=Special Uses of Optical Pyrometers.=—The advantageous use of optical pyrometers is restricted to observations at temperatures beyond the scope of instruments which have the working part in the furnace; or to cases in which occasional readings of temperature suffice. To follow a changing temperature continuous adjustment is necessary, involving labour, and therefore costly. Amongst workshop uses may be mentioned: (1) ascertaining the temperature of pottery kilns and glass and steel furnaces; (2) in the treatment of steels at very high temperatures, to which end the pyrometer may be set to a given reading, and the process carried out when the steel is observed to attain such assigned temperature; (3) to take casual readings when a number of furnaces are in use, or when a number of sighting-holes are provided, as in large brickmaking furnaces; and (4) for occasional observations of the firing end of rotary cement kilns. As an instrument of research in the laboratory, a good form of optical pyrometer is very useful, as, for example, in investigating the working temperatures of electric lamps, and taking observations in electric furnaces. It is a great drawback that records cannot be taken by optical pyrometers, as much valuable information can be gathered from an accurate knowledge of temperature fluctuations in most operations. This disadvantage must always militate against the general use of these instruments. CHAPTER VII CALORIMETRIC PYROMETERS =General Principles.=—If a piece of hot metal, of known weight and specific heat, be dropped into a known weight of water at a temperature _t_{1}_, which rises to _t_{2}_ in consequence, the temperature of the hot metal, _t_{0}_, can be obtained by calculation, as shown by the following example:— _Example._—A piece of metal weighing 100 grams, and of specific heat 0·1, is heated in a furnace and dropped into 475 grams of water, contained in a vessel which has a capacity for heat equal to 25 grams of water. The temperature of the water rises from 5° to 25° C. To find the temperature of the furnace. The heat lost by the metal is equal to that gained by the water and vessel. Equating these, 100 × 0·1 × (_t_{0}_ - 25) = (475 + 25) × (25 - 5) from which _t_{0}_ = 1025° C. The above calculation, which applies generally to this method, depends for its accuracy upon a correct knowledge of the specific heat of the metal used. This value is far from constant, increasing as the temperature rises, and the result will only be correct when the average value over a given range is known.