HOROLOGY. Horology is that branch of science which enables us to measure the portions of time. We judge of the lapse of time by the succession of sensi ble events ; and the most convenient and accurate measures of its quantity are de rived from motions, either uniform, or else repeated at equal intervals. Of the former kind, the rotation of the earth on its axis is the most exact, and the situa tion of its surface with regard to the fix ed stars, or, less simple, with regard to the sun, constitutes the means for deter mining the parts of time as they follow each other. See ASTRONOMY and DIAL LINO.-Of the latter kind, the rotation of machinery, consisting of wheel-work, moved by a weight or spring, and regulat. ed by a pendulum or balance, affords in struments of which the utility is well known. The term horology is at present more particularly confined to the princi ples upon which the art of making clocks and watches is established. A considera ble portion of this extended subject of research has been given under the articles CLOCK and CatiorrostErEn. In the present, we shall chiefly attend to the means by which the train of wheel-work is made to make a number of successive advances, all so very nearly equal in the measure ment of time, that a surprising degree of precision is obtained in ascertaining the intended object.
The machines which, for centuries, have been commonly used to measure time, consist of a movement, or train of wheels, drawn by a weight or spring, and a regulator, the object of which is to keep the motion of the train within the required degree of uniformity. A conti nual rotatory motion, which constantly tends to accelerate, is thus corrected by means of an alternate motion ; while the power which carries round the move ment restores also, to the regulator, the action lost by friction and other causes. The mechanism, by which the two prin cipal parts act on one another, is called the escapement ; and this most admirable contrivance may be reckoned the distin of the modern art of time-piece making.
One of the most ancient escapements is that which is now applied in almost all common pocket watches. It is represented in fig. 1. Plate HOROLOGY, and is best suit ed to the long vibrations of the balance, which was invented earlier than the pen dulum. A B denotes the rim of a contrate wheel, called a crown wheel, having its teeth pointed and sloped on one side only, so that the points advance before any other part of the teeth during the motion. C and D are two pallets or flaps proceeding downwards from the verge E F. The pallets are nearly at right angles to each other ; and when the balance F G, fixed to the verge, is at rest, the pallets remain inclined to the plane of the wheel, in an angle of about forty-five degrees ; but when it is made to vibrate, one of the pal lets is brought nearer to the perpendicu lar position, while the other becomes more nearly parallel. The wheel must be supposed to have one of its teeth resting against a pallet, by virtue of the maintain ing power. This tooth will slip off or es
cape, as the pallet rises towards the hori zontal position, at which instant a tooth on the opposite side of the wheel will strike against the other pallet which is down.— The returning vibration, by raising this last pallet, will suffer that tooth to escape, and another tooth will apply itself to the first-mentioned pallet. By this alterna tion, the crown-wheel will advance the quantity of half a tooth each vibration, and the balance or pendulum will be pre vented from coming to rest, because the impulse of the teeth against the pallets will be equal to the resistances from fric tion and the re-action of the air.
The common escapement here describ ed was well known to Leonardo de Vinci, who describes an instrument acting byan escapement of this kind, similar, as he says, to the verge of the balance in watches, which he does not seem to men tion as a new thing : he died about 1513. The isochronism of the pendulum was known to Galileo, in 1600, who, before his death, namely, about 1633, proposed to apply it to clocks. The actual applica tion by Huygens was made before 1658, when he published his " Horologium Os cillatorium." He applied it by means of the common escapement already in use with the balance, and still retained in our table-clocks. Sanctorius had made the same application about forty years before that time, as appears by his " Commen tarii in Avicennam," (quest. 56,) printed in 1625, in which several instruments are described as having been publicly exhi bited and explained to his auditors, at his lectures in Padtta, for thirteen years pre vious to that time This escapement not being adapted to such vibrations as are performed through arcs of a few degrees only, another con struction has been made, which has been in constant use in clocks for this century past, with a long pendulum beating se conds. (Fig. 2.), A B represents a verti cal wheel, called the swing wheel, having thirty teeth. C D represents a pair of pallets connected together, and movea ble in conjunction with the pendulum, on the centre of axis P. One tooth of the wheel,in the present position,rests on the inclined surface of the inner part of the pallet C, upon which its disposition to slide tends to throw the point of the pal let further from the centre of the wheel, and consequently assists the vibration in that direction. While the pallet C moves outwards, and the wheel advances, the point of the pallet D, of course, ap. proaches towards the centre, in the open ing between the two nearest teeth ; and when the acting tooth of the wheel slips off„ or escapes from the pallet C, another tooth on the opposite side immediately falls on the exterior inclined face of D, and, by a similar operation, tends to push that pallet from the centre. The returning vibration is thus assisted by the wheel, while the pallet C moves towards the cen tre, and receives the succeeding tooth of the wheel after the escape from the point of D. In this manner the alternation may be conceived to go on, without limit.