SCIENCE AND PRACTICE. 415 



(8.) The conductors and coverings being equal in both 

 cables, the times of charge, according to 10), are as the 

 squares of the lengths. 



With the help of these formulao it is easy to compare the 

 quantity and time of charge in any submarine line with 

 those of a given line. 



When calculating the speed of telegraph signals, another 

 condition, discovered by Faraday, steps in, viz., the formation 

 of electric- charge waves in the cable. 



As may be deduced from what has gone before, the charge 

 precedes the current in going to the extremity of the cable. 

 If the communication between the battery and the line be 

 interrupted before the current has reached the end, the 

 electricity diffuses itself and causes a deflection of the gal- 

 vanometer needle, although the battery at that moment may 

 not be in circuit. By reversing the" pole of the battery, 

 instead of cutting it off, that part of the line nearest to the 

 battery will be charged with the opposite electricity, while 

 the electricity already in the more distant part of the line 

 will discharge itself towards both ends of the cable, that is, 

 one part will pass through the instrument and the other 

 combine with, and be neutralised by, the opposite current. 

 An electric wave is therefore formed at the same time, which 

 is neutralised by degrees by the opposite one which follows 

 it, but which, nevertheless, reaches the end. In this manner 

 a succession of signals may be made to pass through a line 

 by rapidly changing the batteries, which will set in motion 

 the receiving instrument at the farther end, if they are of 

 sufficient power on arriving there. If the currents which 

 succeed each other are of the same strength and duration, 

 these waves will all be useful in long lines ; but if long and 

 short signals alternate, the latter will be more or less neutral- ' 

 ised by the former, and very short signals will be absorbed 

 to an extent which will render them powerless to move the 

 armature of the receiving instrument. 



106. The Wippe or Self-acting Make and Break. In order 

 to overcome the inconvenience and difficulty attending the 

 observation of angles by the sudden throw of the needle, Dr. 



