EXPANSION OF WOOD by HEAT. 31 



May Tith 1796. 



P. S. 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 gra- 

 duated fcale of ivory, 360 divifions whereof were equal 

 to one inch. This fcale was viewed with the microi- 

 cope, 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 tem- 

 perature of the air was about 60°. On pouring fand 

 around it, heated a little higher than boiling water per- 

 haps, it immediately expanded ^ ^ divifion, but in lefs 

 than a minute it began to contradi, and continued to do 

 fo for an hour, when I drew off the fand. It was now 

 full 10 divifions fliorter than at firft, fo that it had im- 

 bibed a great deal of moifture from the air which it again 

 parted wdth to the heated fand. Three hours afterwards 

 when the red was cool, I again poured the fand on it, 

 heated as before. It now continued to expand for about 

 three minutes, when its length was encreafed ^~ divi- 

 fions, it then began to contradi, and became full 3 divi- 

 fions lliorter than when the fand was poured on it. I 

 caufed the fand to run off once more, and let the rod 

 cool. Then heating the fand 250" by a thermometer, I 

 poured it on the rod, and in a few minutes it expanded 

 3^ divifions, it then began to contract flowly, and in 15 

 minutes it became t of a divfion lliorter than at firft. On 

 the whole I concluded that the expanfion of wood, in its 

 length, will be irrregular, ccrrefponding partly 'to the 

 warmth, and partly to the moiflure of the atmofphere. 



D. RITTENHOUSE. 



E 2 J Letter 



