160 



HOROLOGY. 



o f an inch. The centre of the ball hangs on the end of 

 < lon the tube of the regulating nut, where it was tapped, to 

 ^ -" "!!1*' wor k on i te corresponding screw, made near the lower 

 Ward's part of the back bar, formed here into a round rod, the 

 pendulum, lower end of which is a point, or index, to a graduated 

 plate fixed to the back of the case, and .5.25 inches be- 

 low the centre of the ball. The weight of the ball is 

 13 Ib. 2oz. ; that of the zinc and steel bars, nut, pen- 

 dulum spring, and connecting screws, 2 Ib. 1 3$ oz. ; 

 weighing in all nearly 16 Ib. la making up steel bars 

 or rods for any compensation pendulum, it is proper to 

 heat or blue them after they are finished, which will 

 dispel whatever magnetism they may have acquired 

 in working them up. The zinc bar of this pendulum, 

 when brought near to the length of compensation, was 

 about 21 inches. Taking the length of steel to be 

 compensated by this at 61.75 inches, we may find what 

 the compensation of the .zinc should be, if the steel is 

 rightly taken at l.'iS. 



Steel in inches 61.75, Log. 1.7906370 



Jlatio 138 Log. 2.1398791 



3.93051-61 



Zinc in inches 21 Log. 1.3222193 



Piatio 405.7 Log. 2.6082968 



3.9305161 



The expansive ratio here is greater than 373, as given 

 by Mr Smeaton ; but is not equal to 420, as given by 

 Mr Ward, from trials made with his pendulum. 



' 



Kemark on The three zinc pendulums which have been deScri- 

 zinc pendu- bed, have each their peculiar properties. The zinc rods 

 unSt of the gridiron one are very troublesome to make ; 

 but they are more exposed to the air, or to changes 

 of temperature, and are adjustable by means of the 

 shortest traverse, and the sets of holes which are in 

 them and the centre rod. When this pendulum is well 

 executed, it is pa-haps the best of the three. The one 

 with the zinc tube is the strongest, the bearing on 

 it being more firm and direct than in either of the 

 other two ; only it has no means for adjusting the com- 

 pensation, unless by shortening the tube from time to 

 time. According as the excess of its compensation is 

 shown, something might be contrived to adjust it, 

 without taking it from its place, but this would be too 

 complicated; so that the shortening of the tube by 

 degrees is rather the better way. Ward's is much 

 more easily made than the other two. Those who use 

 gridiron pendulums should have a half traverse, with 

 three pins on it, similar to the shortest one in the 

 pendulum, which will be found very convenient, when 

 it is necessary, to shift for compensation. The half 

 traverse and pins should be put into the holes, where 

 the traverse in shifting is not to come. This will 

 keep the pendulum rods in their places, and serve in 

 the same way as the spare screw proposed for Ward's 

 pendulum. 



CHAP. XI. 



On the Wooden Pendulum Rod. 



Woodea THE wooden pendulum rod does not come under the 



pendulum clasn of those which have just now been described ; 

 nor can it be supposed equal to any metallic compen- 

 sation one. Having a good opinion of it, however, we 

 put to trial one of them made of a very fine piece of 



straight-grained deal, that, for the purpose of seasoning, Woodem 



had been kept for five years near a parlour fire, which Pendulum 



was almost constantly lighted throughout the whole 



year. The rod, wlien dressed up and fitted to the ball, 



and the pendulum spring put to it, was well varnished, 



go as to exclude any possibility of its being affected by 



damp. It was then applied to the clock, which, when 



regulated, went for about sixty days, during the 



months of June and July, without any apparent devia- 



tion from time ; the very dry weather made the fixings 



for the clock case shrink a little. When these were 



again made more secure, the clock, during a trial of 



many months, could not be brought to give the same 



satisfaction. Whether this was owing to the wooden 



rod, or to what cause, we shall not at present pretend 



to determine. On this pendulum being taken away 



from the clock, a mercurial one was put in its place, 



having the same pendulum spring which was at the 



wooden rod, and every thing else being in the same 



state as before. The difference in the good going of 



the clock after this became truly astonishing, and may 



be considered as a striking proof of the great superio- 



rity of the one pendulum over the other. 



It must here be observed, that, although the compa- 

 rative trial by the some clock with the mercurial and 

 wooden rod pendulums was in favour of the former, yet 

 this clock and another were fixed on two planks, exactly 

 the same as those described in the next Chapter, and 

 strongly fixed to a stone wall, opposite the brick wall 

 where the other two clocks were, which gave rise to the 

 discovery of their pendulums affecting each others mo 

 tions. Not being aware of this at the time of trial, the 

 errors of the going of the clock, while the wooden rod 

 pendulum was used, and the good going of it when the 

 mercurial pendulum was applied, may have arisen from 

 various causes, such as the elasticity of the upper plank, 

 or the pendulums being of unequal length and weight. 

 This much may positively be affirmed, that they were 

 not going under such circumstances as to have a fair 

 trial. We propose however to repeat the experiment with 

 the wooden rod pendulum, applied to another clock, 

 placed in a more insulated situation. An eminent A- 

 merican philosopher says that deal has little or no lon- 

 gitudinal expansion, making it less than glass, as may 

 be seen in the Table under the article EXPANSION in 

 this work. 



In the Astronomical Observations published at Cam- 

 bridge in 1769, by the late Reverend William Ludlam, wooden 

 Professor of Mathematics in that university, he has pendulu 

 described a very neat and ingenious method of fit- r d* 

 ting up a pendulum with a wooden rod, construct- 

 ed for the purpose of preventing any gyratory motion 

 from taking place, as well as to have some resistance 

 from the air. This was effected by having the pendu* 

 lum ball of an equal mass round the centre of a round 

 wooden rod, and by a thin flat hard steel crutch, to 

 give impulse on the hardened ends of two screws put 

 through the rod, which screws were to keep the flat 

 crutch as near as possible in the plane or line of the dia 

 meter of the pendulum rod, or at right angles to the 

 middle plane of the pendulum ball. This ball was near- 

 ly of the form of a cheese, or the middle frustum of a 

 globe. For a more particular description of it, see Lud- 

 lam's Observations, page 81, Plate v. 



From the description given of this pendulum by Mr 

 Ludlam, it appears to be a very complete one, and se- 

 veral persons were on that account led to adopt it; but, 

 from our experience, it was found to be much inferior 

 to what might have been expected, and to possess, r- 



