268 Mr. Daniell on a Nev; Register-Pyrometer, 



Hemitrope crystals occasionally present themselves, the 

 plane of revolution being parallel to the plane h, which trun- 

 cates the edge of the prism. The angle at which the two 

 planes P then intersect each other, is 1 54° 30'. 



XXXV. On a ne'd> Register-Pyrometer, Jor Measuring the 

 Expansio7is of Solids, and determining the higher Degrees 

 of Temperature upon the common Thermometric Scale. By 

 J. Frederic Daniell, Esq. F.R.S. 



[Continued from p. 199.] 



T SHALL now proceed to show the degree of confidence 

 -*■ which may be placed in this new pyrometer, by comparing 

 the result of its indications with those of the best experiments 

 upon tlie expansion of metals. Those of MM. Dulong and Pe- 

 tit* are well adapted to this purpose. These able philosophers, 

 in their celebrated prize Memoir on the Measure of Tempe- 

 ratures, and on the Laws of the Communication of Heat, have 

 given, from experiment, the expansion of rods of platinum 

 and iron at different intervals between the freezing point of 

 water and the boiling of mercury. Their mode of experi- 

 menting was unexceptionable ; but it is to be regretted that 

 they have not corrected their final results for an error of cal- 

 culation which has been pointed out by Mr. Crichtonf which 

 is by no means unimportant to the reasoning which they have 

 founded upon them. The error, however, affecting the amount 

 of expansion in volume, is reduced to one-third in the linear 

 expansion, which is the subjectof the present investigation, and 

 may therefore be disregarded. 



The following Table of the expansion of iron and platinum 

 is extracted from their work. 



Table IL 



Whence we deduce the linear expansion of platinum for 

 180° Fahrenheit, from 32° to 212° -00088420: and for 180°, 



• Ann. de CIdinie et Physique, vii. IIJJ. 



t Annals of Philosophy, New Series, vol. vii. p. 241. 



from 



