Jrom an Increase of Temperature. 365 
most sensible to a change of temperature, I commenced the ex- 
periments for which the instrument was made, but, before giving 
the results of those on stone, I may mention what took place in 
trying the expansion of a rod of oak, cut from the same tree of 
which the pyrometer stand was made. On account of a very 
small quantity of steam escaping through a hole in the inner 
case, the wood expanded very much the first time it was heated, 
and when taken out, and allowed to stand some days to dry, I 
found it lengthened about the thirtieth part of an inch. The 
inner case was made perfectly air-tight, even under considerable 
pressure, as it appeared absolutely necessary to prevent steam 
from escaping into it, for all experiments to ascertain the effect of 
heat on different kinds of wood; and I also wished to keep the 
rods of stone quite dry, to be certain what effect moisture had on 
their expansibility. When the rod of oak was again thoroughly 
dried, and the length of 23 inches laid off anew on the studs, 
it was found that for an increase of temperature of 180° Fahr. 
it only expanded .001426177 of an inch, or .000062007 in de- 
cimals of the length of the rod; which is just one-fifteenth 
part of the amount of the expansion of platinum, the least ex- 
pansible of the metals. This insensibility to change of tem- 
perature in this wood, provided it be kept free from moisture, 
has induced me to investigate the subject a little farther I 
have procured rods of many of the most straight-grained kinds 
of wood, and after I have determined their rates of expansion, I 
intend to varnish or cover them with different substances, to 
find their expansion again, and then to see to what extent, and 
for how long, they will resist moisture, as it has been said that 
a wooden rod answers best for a pendulum when unvarnished, 
and left in its natural state. 
The following Table gives the expansions of the different 
kinds of stone I have examined, and also of two rods of east- 
iron. The one which expanded least, was cast half an inch 
square, and was put into the pyrometer with the outer surface 
342 
