314 



Daniell on a New Pyrometer. 



The exact point of 600° it was impossible to compare with 

 any safety to a close thermometer. The point of 580° agreed 

 very well in the ascent, but too rapid cooling prevented me 

 from catching it on the return. 



When the difficulty of comparing the fine divisions of two 

 instruments so situated, and both in progressive motion, is con- 

 sidered, this will no doubt be regarded as a very close agree- 

 ment, and quite sufficient to establish the fact of the equability 

 of expansion of both the tube and the metal bar. The experi- 

 menthasbeen often repeated with a perfect accordance of result. 

 On one occasion, the lamps employed to heat the mercurial 

 bath had been placed accidentally nearer to one extremity than 

 the other: not having attended to this circumstance, I was 

 surprised not to find the usual accordance between the pyro- 

 meter and the thermometer ; but, upon considering the matter, 

 I placed a thermometer at each end of the bath, and found that 

 there was a difference of 30° between the two, and the pyro- 

 meter indicated as nearly as possible the mean. I was not pre- 

 viously acquainted with the possibility of such a difference 



