Chronometers and docks for astronomical purposes, in which extraordinary nicety in the exact measurement of time is necessary, have, besides the com pensation pendulums or balances, and detached escapements, before described jewelled pallets, and all their pivot-holes jewelled : they are likewise provided with a contrivance for continuing their motion during the time of winding up, when the action of the maintaining power is suspended. For this purpose a second larger ratchet wheel is added on the same arbor which admits the clock to be wound up, but with teeth pointing the contrary way; a strong spring, usually the greatest portion of a circle, connects this large ratchet wheel with the great wheel of the clock, which is on the same axis with it, one end of this spring being attached to the great wheel, and the other to the large ratchet; and a catch proceeds from the inner face of this back plate to the teeth of this ratchet, which prevents its moving back when the clock is winding up, and serves as a support for the reaction of the maintaining spring. When the clock is left to the action of the weight, the small ratchet turns round the larger one, and con tracts or coils up the spring till it has strength sufficient to impel the great wheel and train; and when the action of the weight is suspended, as in wind ing up, the spring, freed from the contracting power of the weight, expands itself and forces round the great wheel, its action in the contrary direction on the peat ratchet being prevented by the catch before mentioned. Leroy is considered to be the inventor of the spring impeller to prevent loss of time in winding up, but the idea of continuing the motion of the train during this time originated with Huygens, for be contrived a method by which the weight of his clock should continue to act on his train whilst it was drawing up ; the weight in his clock having been made to draw up in a similar manner to that used in common wooden clocks, instead of being wound up as in our metallic clocks.
What has been already advanced, will, we trust, be sufficient to convey a clear idea of the nature and general construction of horological machines • but peat diversities in their form and a variety of curious movements have been introduced by the taste and ingenuity of artists in this branch : some of these excite admiration by the number and intricacy of the component parts, and the admirable precision with which they act, whilst others evince equal ingenuity by the simplicity of the means adopted to obtain the end proposed. Of this last description of clocks, none have as yet exceeded that invented by the cele brated Dr. Franklin ; it shows the hours, minutes, and seconds, and yet con tains but three wheels and two pinions in the whole movement. The lowest wheel contains 160 teeth, and goes round once in four hours ; it carries the band on its axle, which points out both the hours and the minutes, as will be described, and it turns a pallet above it of 10 leaves, on the same axis with which is a wheel of 120 teeth, that gives motion to a pallet of 8 leaves : the second-hand is annexed to the same axis with this latter pallet, as also the swing-wheel, which carries 30 teeth, that gives motion to the pallets of an anchor escapement, and to its pendulum that vibrates seconds. The dial of this
clock is of singular formation ; the external circle on it contains 240 divisions, numbered from 1 to 60, in four successive notations ; this circle shows the minutes ; within it the hours are arranged in three concentric circles, or in a volute of three revolutions along four radii, which form right angles with each other. By this arrangement, while the point of the hand shows the minute, its side exhibits the hour; or, more strictly speaking, shows that the hour is one of three; but so that it will hardly ever happen that any doubt will remain of which it may be, as there are four hours difference between the figures next to each other on the same radial line. A small circle is placed above the great one, and divided into 60 parts for the seconds. This clock was wound up by a line going over a pulley and ratchet, on the axis of the great wheel, by which the weight was drawn up in the same manner as in the common wooden docks. Many of these clocks have been made which are found to measure time exceed ingly well. The small imperfection in this clock, of its leaving the uncertainty mentioned as to which of three hours it denotes, though so easily corrected by the judgment, has given rise to some ingenious contrivances to obviate it. Of these the most curious is that which forms the subject of the engraving on page 698. To the great wheel of this clock two concentric plates are annexed, the external one of which has a groove cut through it along the line of a volute of three revolutions. This groove forms a trough, in which a metal ball is placed, part of which is seen through the excavation beneath the hour 11 in the figure ; as the plate and groove turn round, the ball rolls along the volute, still approaching nearer the centre as it proceeds ; and when it at last arrives at the centre, it falls into another trough, by which it is again conveyed to the external part of the volute. The hours are engraved between the revolutions of the volute, and the minutes are marked in four divisions of 60 each, upon an external fixed circle, to which an index annexed to the volute plate points. Several of these clocks were to be seen in the watchmaker's shops m London some years ago, but we believe the author of this ingenious contrivance is unknown.
In addition to the parts which measure and register time, and therefore alone constitute a watch or clock, various appendages are at times connected with them for pleasure or convenience, as chimes and puppets to clocks, musical andting movements, &c. to watches; but as these are not strictly comprised under the head of horology, we shall merely notice one or two inventions of this class, our limits not allowing us to do more.