ON TIMEKEEPERS. 145 



appears from Vitruvius's account that wheelwork was employed,* and the 

 hour was shown on a graduated scale ; the graduations were also probably 

 so adjusted as to correct the error arising from the inequality of the velocity 

 occasioned by the variation of the height of the water in the reservoir. This 

 inconvenience was however sometimes wholly avoided by means of a con- 

 stant stream, which kept the vessel full, or still more elegantly, by the 

 siphon of Hero, which was a bent tube supported by a float, so that its 

 lower orifice, at which the water was discharged, was always at a certain 

 distance below the surface. Dr. Hooke proposed to keep the reservoir full, 

 by means of a semicylindrical counterpoise, t so that the time might be 

 determined either from the measure or weight of the quantity of water 

 discharged, or from the position of the counterpoise. Various other modes 

 might also be devised for making cheap and simple timekeepers on similar 

 principles, dependent on the motion of various liquids or elastic fluids ; but 

 great accuracy could scarcely be expected from them. A candle sometimes 

 serves as a coarse measure of time ; and by burning a thread which passes 

 through it, it may easily be made to answer the purpose of an alarm. 



Clocks and watches are machines in which wheelwork is employed for 

 the measurement of time, being driven by a weight or by a spring, and 

 regulated by a pendulum or a balance. Watches differ from clocks, in 

 being portable, and this condition excludes the pendulum and the weight 

 from their construction. 



It is conjectured that the Saracens had clocks which were moved by 

 weights, as early as the eleventh century. J Trithemius mentions an orrery, 

 moved by a weight, and keeping time, which was sent, in 1232, by the 

 Sultan of Egypt, as a present to the Emperor Frederick II. Wallingf ord ? 

 in 1326, had made a clock which was regulated by a fly. The use of such 

 a fly in equalising motion depends on the resistance of the air, which 

 increases rapidly when the velocity is increased, and therefore prevents any 

 great inequality in the motion as long as the moving power varies but 

 little ; and if the action of the weight were transmitted with perfect regu- 

 larity by the wheels, and the specific gravity of the air remained unaltered 

 by pressure or by temperature, a fly clock might be a perfect machine, the 

 weight being always exactly counterbalanced by the resistance of the air, 

 attending a certain velocity of the fly ; and it might even be possible to 

 regulate the inequalities of the action of the weight, by causing the fly to 

 open and shut or to turn on an axis, by means of a spring, according to the 

 magnitude of the resistance. The unequal density of the air would how 

 ever still remain uncompensated ; and in this respect a liquid would be a 

 better medium than an elastic fluid. For experiments which are but of 

 short duration and which require great precision, a chronometer regulated 

 by a simple fly is still a useful instrument. Mr. Whitehurst's || apparatus 

 for measuring the time occupied in the descent of heavy bodies, is governed 



* See Derham, The Artificial Clockmaker, 1696, p. 85. 

 f Lampas, 4to, 1677, p. 42. 



J Beckmann, History of Inventions, 4 vols. translated by Johnstone, vol. i. 

 Epitome Conrardi Gesneri, p. 604. 

 || Ph. Tr. 1794, p. 2. 



