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ELECTRICITY FOR BLASTING 



by the full lines at the top of the dial, and, in doing so, it encounters the pin and 

 forces the hand into the vertical position, and sets the clock to true time, providing 

 the signal conies from a standard clock, or is sent by hand at true time. A dial 

 of moderate character keeps so near to time that once or twice a day would be, for 

 all common purposes, often enough to correct it. 



Fig. 777 is an arrangement of Bain's, by which a principal clock, showing seconds, 



\ 



sends electric currents at minute intervals to other clocks, and causes the hands to 

 move minute by minute. A is a voltaic battery ; B is the principal clock, which may 

 be an electric clock or not at pleasure; G and H are two out of many subordinate 

 clocks. The seconds hand of the principal clock completes a voltaic circuit twice 

 (for the case of two clocks) during the minute ; at the thirty seconds for the clock o, 

 and at the sixty seconds for the clock H. The clock H shows time in leaps from one 

 minute to the next ; and the clock G from one half minute to the next half minute. 

 As many more contacts per minute may be provided for the seconds-hand of the 

 prime clock as there are subordinate clocks. 



Next akin to the time signals above described, and which act automatically upon 

 clocks, either to drive the clock-train or to correct the clock errors, are mere time 

 signals, which are extensively distributed throughout the country by the ordinary tele- 

 graph wires, and are looked for at the various telegraph stations, in order to compare 

 the office dials with Greenwich mean time, and to make the necessary correction ; 

 they are also re-distributed by hand the moment they appear, through sub-districts 

 branching from junction-stations. Large black balls, hoisted in conspicuous stations, 

 are also dropped daily by electric currents in various places, for the general infor- 

 mation of the public, or of the captains of ships. C. V. W. 



ELECTRICITY FOR BLASTING- in Mines and Quarries. Professor Hare 

 was the first who entertained this idea, but Mr. Martin Roberts devised the following 

 process. In order not to be called upon to make afresh a new apparatus for each explosion, 

 Mr. Roberts invented cartridges, which may be constructed beforehand. With this view, 

 two copper- wires are procured, about a tenth of an inch in diameter, and three yards 

 in length, well covered with silk or cotton tarred, so that their insulation may be 

 very good. They are twisted together (jig. 778) for a length of six inches, care being 

 taken to leave their lower extremities free, for a length of about half an inch (sepa- 

 rating them about half an inch), from which the insulating envelope is removed, in 

 order to stretch between them a fine iron wire, after having taken the precaution of 

 cleaning them well. The upper extremities of the two copper- wires are likewise 

 separated, in order to allow of their being placed respectively in communication with 

 the conductors, that abut upon the poles of a pile. The body of the cartridges is a 

 tin tube, three inches long, and three quarters of an inch in diameter, the solderings 

 of which are very well made, in order that it may be perfectly impermeable to water. 

 A glass tube might equally well bo employed, were it not for its fragility, which h;is 

 caused a tin tube to be preferred. The system of copper-wires is introduced into the 

 tube, fixing them by means of a stem that traverses it at such a height that the fine 

 iron wire is situated in the middle of the tin tube, so arranged that the ends of the 

 copper- wire do not anywhere touch the sides of the tube (Jig. 779). -The cork is firmly 

 fixed at the upper extremity of the tube with a good cement. Mr. Roberts recom- 



