Jrom an Increase of Temperature. 363 
rod to become drier; but the whole of the inner case is so tight 
as to prevent any evaporation, unless the upper pipe V, and the 
under one of the same size, V, Fig. 1, which passes through the 
bottom plate, are left open. The springs and roller represented 
in this figure were firmly tied on the rods, and could easily be 
shifted from the one to the other It was my intention to have 
ascertained the expansion of the different rods between the freez- 
ing and the boiling points, by first filling the space SS, Fig. 4, 
with melting ice, and then by passing a current of steam to get 
the higher point, but the difficulty of getting ice until a few 
weeks since made me abandon this; besides, I found that, to 
keep the thermometers above 208° F., sent a great heat through 
the room, on account of the strong fire it was necessary to keep 
up. As the thermometers were made with every precaution to 
insure their accuracy; I contented myself with the range I could 
get between the temperature of the room, which was generally 
under 50°, and 207° or 208°. 
The rod was put into the pyrometer, the micrometers set, and 
allowed to stand over the night, and the lowest point taken at the 
height at which the thermometers stood next day, by making the 
line on the under stud intersect the angle of the spider threads 
of the lower micrometer; and the index error of the line on the 
upper stud was read from the zero of the upper micrometer: This 
was repeated four times, by deranging and again adjusting the 
height of the under line by means of the screw G. 
After completing the readings, the steam was turned into 
the case, and the expansion of the rod measured by a similar 
set of readings, as soon as the line on the upper stud was found 
to be stationary, which sometimes required the temperature of 
the pyrometer to be kept at the same degree for an hour and 
a half. 
I thought that my application of steam for heating a pyro- 
meter was the first time it had been done, as I could find no ac= 
count of its having been formerly used in any of the descriptions 
VOL. XIII. PART II. 3A 
