=Special Uses of Radiation Pyrometers.=—For regular use at temperatures above 1000° C. or 1850° F. the radiation pyrometer will be found to be more useful than instruments of the thermo-electric or resistance type as the latter undergo deterioration owing to the continuous action of the furnace gases, which becomes more marked as the temperature increases. Examples of industrial processes in which 1000° C. is considerably exceeded are the manufacture of glass, pottery, and cement, the treatment of special steels, and the casting of metals and alloys. Even for temperatures between 750° and 1000° C. a radiation pyrometer may be used, but is not so convenient for this range as a thermo-electric instrument. There is no upper limit to the instrument, which may be calibrated by the fourth-power law to the highest temperature attainable, that of the electric arc, which has been found to be 3720° C. by the use of a Féry radiation pyrometer. Measurements may therefore be made beyond the limits of thermal junctions, such as the temperature of electric furnaces and of thermit in the mould, and of molten steel before pouring, thus opening out the possibility of accurate control at extremely high temperatures. There is always a danger, however, of the cold junction becoming unduly heated when near to large masses at very high temperatures, and serious errors may arise from this cause. Two examples may be cited to illustrate the usefulness of the radiation pyrometer in practice: (1) the hardening of steel projectiles; and (2) the determination of the temperature of the clinkering zone in a rotary cement kiln. In (1) the projectile is brought to a given spot near the brink of the furnace, where it is in the focus of a radiation pyrometer, and when at the specified temperature is raked out of the furnace and drops into an oil-trough. It has been found that a difference of 10° C. from the standard temperature at which the projectiles should be quenched may cause a serious lowering of the penetrative power of the finished projectile; and hence a radiation pyrometer, which may readily be sighted on each individual shell, is the best to use for this purpose. In (2) the hottest spot may be found by focusing the pyrometer to different distances up the kiln, and, by taking a record, any fall in temperature due to defect of coal or air supplies, or to excessive feed of raw material, may be detected, thus furnishing information from which the process may be regulated to the best advantage. At the temperatures prevailing in such kilns—1300° to 1450° C., or 2370° to 2640° F., according to the nature of the kiln—a Féry radiation pyrometer is quite sensitive to changes of 10° C. or 18° F., and the author has found it to be entirely satisfactory in this connection. The adaptability of radiation pyrometers to all temperatures above a red heat, combined with the absence of deterioration, renders these instruments of great value, and the possibility of obtaining records is