=Siemens’ Calorimetric or “Water” Pyrometer.=—Fig. 66 shows this instrument in longitudinal and transverse section. It consists of a double copper vessel, the inner containing water, and the outer provided with a handle. The space between is lagged with felt, to prevent escape of heat from the water. The thermometer, _b_, is protected by a perforated brass tube from damage that might be caused on dropping in the hollow nickel cylinder, _d_. Opposite the stem of the thermometer is placed a sliding-piece _c_, on which a temperature scale is marked. In using the instrument, the specified quantity of water is placed in the inner vessel, and the pointer on _c_ brought opposite to the top of the mercury column in the thermometer. The nickel cylinder, which has been heated in a crucible or muffle in the furnace, is then dropped in, and the vessel shaken to secure an equal temperature throughout the water. When the thermometer is stationary, the mark on _c_ opposite the top of the mercury gives the temperature of the furnace, the scale on _c_ having previously been marked from calculations made for each 50 degrees. The correctness of the reading evidently depends upon the accuracy with which _c_ has been calibrated, an operation which involves taking into account the water equivalent of the vessel and the variation of the specific heat of nickel at different temperatures. Allowing for the sources of error attaching to the method, results by this pyrometer cannot be guaranteed to better than 2 or 3 per cent, at 900° or 1000° C., but in cases where this degree of inaccuracy is not of importance, the instrument may be used with advantage. As no calculation is necessary, the determination may be made in the workshop by any workman who exercises care in conducting the operation. Copper and iron cylinders are sometimes supplied instead of nickel, but are not to be recommended, as they decrease in weight with each test, and necessitate the use of a multiplying factor to convert the reading on _c_ into the true temperature.