EXPANSION OF WOOD by HEAT. 29 



No. V. 



0/1 the Expafijion of Wood by Heat, in a Letter from 

 David Rittenhouse L. L. D. Prefident of the 

 Society. 



May 15 th, 1795. 



IN the prefent ftate of experimental philofophy it is 

 well known that bodies in general enlarge their dimen- 

 lions, or expand, on being heated, and coatrait in cooling. 

 From fome experiments heretofore m ide, wood has been 

 thought to make an exception to the general rule, and 

 this opinion has lo far prevailed that many curious per- 

 fons have applied wooden pendulum rods to their time- 

 pieces, to prevent the variation in their rate of going, 

 ariling from the expanfion and contracting of a metal 

 rod. From my own obfervations, however, as well as 

 thofe of fome of my friends, the wooden pendulum rod 

 does not appear to anfwer the expe£lations formed on it. 

 I had in my poffefiion for feveral years an excellent time- 

 piece made for this fociety by an ingenious v/orkman 

 and worthy member of the fociety. The refult of my 

 conftant attention to this clock was, that though its re- 

 gular variations with heat and cold were probably much 

 lefs than thofe of metal pendulums, it neverthelefs always 

 went fafter in winter than in fummer, and was liable to 

 very fudden and confiderable variations ; arifrng, no 

 doubt, from the combined efteds of heat and cold, moif- 

 turc and drynefs. This determined me to make fome 

 careful experiments with a pyrometer capable of receiving 

 a piece of wood of the length of a fecond pendulum. 

 Several years ago I made fome experinients of this kind, 

 perfedly correfponding with thofe I have lately made, 

 and which 1 now communicate to the fociety. 



E I took 



