DR. C. W. SIEMENS* ADDRESS. 211 



ployed in modern clocks and watches for compensating 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 

 convey 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 discharges, which was em- 

 ployed 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 regu- 

 lating 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 denned by Watt. The machines for the measurement of 

 work, here exhibited, are not numerous, but are interesting. Among 



