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.

In 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 purpose the weight at its upper 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 he 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 in a glass tube; d is a cap or stopper to the upper end of the tube, and carrying underneath a small graduated scale, over which ranges the index at the top of the weight. For this invention Mr. Hardy received a reward from the Society of Arts.

His communication to them was accompanied by testimonials to the value of the invention 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the application of the pendulum as a regulator in clocks, balances had been employed for that purpose, but were quickly superseded after the introduction of the pendulum; but as external motion is destructive of the regularity of the pendulum's performance, the balance was still the only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 pendulum 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 vibrations will be farther quickened. There are two ways of correcting these irregularities; 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 different 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. Harrisons 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 convex 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 ingenuity, is now seldom used, owing to the extreme difficulty of effecting an accurate 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 flat 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 into a crucible, is made sufficiently hot to melt the brass, which in these circumstances adheres firmly to the steel without requiring any solder. The face of the steel is then cleaned, and by proper application of the mechanical means of turning, boring, and filing, the superfluous steel is taken away, and the balance is left, consisting of two, or sometimes three, radii, and a rim, the external part of which is of brass, and the internal part of steel, the former metal being about twice the thickness of the latter. In this state the arms of the rim are then cut through and diminished in their length, as in the figure; and near that extremity of each arc which is farthest from its radius, is put on a piece or weight, which can be slided along the arm, so as to be adjusted at that distance which shall be found, upon trial, to produce a good performance under the different changes of temperature; the flexure of the arms by the change of temperature carrying the weights nearer to the centre in hot than in cold weather, and the more the greater the distance of the weights from the radius.