer furnishes an approximate clue to the temperature of the furnace, rising or falling with corresponding changes in the hot space. A calibration might be effected by comparison with a standard; but the method is only applied to the production of a prescribed condition, known by experience to be attained when the thermometer reading has a certain value—say 120° C. Changes in atmospheric temperature, or currents of air, seriously affect the readings, and the method at best is only approximate.