CHAP, vi.] ELECTRIC HOROLOGY. 649 



degree these causes of error by employing in the beginning a current 

 of very feeble intensity, and by so arranging the wires of the circuit 

 that at the instant of the impact on the target a much stronger 

 batteiy should be put into action to close the circuit and give the 

 desired motion to the armature. 



M. Hipp has also modified Wheatstone's chrorioscope by making 

 the motions of the clock and the indicating needles independent, so 

 that whether the latter is at rest or no, the former continues going. 

 The needles only move during the time of flight of the projectile. 



We can only mention further : M. Pouillet's chronoscope, which 

 was founded on the amount of deviation which a current of known 

 intensity can give to the needle of a galvanometer during the time the 

 current is passing ; the chronograph of Messrs. Breguet and Con- 

 stantinoff, which consists of a revolving cylinder, on the surface of 

 which two pens, maintained by electro-magnets, trace in succession a 

 line, when the projectile breaks two wires, at the time of departure 

 and arrival, and so interrupts the circuits, and the position of the 

 lines traced on the cylinder indicates what fraction of a turn the latter 

 has made during the transit of the projectile ; the chronographs of 

 Captain Noble and Captain Navez, which have been used with 

 success in numerous experiments on projectiles in this country, 

 Belgium and Holland; the chronographs of M. Martin de Brettes 

 and Boulanger, by means of which the initial velocity of a projectile 

 and its velocity at any point of its path can be measured ; and lastly, 

 the levelling chronoscope of M. Breguet. Space fails for a detailed 

 description of these ingenious and useful apparatus, of which we 

 have not even mentioned the whole. It is enough for our purpose 

 to have explained by an example the possibility of making use of 

 electricity for the precise measurement of elements so difficult to 

 determine as those connected with projectiles. Wheatstone has 

 applied chronographic methods to the study, and proof of the laws^ 

 of falling bodies. 



Chronographs are also in daily use in astronomical observatories. 

 A barrel which rotates and travels along at an even rate receives a 

 puncture every second from a pointer or pen in electrical connection 

 with the sidereal clock. In this manner a spiral line of dots is traced 

 on the paper covering the barrel, and the commencement of each 

 minute is marked by the absence of the dot. An observer at any of 



