contrivance, by or through which the successive impulses,or actions, are made, is Called a 'scapement or ESCAPEXENT, several of which are described at the ar ticles referred to.
According to the nature of the escape ment, and the part of the vibrating arc at which the impulses are applied, the vibrations of the balance may be made to employ a longer or shorter time than they would have employed if the balance had been separate from the works. Thus, in the common watch, these impulses quicken the vibrations ; and consequent ly an increase in the maintaining force will make the watch go faster; as may be easily tried by gently forcing the key in the opposite direction to winding up.
If the balance and its spring were to continue unchanged in all temperatures, and under all circumstances, and if its long and short vibrations measured equal times when separate from the machine, it would only be required that the escape ment should be so constructed as neither to accelerate nor retard them. But none of these conditions can be had in the or dinary structure of watches, and in-the superior time-pieces considerable diffi culties are found in the attempt to obtain them.
By the natural contraction, to which all bodies are subject when cooled, the di ameter of the balance will be less the lower the temperature : it will therefore be more easily carried by the vibrating forces, and will then vibrate more quick ly.
The spring attached to the balance, which is called the pendulum spring, will likewise act with greater force when cold, and on this account also the vibra tions will be quicker.
The remedies for these causes of im perfection are the following: 1. The Remontoire. As the irregu larities in the transmission of force from the main spring are certainly increased by the number of wheels in the train, it was proposed, in the infancy of the art, to detach the last wheel, or that nearest the balance, or time measurer, from the rest, and to move this by a separate spring or weight: so that in this contrivance the time measurer is acted upon by one single wheel, and the rest of the,train is employ ed in winding up the secondary first mover at short intervals, such, for exam ple, as every half minute. We shall also have to mention some escapements, in which the winding up is performed in every single vibration. With regard tc remontoires, it maybe remarked, that they either greatly shorten, or else de stroy the periodical irregularities of the train, and those of the main-spring; but that with regard to the in`inence of oil, and other causes of more permanent dif ference, their advantage is not very con siderable, because the remedy is not ap plied where the motion is quickest.
Whether the irregular action of the maintaining power be diminished by the remontoire or not, it is desirable that the impulse on the balance, through the es capement, should affect the natural mea sure of its vibrations as little as possible; or rather that it should tend to equalise them when the arcs of vibration vary. Some attention, but not much, has been paid to the equalizing quality of an es capement, principally by making the faces of the pallets of a figure suitable to that effect ; but these are now for the most part abandoned, and the method of applying the force constitutes the dis tinguishing feature in this part of our mo dern chronometers. If a balance be set to vibrate by the mere action of its pen dulum spring, its motion will soon de cay ; but if we suppose a lever or pallet to proceed from its axis, and a maintain ing power to be applied to this, it is ob vious, that if the power meet the pallet in its progress from the point of quiescence, it will shorten the time, and also the arc of that semi-vibration ; and, again, that if the power follow the pallet in its pro gress towards the point of quiescence, it will drive it home sooner, and conse quently will shorten the time of that se mi-vibration; and that actions contrary to these would lengthen the times. If,there fore, the action itself, which may be con sidered as an accelerating force, be not applied on both sides of the point of qui escence through a certain arc, determin able from the circumstances, the main taining power, when it comes to be ap plied, will alter the time ; and if this va ry, the time must also vary. Now the remedy at present adopted is, to make the balance vibrate through a very large arc, such as a semicircle or more, and to follow the pallet in each returning vibra tion by a strong power exerted through a very small arc, as, for example, 15° or 16°. By this contrivance the balance will vibrate at perfect liberty, out and home, through two semicircles, or 360°, except ing the small part during which the im pulse is given ; and if the impulse vary, the arc of vibration will vary, and with it the time, unless the spring be made of a certain definite length, or tapered in its thickness according to the experi ence which many artists in this country possess.