1 94- Mr. Daniell on a Ne'vc Register-Pyrometer^ 



bar pressed, being of such very slender dimensions, would 

 obviously be liable to bend, and thus frustrate the experiment: 

 in addition to which, I can speak from my own experience, 

 the platinum spring plate and the centre pin would be liable 

 to a change of texture, which would impede the motion of the 

 lever, and it would finally become welded to the index ; for a 

 very moderate pressure at a high temperature would produce 

 this effect. 



The conclusion, indeed, of these Essays seems to admit that 

 the author did not expect that the platinum pyrometer could, 

 ever come into general use: "enfin, ces corrections ne peuvent 

 manquer d'ajouter a Tutllite du pyrometre d'argile, soit dans 

 les travaux chimiques, soit dans les aits; quand meme le 

 pyrometre de platine, plus exact mais moins usuel, serait re- 

 serve pour en assurer la marche, et pour servir a des recher- 

 ches plus importantes." 



M. Guyton, however, although he abundantly proves the 

 incorrectness of Mr. Wedgwood's estimate of the higher 

 degrees of temperature, is very far indeed from establishing 

 the point at which he so earnestly laboured, namely, the re- 

 gularity of the contraction of the clay pieces; or from substi- 

 tuting a more correct value of the degrees throughout the 

 whole range of the gauge than the one which he so completely 

 overturned. His comparative experiments with the platinum 

 pyrometer, at the boiling points of mercury and linseed oil, 

 and the melting point of antimony, led him to reduce the 

 equivalent of each degree from 130° Fahr. to 62°'5. The 

 zero point of the clay pyrometer was thus cai'ried back to 517° 

 instead of 1077°; but it seems to have escaped his notice that 

 this zero point was declared to be a red heat visible in the day- 

 light, — a description which cannot be mistaken, and which 

 clearly could not be below the temperature of boiling oil, 

 melting lead, or boiling mercur}' ; all of which are, however, 

 placed above it in M. Guyton's table. M. Guyton also places 

 the melting point of silver at the 22nd degree of Mr. "Wedg- 

 wood's scale instead of the 28th, which was, according to his 

 own determination, a correction first suggested by Sir James 

 Hall in the 9th volume of Nicholson's Journal. Taking the 

 value of each degree at 62°-5 Fahr., it fixes this point at 1892° 

 Fahr., which agrees very nearly with my own experiment in 

 the paper before alluded to ; but continuing the calculation 

 up to the melting point of iron, upon the supposition of an 

 uniform pi'ogression, the 130th degree corresponds with 8696° 

 Fahr., which, although only about half the amount 17977° 

 assigned by Mr. Wedgwood, is very far removed from the 

 result of my calculation 3479°. 



Never- 



