On the Expanfion of Wood by Heat. 343 



fea, the blades, which the acidity of the atmofphere rufts and 

 deftroys when made of iron, would be preferved from that 

 inconvenience. 



I (hall terminate this extract with fome reflections on the 

 price of metals in the time of Pliny. Lead at Rome, during 

 thefirft century of our gera, was worth twenty-four times the 

 price at which it is fold at prefent at Paris, and tin c iight times 

 its prefent price ; {o that lead was three times as dear there as 

 tin, while among us it does not coll a third of the price. 

 The comparative dearnefs of thefe metals among the antients 

 muft make us fenfible of the advantage which more extenflve 

 commerce, and metallurgic labours conducted in a more 

 feientifje manner, give us over them. But we ought not to 

 forget that this advantage is the rcfult of eighteen centuries of 

 experience and improvement ; and that, notwithstanding this 

 great number of centuries, the antients are ftill our mailers in 

 that part of literature, and the fine arts, which forms the pe- 

 culiar domain of the imagination. 



LV. On the Expanfion of Wood by Heat. By David Rit> 

 tenhousk, LL. D. Prejident of the American Philofor- 

 phis a I Society '" 



,« 



J-N the prefent (late of experimental philofophy it is well 

 known that bodies in general enlarge their dimenfions, or ex- 

 pand, on being heated, and contract in cooling. From fome 

 experiments heretofore made, wood has been thought to make 

 an exception to the general rule, and this opinion has fo pre- 

 vailed that many curious perfons have applied wooden pen- 

 dulum rods to their time-pieces, to prevent the variation in 

 their rate of going, arifing from the expanfion and contra6ling 

 of a metal rod. From my own observations, however, as well 

 as thofe of fome of my friends, the wooden pendulum rod does 

 not appear to anfwer the expectations formed on it. I had in 

 my polfcilion for feveral years an excellent time-piece made 

 for this fociety by an ingenious workman and worthy mem- 

 ber of the foctet\\ The refult of my conftant attention to this 

 clock was, that, though its regular variations with heai and 

 cold were probably much lefs than thofe of metal pendulums^ 

 it nevertheless went always fa iter in winter than in fummer, 

 and was liable to very fudden and confiderable variations ; 

 aiifing, no doubt, Irom the combined effects of heat and eold, 



' : From the Yrcwfiflivns of the American Fbtiojbfbiccd Sj iety. 



Y 4 moifture 



