WILLIAM SIEMENS, />\K.S. IIQ 



clock, supposed to have been made in 1348 ; it has the verge 

 escapement which is said to have been in use before the pendulum. 

 The methods employed in modern clocks and watches for com- 

 pensating for variation of the thermometer and barometer, are 

 illustrated by numerous exhibits, notably the Astronomical Clock, 

 with Sir George Airy's compensation, which will form the subject 

 of a special demonstration by Messrs. Dent and Co. 



The measurement of small increments of time has been rendered 

 possible only in our own days by the introduction of the conical 

 pendulum, and other apparatus of uniform rotation, which 

 alone conveys to our minds the true conception of the continuity 

 of time. Among the exhibits belonging to this class, must be 

 mentioned Sir Charles Wheatstone's rotating mirror, moved by a 

 constant falling weight, by which he made his early determination 

 of the velocity of electricity through metallic conductors ; the 

 rotative cylindrical mirror, marked by successive electrical dis- 

 charges, which was employed by Dr. Werner Siemens in 1846, to 

 measure the velocity of projectiles, and has been lately applied by 

 him for the measurement of the velocity of the electric current 

 itself, and the Chronometric Governor, introduced by him in 

 conjunction with myself, for regulating chronographs, as also the 

 velocity of steam engines under their varying loads ; Foucault's 

 Governor, and a considerable variety involving similar principles 

 of action. 



Another entity which presents itself for measurement is, sixthly, 

 that of Velocity, or distance traversed in a unit of time, which 

 may either be uniform or one influenced by a continuance of the 

 cause of motion, resulting in acceleration, subject to laws and 

 measurements applicable both in relation to celestial and terrestrial 

 bodies. I may here mention the instruments latterly devised for 

 measuring the acceleration of a cannon ball before and after 

 leaving the mouth of the gun, of which an early example has been 

 placed within these galleries. Other measurers of velocity are to 

 be found here, ships' logs, current meters, and anemometers. 



In combining the ideas of weight or pressure with space, we 

 arrive at, seventhly, the conception of work, the unit of which is 

 the foot-pound, or kilogrammetre, and which, when combined with 

 time, leads us to the further conception of the performance of 

 duty, the horse-power as defined by Watt. The machines for the 



