on some of the leading doctrines of caloric, &c. 373 
such as to allow their extremities, with the attached rod, to 
be plunged beneath the surface of oil or water , about an inch, 
contained in a 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 thermome- 
ters, mutually comparable, were immersed into it at regular 
distances. The micrometers were screened from the influence 
of the heat. They rendered the of an inch discernible, 
and even a smaller quantity, by an experienced eye. 
A rod of pure Swedish iron, or of such pure copper as 
jewellers use for alloying gold, being adjusted to the appara- 
tus, 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 0 . The value and truth of the micrometrical indications 
had been previously ascertained, by viewing through the 
microscopes a given surface or aperture, moved laterally, so 
as to make its image successively 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 io° or 20 0 of the Fahr. thermometer. But as the pyro- 
metrical details will appear in a separate memoir on the ex- 
pansions of bodies, I shall state here merely what concerns 
the present subject. 
