MECHANICS AND USEFUL ARTS. 109 



THE OCEAN TELEGRAPH. 



Expert operators are able to transmit from 15 to 20 words per 

 minute through the Atlantic cable. The velocity with which a 

 current or impulse will pass through the cable has been ascer- 

 tained to be between 7,000 and 8,000 miles per second ; the for- 

 mer being the velocity when the earth forms a part of the circuit, 

 and the latter when it does not. 



FRENCH MILITARY TELEGRAPHS. 



The following are details of the military electric-telegraphic 

 apparatus used in the experiments in the camp at Chalons, last 

 summer : Electric Telegraph. — For military purposes it is desirable 

 that the apparatus should not only be simple in itself, but should 

 be capable of being used in connection with the permanent lines 

 of telegraph already established. Keeping these ends in view, a 

 modification of Morse's recorder, constructed by M. Duguy, from 

 designs furnished by the Bureau des Telegraphic, and known as 

 Le Poste Militaire, was adopted and found to answer well. This 

 apparatus is contained in a box, to the bottom of which it is at- 

 tached by^ slides. The manipulator is placed on the right of the 

 small shelf supporting the recorder ; on the left are the galva- 

 nometer to show the strength of the current, and a paratonnere to 

 protect the operator from the shock of unforeseen accumulations 

 of electricity in the wires in stormy weather. The sides and 

 front of the box fold down, so as to permit of the instrument 

 being used without necessitating its removal from its case. The 

 connection between the stations was kept up partly by wires, 

 partly by a cable laid along the ground. Wires. — These were of 

 copper, 1.6 mil. in diameter, weighing 22.5 kilog., and costing 

 about 100 francs per kilom. This wire proved an excellent con- 

 ductor, and, with care, could be used with intervals of 200 and 

 300m., or even more, between the supports. Cables. — Several 

 kinds were tried. In the- last experiments the cable was formed 

 of a core of 5 annealed copper wires, bound round with white 

 cotton thread, over which was a coating of gutta-percha, and 

 then a layer of oakum, the whole being bound round twice with 

 cotton tape steeped in vulcanized India-rubber. It weighed 35 

 kilog., and cost 320 francs per kilom. It was perfectly insulated, 

 and a good conductor. When laid along the ground it sufiered 

 little from wheels and the feet of horses passing and repassing 

 over it. But it had serious defects. It was rather too large in 

 its diameter, and very ^\eak, stretching sufficiently to injure the 

 core with a strain of 30 kilog. The wires of the core were so 

 fine as to be frequ(Mitly cui tlirough in removing the covering for 

 the purpose of splicing. Supports. — The wires were supported 

 on light staves called lances, 3m. 80c. in length, 200 of which 

 made a military wagon load. They were sunk 12 inches in the 

 ground, and weighed up with wooden pickets. Wliere the line 

 made an angle, the lances were strengthened with guy-ropes, 



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