from 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 cast- 

 iron. The one which expanded least, was cast half an inch 

 square, and was put into the pyrometer with the outer surface 



3a2 



