=General Principles.=—It is a common experience that the heat radiated by a substance increases as its temperature rises; and it would obviously be an advantage if the temperature of a hot body could be deduced from the intensity of its radiations, as the measurement could then be made from a distance, without the necessity of placing a pyrometer in contact with the heated substance. At temperatures above 1000° C., when difficulties are experienced either with the metals or protecting sheaths of thermo-electric or resistance pyrometers, the advantage gained would become more conspicuous as the temperature increased. A brief survey of our knowledge of the relations between radiant energy and temperature will indicate how this desired end may be achieved. Any substance at a temperature above absolute zero (-273° C.) radiates energy to its surroundings by means of ether waves. Below 400° C. these waves produce no impression on the retina of the eye, and the radiating body is therefore invisible in a dark room. Above 400° C., however, a proportion of visible waves are emitted; and as the temperature rises the effect on the retina is enhanced, and the body increases in brightness. The difference between the non-luminous and luminous waves is merely one of wave-length, the shorter wave-lengths being visible to the eye; and both represent radiant energy. In addition to giving out radiant energy, a substance receives waves from its surroundings, which it absorbs in greater or less degree, and which when absorbed tend to raise the temperature of the receiving substance. A number of objects in a room, all at the same temperature, are therefore radiating energy to one another, and equality of temperature is established when each object receives from its surroundings an amount of energy equal to that which it radiates. A hot substance radiates more energy than a cold one; thus if a hot iron ball be hung in a room it will radiate more energy to its surroundings than it receives from them, and will therefore cool until the outgoing energy is balanced by the incoming, when its temperature will be equal to that of the other objects in the room.