CHAP. vi.l- ELECTRIC HOROLOGY. 64' 



valuable and practical mode of communicating time audibly over an 

 extended area, due allowance being made for the rate at which sound 

 travels, and the position of the observer as regards direct distance 

 from the gun. Sound travels at the rate of a little under a quarter of 

 a mile in a second. From the rapidity of the motion of light, the 

 flash of the Edinburgh time gun can be seen from the shipping in the 

 Firth of Forth, and true time indicated long before the report of the 

 discharge of the gun has reached the ear of the observer. 



The Greenwich hourly time-currents over London are distributed 

 chiefly by wires in connection with the lines of the South-Eastern 

 Eailway Company, the elaborate and delicate adjustments for which 

 are entrusted to Mr. Charles V. Walker, F.R.S., to whom practically 

 the vast metropolis of London has to look for the daily accuracy of 

 her chronometrical arrangements and time measurements. Electric 

 time-signal systems are now very extensively employed in the. 

 United States of America, the continent of Europe, and also in some 

 of our Colonies. 



IV. CHRONOGRAPHS AND CHRONOSCOPES. 



Another use that has been made of the property possessed by 

 electricity of propagating itself almost instantaneously, is to measure 

 with precision very short intervals of time ; for example, to measure 

 the time which artillery projectiles take to clear the distance between 

 the mouth of the cannon and the object struck. Instruments con- 

 structed for this purpose are called chronoscopes or chronographs, the 

 second of the names being particularly reserved for those which 

 register this interval and preserve a written mark of it. 



Again, the name of Wheatstone presents itself in the first inven- 

 tion of this ingenious application of electricity. The chronoscope 

 which he invented in 1840 was at first arranged in the following 

 manner. 



At the firing- station A, Fig. 424, is fixed a time-keeping apparatus 

 c, having a weight for its motor, and capable of giving on two distinct 

 dials, E D, the lOths and l,GOOths of a second. An electro-magnet 

 placed behind the box containing the wheelwork is provided with an 

 armature, which is attracted when the 'current of the battery passes, 



