184 New experimental Researches 
alout an inch, contained ina copper trough. This was placed 
parallel to the large trough, and a few inches distant from it. 
The copper vessel was slowly and equably heated, by a series 
of Argand lamps placed beneath. One micrometer watched a 
point projecting from the arm that held the fixed extremity of 
the rod. The oil was carefully agitated during the application 
of the heat ; and the bulbs of three thermometers, mutually com- 
parable, were immersed into it at regular distances. The mi- 
crometers were screened from the influence of the heat. They 
rendered the +54, of an inch discernible, aud even a smaller 
quantity, by an experienced eye. 
A rod of pure Swedish iron, or of such pure copper as jewel- 
lers use for alloying gold, being adjusted to the apparatus, the 
point on the micrometer scale, that appeared a tangent to the 
small luminous aperture in the thin index plate of steel, was 
noted down, when the liquid in the trough was at 32°. The 
value and truth of the micrometrical indications had been pre- 
viously ascertained, by viewing through the microscopes a given 
surface or aperture, moved laterally, so as to make its image suc- 
cessively coincide with the different points of the interior notched 
scale. 
Heat being now applied, the progressive march of the index 
across the field of view of the micrometer microscope was closely 
observed, and its position written down at intervals of 10° or 20° 
of the Fahr. thermometer. But as the pyrometrical details will 
appear in a separate memoir on the expansions of bodies, I shall 
state here merely what concerns the present subject. 
If we denominate the absolute elongation of the heated me- 
tallic rod from 32° to 122°-10, then its elongation from 122° to 
212°; from 212° to 302°; from 302° to 392°; from 392° to 
4§2°, was in each successive interval of 90° F. as nearly as pos- 
sible 10 also. The slight irregularities incident to all delicate 
experimental investigations being often in opposite directions, 
in different repetitions of the same experiment, or those which 
manifested themselves in the ascending or elongating range, were 
neutralized, so to speak, by others of an inverse nature, which 
appeared in the cooling retrocession. Here, the movements of 
the liquid mercury and of the solid rod by heat proceeded, pari 
passu, through a very great extent of temperature. Let us now 
recollect that these five increments,which on our thermometer are 
equivalent to 5 x 90° =450°, and which altogether produce five 
times the elongation that the first interval occasions, constitute, 
on Mr. Dalton’s scale, only 350°. If we call the first interval 
given by this philosopher 1-00, then the four succeeding intervals 
contain a range of temperature, on his scheme, of only two and 
a half times the first; and therefore only two and a half times 
additional 
