Wedgwood, the famous potter, appears to have been the first to apply this method of determining the condition of a furnace, his test-pieces consisting of special clay compositions. The effect of the furnace on these was noted, and the suitability of the temperature for the work in hand deduced from the observations. Wedgwood in this manner investigated the variations in temperature at different levels in his firing-kilns, and was thus enabled to place the various wares at the positions best suited for their successful firing. Modern potters still use such test-pieces, as the information gained is not merely the degree of heat, but the effect of such heat on the articles undergoing firing. The fusion method, however, is now used to determine the temperature of all kinds of furnaces, and the chief modifications will now be described. [Illustration: FIG. 67.—SEGER PYRAMIDS OR “CONES.”] =Seger Pyramids or “Cones.”=—Seger, of Berlin, published in 1886 an investigation dealing with the production of silicates of progressive melting points. By varying the composition, he was able to produce a series of materials with melting points ranging from 1890° C. to 590° C., the interval between successive compositions being 20° between 1890° and 950°, and 30° from the latter temperature to 590°. The highest member of the series has the composition Al_{2}O_{3}, SiO_{2}; and the lowest member 2SiO_{2}, B_{2}O_{3}. For convenience in use the materials are made in the form of triangular pyramids, 5 cms. in height, and each side of the base 1·5 cms. long. Each pyramid is stamped with a distinguishing number, and altogether 60 are made to cover the range 1890° to 590°. When conducting a test, several pyramids are selected with melting points known to be near the temperature of the furnace, as discovered by previous trials. These are inserted in the furnace standing on a slab of refractory material, as in fig. 67, and may be watched through a sight-hole or withdrawn from the furnace for examination after attaining the existing temperature. If the right pyramids have been chosen, the appearance presented will be as in fig. 67, in which D is seen to have collapsed completely, C has bent over, B has been rounded at the top, whilst A is intact. The temperature of the furnace is then taken to correspond to the melting point of C, which is found by reference to a table in which the melting points corresponding to the different distinguishing numbers are given. The pyramids are extremely cheap, and only those with melting points near to the working temperature need be purchased. In cases where it is desired to increase the heat to a specified point, and then to allow the furnace to cool, these pyramids fulfil all requirements; an examination through a sight-hole closed with darkened glass enabling the furnace attendant to discover when the requisite temperature has been attained. The procedure is more difficult when it is desired to maintain a steady