30 



THERMOMETER AND PYROMETER. 



bar rests or abuts against the bottom of 

 Fig. 21. 



the groove ; the other presses against 

 the short arm of a bended lever, Ihe 

 long arm of which, moving on a pivot, 

 becomes the index of the instrument. 

 The short arm of this lever is just one 

 twentieth ot the length of the long 

 arm, which in the original instrument 

 was equal to 1.8 inch; consequently 

 the space moved over by the long arm 

 will be twenty times as great as the mo-* 

 tion caused in the short arm by the ex- 

 pansion of the bar. 



A finely graduated arc of a circle, of 

 which the index is a radius, is fixed on 

 the porcelain ; and each degree of this 

 arc is subdivided into ten parts by a 

 vernier on the extremity of the index 

 itself, and thus the instrument is capable 

 of indicating an expansion of s^oth 

 part of the radius. All these parts are 

 of platina. 



With this instrument Guyton made 

 many experiments, the general result of 

 which proves, that Wedgwood has 

 greatly erred in assigning too high a 

 temperature for the degrees of his scale, 

 a result confirmed by the later experi- 

 ments of Daniell. Guyton ascribes 

 Wedgwood's error to his estimating the 

 fusing point of silver much too high. 

 It was by means of a pyrometric piece 

 of fine silver that Wedgwood connected 

 his scale with that of Fahrenheit ; and 

 an error with respect to that metal must 

 viciate all the results. According to 

 Morveau, the fusing point of silver 

 ought to have been at 22 W. instead of 

 28 : and each degree, instead of being 

 equivalent to 130 of F., ought to have 

 been no more than 62.5 ; while the 

 commencement of his scale should have 

 been at, 517 F., instead of at 1077.5. 



There is some reason, however, to 

 believe, that Morveau has stated a red 

 heat in day rather too low ; for thermo- 



meters of mercury and of oil can sustain 

 a temperature of 517 F. without any 

 luminousness even in the dark. 



Morveau appears to have taken great 

 pains to connect the scale of his pyro- 

 meter with the common thermometer ; 

 and he is probably nearer the truth 

 than Wedgwood.* 



His corrected table of Wedgwood's 

 temperatures is as follows : 



AVedg. 



Mercury boils 2 = 



Zinc melts 3 = 



Antimony melts 7 = 



Silver mrlts 22 = 



Copper melts 27 = 



Gold melts , . 32 = 



Iron welds 95 = 



Cast iron melts ISO = 



Porcelain melts ,. 155 = 



Manganese melts 160 



Malleable iron melts 175 = 



Nickel melts 175 = 



Platina melts ..175 = 



Fah. 

 642.75 

 705.26 

 95523 



1822.67 

 2 03. 18 

 2517.63 

 6508.88 

 8696.24 

 9633.68 

 1051712 

 1 1454.56 

 1145456 

 11454.56 



16. In 1803, Mr. James Crighton, of 

 Glasgow, published a new "metallic 

 thermometer," in which the unequal 

 expansion of zinc and iron is the moving 

 power. A bar is formed by uniting a 

 plateofzinc,/g-.22, c, d, 8 inches long, 1 



Fig. 22. 



inch broad, and | inch thick, to a plate of 

 iron a, b of the same length. The 

 lower extremity of the compound bar 

 is firmly attached to a mahogany board 

 at e, e ; a pin / fixed to its upper end 

 plays in the forked opening in the short 

 arm of the index g t g. When the tem- 

 perature is raised, the superior expan- 



* Annales de Cbimie, torn. Ixxiv. Ixxviii, xc. 



