In the year 1818 a reward was given to Mr. Reid for a compensation pen dulum, in which the bob rested in a hollow cylinder of zinc, through which the rounded end b c of the steel pendulum rod is passed, the zinc itself being supported by the nut a at the end of the rod. As, therefore, the rod lengthened by heat, carrying the bob downward, so the upward expansion of the zinc raised the bob; and if the rela tive lengths of the steel and zinc were so propor tioned that the amount of their expansions was equal, it is evident that the compensation above described would be perfect ; but it is extremely difficult to effect this accurate proportioning of the lengths of the two metals. The length of the zinc at first must be such that its rate of expansion shall be in excess, and it must be cautiously reduced by repeated trials till the requisite accuracy is attained; this, however, is not done, except at a considerable expense of time and attention, to avoid which Mr. Reid has introduced the following modification. He forms a hollow screw in the cross bar f of the bob g, and an external screw on the same rake on the end of the zinc cylinder d; this latter is pur posely made too long for due compensation, but its effective length may be commodiously and accu rately reduced to what is required by screwing it up as represented in the figure; but after this has been done, supposing the nut a to have remained stationary, it is evident that the extent of gravity of the pendulum itself will have been lowered by the bob descending exactly as much as the upper end of the cylinder has advanced through the hole in the cross-bar f; an adjustment for time is therefore required after that for compensation has been effected, which is done in the usual way, by screw ing up the nut e. This latter compensation, how ever, will not be required, if the rakes of the screws f and c are proportionate to each other, as the weight of the bob alone is the sum of the weights of the bob, the zinc cylinder, and the nut. Thus, if the former weight be assumed as ten, and the latter as eleven, the screw at f must have ten threads, in the same length that the screw at c has eleven threads. Care must be taken in screwing the cylinder of zinc up or down, to place the finger and thumb at the same time on the nut, so that the two may turn together; or the nut may be fastened to the cylinder.
connexion with this branch of the subject, we may notice the inverted spring pendulum, invented by Mr. W. Hardy, of Wood-street, Clerkenwell. The object of this invention is to ascertain the stability of the support of clocks, for which pur pose the weight at extremity is screwed down until it will perform its vibrations in the same period of time as the pendulum of the clock. The inventor states that when the weight a is screwed up to the top of the steel rod b, it vibrates only once or twice in eight or nine seconds, which renders it remarkably susceptible, and that its sensibility is so great that the attraction of the sun and moon, or of mountains, may be observed by it : it serves also for ascertaining the stability of buildings. a is the weight screwed upon a steel rod b, which is supported at bottom by a small piece of steel watch-spring c, and contained a glass tube ; d is a cap or stopper to the upper end of the tube, and carrying underneath a small gra duated scale, over which ranges the index at the top of the weight. For this invention dr. Hardy received a reward from the Society of Arts. His com munication to them was accompanied by testimonials to the value of the inven tion from several eminent scientific characters, amongst whom was Captain Kater, who employed the instrument to assure himself of the stability and freedom from tremor of the base upon which his clock rested during his experiments.
Before the application of the pendulum as a regulator in clocks, balances had been employed for that purpose, but were quickly superseded after the intro duction of the pendulum ; but as external motion is destructive of the regularity of the pendulum's performance, the balance was still the only regulator which could be applied in portable machines, and the great improvement made in that regulator by the addition of a spring has caused it to approach the pen dulum in point of correctness. The first invention attaching a spring to give
to the balance, by its elasticity, a power which renders the action of this sort of regulator similar to that of gravity in a pendulum, is undoubtedly due to Dr. Hooke, but he appears to have applied it only in a straight form. Huygens, improving upon this idea, substituted a spiral spring, which is much more favourable to the vibrations of the balance.
The alterations to which the length of the pendulum is exposed by variations of temperature, and which affect the going of clocks, have already been noticed; but watches with a balance are still more exposed to irregularity from that cause, as not only the balance expands or contracts according to the rise or fall of the thermometer, but the regulating spring itself suffers the same changes. As the balance contracts, and its diameter becomes less, it will be more easily carried round by the vibrating forces, and will then vibrate more quickly ; and as the spring attached to the balance becomes contracted at the same time, it will likewise act with greater force when cold, and on this account the vibra tion; will be farther quickened. There are two ways of correcting these irregu larities ; the first, which was invented by Harrison, consists in lengthening or shortening the spring when heat or cold may have given it more or less force ; the other method is to cause the balance to expand instead of contracting by cold, by which means the spring, when in the state of great rigidity, has more work to do; this method originated with Peter Leroy, and has since been carried to great perfection by Arnold. Harrison (whose application of the dif ferent expansion of two metals to correct the variations in the length of the pendulum, has been already noticed), applied the principle in a manner not before thought of, and made it act on the spiral spring so as to produce the desired compensation in the regulator. His method is described as follows, (Principles of Mr. Harrison's Time-keeper, p. xii—Notes.) " The thermometer kirb is composed of two thin plates of brass and steel, rivetted together in several places, which, by the greater expansion of brass than steel by heat and contraction by cold, becomes convex on the brass side in hot weather, and con vex on the steel side in cold weather; whence, one end being fixed, the other end obtains a motion corresponding with the changes of heat and cold, and the two pins at the end, between which the balance spring passes, and which it alternately touches as the spring bends and unbends itself, will shorten or lengthen the spring, as the change of heat or cold would otherwise require to be done by hand in the manner used for regulating a common watch.' This method of effecting the compensation, although it evinces considerable inge nuity, is now seldom used, owing to the extreme difficulty of effecting an accu rate adjustment ; recourse is therefore had to the principle introduced by P. Leroy, a modification of which is seen in the figure on the following page, which represents the balance of a chronometer as commonly constructed. A circular groove is turned in the fiat face of a piece of steel, and into this groove a piece of good brass is driven, and a little of the solution of borax is applied to prevent oxydation. This compound piece being then put mto a crucible, is made sufficiently hot to melt the brass, which in these circumstances ad heres firmly to the steel without requiring any solder.