ON TIMEKEEPERS.

189

semicylindrical counterpoise, so that the time might he determined eitherfrom the measure or weight of the quantity of water discharged, or from theposition of the counterpoise. Various other modes might also be devised formaking cheap and simple timekeepers on similar principles, dependent on themotion of various liquids or elastic fluids; but great accuracy could scarcelybe expected from them. A candle sometimes serves as a coarse measure oftime; and by burning a thread which passes through it, it may easily bemade to answer the purpose of an alarm.

Clocks .and watches are machines in which wheelwork is emploj-ed for themeasurement of time, being driven by a weight or by a spring, and regulat-ed by a pendulum or a balance. Watches differ from clocks, in being port-able, and this condition excludes the pendulum and the weight from theirconstruction.

It is conjectured that the Saracens had clocks which were moved byweights, as early as the eleventh century. Trithemius mentions an orrery,moved by a weight, and keeping time, which was sent, in 1232, by theSultan of Egypt, as a present to the Emperor l’rederic IT. Wallingford, in1326, had made a clock which was regulated by a ily. The use of such a flyin equalising motion depends on the resistance of the air, which increases ra-pidly when the velocity is increased, and therefore prevents any great ine-quality in the motion, as long as the moving power varies but little; and ifthe action of the weight were transmitted with perfect regularity by the■"'heels, and the specific gravity of the air remained unaltered by pressure orby temperature, a fly clock might be a perfect machine, the weight beingalways exactly counterbalanced by the resistance of the air, attending a certainvelocity of the fly; and it might even be possible to regulate the inequalities°f the action of the weight, by causing the fly to open and shut, or to turn ona u axis, by means of a spring, according to the magnitude of the resistance.

^ he unequal density of the air would however still remain uncompensated;and in this respect a liquid would be a better medium than an elastic fluid.*'° r experiments which are but of short duration, and which require greatprecision, a chronometer regulated by a simple fly is still a useful instru-meut. Mr. Whitehurst’s apparatus for measuring the time occupied in thedescent of heavy bodies, is governed by a fly; the index is stopped by the