Mr, Baily o/i the Use of Wood for Pendulum Rods, -tl 



on the -Mercurial Compensation Pendulum, read May and June 18.2;? be- 

 fore the Astronomical Society, and just published in the second part of the 

 bociety's Memoirs, p. 385, and p. 41 1.— Edit.] 



On the Use of Wood for Pendulum Rods. By F. Baily, Esq. 



" Having enumerated the principal experiments that have 

 been made on the expansion of merciny, I shall proceed to 

 an examination of those which have been made, by various 

 persons, on those substances of which the rod of the pendu- 

 lum may be composed. Of all these substances, that oi' wood 

 appears to be the least expansible ; but, it is unfortunately so 

 liable to be affected by the moisture of the atmosphere, that, 

 notwithstanding we coat it with varnish, or sealing wax, or 

 paint, or gilding, or even bake it, and impregnate it with oil, 

 It has seldom been found sufficiently accurate for the refined 

 purposes of the modern astronomer. 1 would not, however, 

 wish to discourage any attempts to render this substance more 

 fit for general use. Fitted up with a leaden bob (in the man- 

 ner which I shall hereafter describe), it forms the cheapest 

 pendulum that I know of: and, if placed in a room where 

 there is an uniformity in the atmosphere, it might answer 

 every useful purpose for an economical observatory. At all 

 events, it would form an excellent appendage and improve- 

 ment to the common household clock, and would be far supe- 

 rior to and much cheaper than the usual and absurd mode of 

 hanging a leaden bob to the end of an iron wire. 



" I have already stated that an economical pendulumfor a se- 

 conds clock might be constructed by means of a leaden bob at- 

 tached to a wooden rod: and I shall now show the mode of de- 

 termining the relative lengths of these two substances, I have 

 preferred lead to zinc on account of its inferior price, and the 

 ease with which it may be formed into the required shape : and, 

 as there is no considerable diffbrence in their rates of ex- 

 pansion, it is equally applicable to our purpose. If we take 

 the rate of expansion of deal to be -0000022685, and that of 

 lead to be -0000159200, as given in the table, we shall have 



e 2'268,5 , 



/3 =^ 159200 ^^^ '14'25 = K. Now, if we assume the weight of 



the bob to be 100 times the weight of the rod (which will pro- 

 bably be the case), we shall by means of the equation (D) have 

 x = -2874 (l--2926+-.'3832) =-313: which, being substituted 

 for X in the equation (C), will give r = ^5"i5 and b = H-30*. 



• If the bob were made of zinc, the length of the cylinder would be oniv 



1.3-8H inches. " ^ 



Vol. 65. No. 321, Jaw, 1825, F This 



