162 



HOROLOGY. 



8fTect of 

 the weight 

 when oppo 

 eitc to the 

 pendulum. 



pendicular. The piece of steel should not be less than 

 half an inch thick at the middle, where it should be circu- 

 lar, and about three quarters of an inch broad. In the 

 middle is a hole of about three-tenths of an inch in dia- 

 meter: the two conical arbors are formed from the circu- 

 lar part, so as to be in a line with the diameter of the hole. 

 The pivots of these arbors, which turn in the triangu- 

 lar notches, may be about three-twentieths of an inch 

 thick. In the hole, which is three-tenths of an inch wide, 

 is fitted a steel pivot, having a shoulder on the. under 

 side, which comes so far beyond the upper side as to ad- 

 mit a stout brass collet to be pinned on it, and against 

 the upper flat side of the circular part, besides a sort of 

 screw head on the end, with a slit in it, by which a 

 screw-driver can turn it about, so that the pendulum ball 

 may be made to stand in the plane in which it ought to 

 swing. From the shoulder, below the circular part of 

 the steel piece, to the lower end, may be an inch long. 

 To this the pendulum spring is fixed. This, in some 

 respects, is a very convenient mode of suspension, but 

 we do not think it so strong and so firm as the other. 



When the astronomical clock, formerly mentioned, 

 which goes six weeks without winding up, was plan- 

 ned, care was taken to have the weight kept at as great a 

 distance as possible from the pendulum ball, as we con- 

 ceived that the attraction of the weight would disturb the 

 vibrations of the pendulum. This idea, which appeared 

 to us new, had occurred, we have been told, long before 

 to several very able artists and amateurs, such as Graham, 

 Harrison, Lord Macclesfield, Sir George Shuckburgh, 

 Troughton, and others. In the course of our trials 

 with the clock, the arc of the vibrations of the pendu- 

 lum, when the weight came as far down as the ball, 

 was observed to suffer a sensible diminution, and this 

 was imputed to their mutual attraction. Upon mention- 

 ing this afterwards to one or two persons, supposed to be 

 competent to judge in an affair of this kind, they en- 

 tertained some doubts respecting this explanation of the 

 fact, and thought it might probably arise from some mo- 

 tion communicated to the air by the swinging of the 

 pendulum. Without making any experiments in order 

 to examine the action of the air on the motion of the 

 pendulum, an account of the fact, which was ascribed 

 solely to attraction, was published in Nicholson's Phil. 

 Jour, October 1812| vol. xxxiii. octavo series. Soon af- 

 ter this, Mr Ezekiel Walker of Lynn, in a paper pub- 

 lished in the same Journal, endeavoured to shew, that 

 the cause of this disturbance of the pendulum, ( which 

 he says had been known to him 30 years before,) arose 

 from the motion of the air communicated by the weight 

 to the pendulum, which it certainly did, as we soon af- 

 terwards found from one or two experiments, which 

 did not occupy much time. In a paper of Mr Walker's 

 in Nicholson's Phil. Jour, for May 1802, vol. ii. octa- 

 vo series, p. 76. entitled, " Methods for diminishing the 

 irregularities of Time-pieces, arising from differences in 

 the arc of vibration of the Pendulum," he has assigned 

 several causes for the changes that take place in the arcs 

 of vibration, and proposed different methods to pre- 

 vent them. But no notice whatever is taken of the 

 motion communicated to the air by the pendulum. M. 

 Berthoud mentions, in the first volume of his Essai, 

 published in 1763, No. 642, that the air put in motion 

 by the vibrations of the pendulum, acts against the 

 weight of the clock, so as to. set it in motion, and that 

 tins will in its turn gradually diminish the motion of the 

 pendulum until it stops it altogether. This takes place 

 more readily when the weight hangs by a single ball 



than when it is suspended upon a pulley. This fact, it Pendulums, 

 must be confessed, had either been overlooked by us, '*~~~\~~*' 

 or had entirely escaped our memory. Month clocks, 

 from stopping frequently, have long been very trouble- 

 some to clockmakers, who no doubt assigned for it a 

 different cause from tlie true one. In the old month 

 clocks, the weights are very large and heavy, and the 

 momentum of the pendulum very small, so that they 

 were extremely liable to be stopped. But in clocks 

 where the pendulum has even a considerable momen- 

 tum, this agitation of the air will be sufficient to stop- 

 them altogether. 



Having been called upon to examine a good astrono- 

 mical regulator of Graham's, which had stopped, and 

 which belonged to a nobleman in the neighbourhood 

 of Edinburgh, we informed the man who was sent to 

 put it in order, that he would find the weight opposite 

 to the pendulum, which was actually found to be the 

 case. 



CHAP. XII. 



On the Sympathy or Mutual Action of the Pendulums 

 of Clocks, 



IT is now nearly a century since it was known that Sympathy 

 when two clocks are set agoing on the same shelf, they wdock* 

 will disturb each other; that the pendulum of the one 

 will stop that of the other; and that the pendulum which 

 was stopped will, after a while, resume its vibrations, and 

 in its turn stop that of the other clock, as was observed by 

 the la:e Mr John Ellicott. When two clocks are placed 

 near one another, in cases very slightly fixed, or when 

 they stand on the thin boards of a floor, it has been long 

 known that they will affect a little the motions of each 

 other's pendulum. Mr Ellicott observed, that two clocks 

 resting against the same rail, which agreed to a second 

 for several days, varied 1' 36" in 21 hours when sepa- 

 rated. The slower having a longer pendulum, set the 

 other in motion in- 16| minutes, and stopped itself in 

 36J minutes. It never could have been supposed, how- 

 ever, that when very strong fixtures were made, it was 

 possible for any thing of this kind to take place. About 

 three years ago, in a room where astronomical clocks 

 were placed under trial, two strong deal planks were 

 firmly nailed to a tolerably stout brick wall or partition, 

 the ends of the planks being jambed between the adjoin- 

 ing partitions. The planks were 6 feet long, 6 inches 

 broad, and l{ths of an inch thick. One of them was 

 placed behind the suspension, and the other behind 

 the balls of the pendulums. The pendulums were sus- 

 pended on strong massy cocks, partly of brass, and 

 partly of iron, and, with the back of the cases, ( one 

 of which was of very hard oak,) were firmly screwed to 

 the upper plank, and also to the lower one, the bottom 

 of the cases being free and independent of the floor. 

 Two clocks, whose pendulums were nearly of equal 

 length and weight, and whose suspensions were distant 

 from each other about two feet, kept so unaccountably 

 close together for the greater part of twelve months, as to 

 become a matter of considerable surprise. When the cold 

 weather commenced in November, they made a small de- 

 viation from one another for a few days, and then resum- 

 ed the same uniformity which they hud before. An ac- 

 count of this was published in Mr Tilloch's Philosophi- 

 cal Magazine, where the observations of M. de Luc, 

 which seem to have been a very near approach to 

 the cause, were inserted by way of reply. The pen- 



