42 Mr. Daily on the Use of Wood for Pendulum Rods. 



"This being premised, let us assume 6=14i inches, and 

 make the cylinder of any diameter, according to the weight 

 proposed. The construction then of such a pendulum will be 

 as follows. Take a deal rod, of a convenient size, but not less 

 than 'te inches in length, the lower part of which (14^ inches 

 long) should be made cylindrical, about | of an inch in di- 

 ameter : or the whole rod may be of this size and shape. Pro- 

 cure a leaden weight or cylinder to be cast, with a hole through 

 the centre which will freely admit the cylindrical end of the 

 rod, and of such a length that when put in the lathe it may be 

 reduced to the required standard of 14-3 inches*, and to the 

 required weight f. 



" The lower end of the rod should be formed into a screw, to 

 which a wooden nut may be fitted, in order to adjust the pen- 

 dulum nearly to the given rate : and the final adjustment may 

 be made by means of a slider, as above described:}:. A pendu- 

 lum of this kind will cost but a few shillings, and will answer 

 many useful purposes, as 1 have found by experience. If the 

 expansion of the rod could be depended on, it would be as ac- 

 curate as any other. In this construction I have not consi- 

 dered the expansion of that part of the spring which is below 

 the axis of motion. The effect of this may be taken into ac- 

 count, at the final adjustment of the pendulum. 



" In all the wooden pendulums which I have seen, the bob 

 has been made of a Icntictdar shape, and oftentimes been fast- 

 ened to the rod by means of a pin passed through its centre', 

 and therefore not constructed with any view to compensation. 

 This shape was, I presume, originally adopted with a view to 

 overcome the resistance of the air, and thus reduce the main- 

 taining power of the clock. But it is well known that the air, 

 as a resisting medium, has no sensible effect on the rate of the 

 clock ; neither is the rate affected by the shape of the bob ; 

 we may therefore choose that shape which will bestanswer other 

 useful purposes." 



* It will be more convenient to have this cylinder toe long rather than 

 too short : since we may readily diminish the length of it, if, on trial, it 

 should be found that the pendulum is oiifr-compensated. 



\ The following are the respective weights of a leaden cylinder ] 4'3 in- 

 ches long, and having a hole, through the centre, equal to y of an inch in 

 diameter. 



Diameter of Cylinder. Weight. 



li inch. = C-56'lbs. 



U = 9-73 



H = 1347 



a = 17-80 



2J = 22-7« 



\ A small sliding weight attached to the rod, as described in p. 402 of 

 the Memoirs. 



VIII. On 



