THERMOMETER AND PYROMETER. 



groove is divided into inches and tenths, 

 making 240 degrees in the whole scale; 

 and the higher the temperature to which 

 the pyrometric piece has been exposed, 

 the further will it slide up the scale. 



In order to compare his scale with 

 Fahrenheit's mercurial thermometer, 

 which cannot measure a temperature 

 much beyond 600, Mr. Wedgwood was 

 compelled to make use of the expansions 

 of a pyrometric piece of fine silver, ap- 

 plied to a gage on the same principle as 

 that above described. By this, the ex- 

 pansions of the silver for 50 and 212 

 Fahrenheit were first noted ; and then 

 the silver and clay pyrometric pieces 

 were compared at the same temperature. 

 By such means Wedgwood estimated 

 the value of each degree of his scale at 

 130 of Fahrenheit ; and he reckoned 

 that the of his scale corresponded 

 with the 1077.5 of the common scale. 

 On this principle comparative tables of 

 the two thermometers have been con- 

 structed ; but their accuracy depends on 

 two circumstances which have not been 

 determined to the satisfaction of the 

 philosophic world. Clay being a hete- 

 rogeneous mixture, it by no means fol- 

 lows that its contractions are equable at 

 different temperatures ; and even were 

 this ascertained, there is great doubt 

 how far the means employed by Wedg- 

 wood did accurately estimate the degree 

 of Fahrenheit at which his scale com- 

 mences. 



There is still another serious objection 

 to the general use of such an instrument. 

 It occurred to the ingenious inventor, 

 that different portions of clay would pos- 

 sess different degrees of contractibility ; 

 and he endeavoured to secure unifor- 

 mity, to a certain extent, by laying in a 

 large stock of Cornish clay, which he 

 hoped would supply innumerable pyro- 

 metric pieces of the same quality. It 

 was found, however, that spontaneous 

 changes take place in such clay, which 

 render its indications liable to variation 

 at distant intervals : or pieces, now 

 formed of the same clay, will not give 

 the same indication with pieces baked 

 several years ago. Attempts were 

 made to remedy this inconvenience by 

 forming a clay of uniform quality of 

 fixed proportions of silex and alumine. 

 Fine Cornish clay yielded, on analysis, 

 two parts of silex and three of alumine ; 

 and such a mixture made into a paste 

 with fths their weight of water, has been 

 recommended for the fabrication of 

 pyrometric pieces. The method detailed 



by Wedgwood should then be followed 

 in moulding them. The paste is first 

 to be rammed into a metallic mould 

 0.6 inch wide, 0.4 deep, and 1 inch 

 long : they should be dried in the air, 

 and when quite desiccated, Wedgwood 

 gaged them in another mould exactly 

 0.5 of an inch wide, and of the form 

 given in the figure. Before they are 

 baked they will, of course, just enter the 

 widest end of the scale, resting at 0. 

 When contracted by baking to th of their 

 bulk, they will pass to the 120; and 

 when reduced to fths, they would pass to 

 the 24 (j, or the extremity of the scale ; 

 but Mr. Wedgwood never did obtain a 

 higher temperature than 160. From 

 these proportions each degree of Wedg- 

 wood's scale is equivalent to , contrac- 

 tion of' ooth part of the pyrometric 

 piece. 



The difficulty of obtaining clay of an 

 uniform quality, and not liable to spon- 

 taneous change, has lately given rise to 

 a suggestion of employing pyrometric 

 pieces" formed of Chinese agalmatolite ; 

 a suggestion of Mr. Sivright of Meg- 

 getland, well worthy of attention.* 



A more formidable objection was 

 started by some foreign chemists to 

 Wedgwood's scale ; one, indeed, that 

 would have overturned the theory of the 

 instrument. It was alleged, that the 

 effect of a long continued, or often re- 

 peated, exposure to even inferior degrees 

 of heat, would cause contraction of the 

 clay, after it had undergone the action 

 of a higher temperature. This point 

 has been examined with much care by 

 Guyton de Morveau, who has shown, 

 in his valuable essay ,} the inaccuracy of 

 this opinion ; although he contends that 

 Wedgwood has greatly erred in the at- 

 tempts to convert his scale into degrees 

 of Fahrenheit's thermometer, as we shall 

 immediately notice. 



On the whole, the pyrometer of 

 Wedgwood is an instrument well adapt- 

 ed to the purposes of the potter, or to 

 convey some idea of the relative heat of 

 furnaces ; but we cannot regard the de- 

 termination of the celebrated inventor 

 as giving even a tolerable approximation 

 to relative degrees of high temperatures 

 by other scales. As, however, Mr. 

 Wedgwood's tables of temperature are 

 often quoted, we shall here subjoin 

 them, with the corresponding degrees 

 of Fahrenheit, according to his calcu- 

 lation. 



hclinb. Hiil. Jouinal, vol. vi. p. 179. 



| Annales de Chiinie, vol Ixxiv, Ixxviii. xc. 



