‘266 
MR. DANIELL ON A NEW REGISTER-PYROMETER 
The chord of ten degrees derived from this Table by multiplying .00872 by 
10 M ould therefore be .0872, whereas it is more accurately .0871; but the dif- 
ference being only j^dth of an inch may, in most cases, be disregarded. 
I 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 the expansion of metals. Those of MM. Dulong 
and Petit* are well adapted to this purpose. These able philosophers, in their 
celebrated prize Memoir on the Measure of Temperatures, 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 experimenting was unexception- 
able ; but it is to be regretted that they have not corrected their final results 
for an error of calculation which has been pointed out by Mr. Crichton 
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 subject of the pre- 
sent investigation, and may therefore be disregarded. 
The following Table of the expansion of iron and platinum is extracted from 
their work. 
Table II. 
Temperature deduced from 
the dilatation of Air. 
Mean absolute dilatation 
of Iron for 180 degrees. 
Mean absolute dilatation 
of Platinum for 180 degrees. 
From 32° to 212° 
1 
1 
28200 
37700 
From 360° to 572° 
1 
22700 
1 
36300 
Whence we deduce the linear expansion of platinum for 180° Fahrenheit, 
from 32° to 212° .00088420: and for 180°, from 360° to 572° .00091827 : and 
of iron, from 32° to 212° .001 18203: from 360° to 572° .00146842, showing an 
increasing dilatation in each when referred to an air-thermometer. 
The bars of the different metals used in the following experiments were all 
exactly 6.5 inches in length. 
Exp. 1 . A square bar of platinum j^ths of an inch thick, was carefully arranged 
in the black-lead register, which was placed in the apparatus represented, upon 
• Ann. dc Chimie et Physique, vii. 113. t Annals of Philosophy, New Series, vol. vii. p. 241. 
