344 Q' n M* Expanjion of Wood by Heat. 



moifture 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 experiments of this kind, perfectly cor* 

 refponding with thofe I have lately made, and which I now 

 communicate to the fociety. 



I took a ftraight-grained piece of white hiccory, green, for 

 I could not procure any feafoned, its length 39 inches, and 

 about 3-8ths of an inch fquare. This I placed in my pyro- 

 meter, and kept it fully extended by a weight fattened to a 

 firing, going over a pulley. To the pyrometer I applied the 

 tube and glafTes of a good compounded microfcope, and a 

 micrometer, the value of the fmaller divifions whereof I found 

 to be nearly .QO053 parts of an inch each. 



The rod of wood being placed in the pyrometer, I poured 

 fand all around it, heated to about 250 of Fahrenheit, which 

 degree of heat I found the wood would bear without fcorching. 

 On pouring in the hot fand, the rod expanded very much, 

 but foon began to contract, even before the fand was fenfibly 

 cooled, which I fuppofe arofe from the hot fand extracting 

 the moifture of the wood. It continued to contract as the 

 whole grew cool, fo that when the rod had acquired its firft 

 temperature it was near 30 of the above divifions fhorter than 

 at firft. I repeated the operation a fecond and third time, and 

 had then reafon to conclude that the wood was nearly as dry- 

 as it would become by lying long in a dry air. I now let it 

 cool to the temperature of the atmofphere,which was J5 , and 

 heating the fand to 200 only, I poured it around the rod. In 

 a few minutes it expanded 16 divifions. In half an hour the 

 fand had cooled to 125, and the rod had contracted 11 divi- 

 sions. In an hour more the fand was 80, and the rod fhort- 

 ened full 4 divifions more, being nearly equal to its length 

 when the fand was firft applied. On the whole I conclude, 

 lhat very dry wood expands with heat pretty regularly, though 

 certainly in a much lefs degree than any of the metals, or even 



glafs. 



The rod above mentioned having been kept in a dry place 

 for twelve months, I again tried it with the pyrometer, having 

 fixed near one end of it a fmall graduated fcale of ivory, 360 

 divifions whereof were equal to one inch. This fcale was 

 viewed with the microfcope, furnifhed with a crofs hair, and I 

 thought this method preferable to the fcrew micrometer ufed 

 before. 



The rod was placed in the pyrometer when the temperature 

 of the air was about 6o<\ On pouring fand around it, heated 

 a little higher than boiling water perhaps, it immediately ex- 

 panded 



