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Miscellaneous Devices. Amongst these may be mentioned: (i) Fusion pyrometers, which consist of pieces of materials of progres- sive melting-points, which are placed in the furnace, the temperature of which is represented by the melting-point of the highest in the series that undergoes fusion (Seger cones, Sentinel and Watkin's pyrometers) ; (2) Calorimetric or Water pyrometers, in which a piece of hot metal taken from the furnace is dropped into a known quantity of water, and the temperature deduced from the extent to which the water is heated ; (3) Expansion pyrometers, based on the linear expansion of solids; and (4) the Clay-Contraction pyrometer of

Wedgwood. All these methods are at best approximate, and are not employed to the same extent as formerly, when accurate instruments were not available.

REFERENCES. Measurement of High Temperatures, Burgess and Le Chatelier (contains bibliography). Transactions of the Faraday Society, vol. xiii., Part 3; discussion on Pyrometers and Pyrom- etry, with bibliography by Sir Robert Hadfield. Pyrometry, Darling (deals with industrial uses). Also accounts of research in pyrometry in the publications of the National Physical Laboratory, the U.S. Bureau of Standards, and the Reichsanstalt (C. R. D.)