THE ELECTRIC TELEGRAPH. 



of the train in which he travels. If he count the number of 

 telegraph posts which pass his eye in two minutes, that number 

 will express in miles per hour the speed of the train. 



49. Since the current of electricity which flows along the wire 

 has always a tendency to pass by the shortest route possible to the 

 ground, it is evident that the supports of the wires upon these 

 posts ought to possess, in the highest attainable degree, the pro- 

 perty of insulation ; for even though the entire stream of electrical 

 fluid might not make its escape at any one support, yet if a little 

 escaped at one and a little at another, the current would, in a 

 long line, be soon so drained that what would remain would be 

 insufficient to produce those effects on which the efficiency of the 

 telegraph depends. Great precautions have therefore been taken, 

 and much scientific ingenuity has been expended in contriving 

 supports which shall possess, in the highest attainable degree, the 

 property of insulation. 



50. To each of these posts or poles are attached as many tubes or 

 rollers, or other forms of support, in porcelain or glass, as there are 

 wires to be supported. Each wire passes through a tube, or is 

 supported on a roller ; and the material of the tubes or rollers being 

 among the most perfect of the class of non-conducting substances, 

 the escape of the electricity at the points of contact is impeded. 



Notwithstanding various precautions of this kind, a considerable 

 escape of electricity still takes place in wet weather. The coat of 

 moisture which collects on the wire, its support, and the post, 

 being a conductor, carries away more or less of the fluid. Conse- 

 quently, more powerful batteries are necessary to give effect to the 

 telegraph in wet than in dry weather. 



In England, and on the Continent, the material hitherto used 

 for the supports of the wires is principally a sort of earthen or 

 stone ware. In the United States it is generally glass. 



51. The forms of these insulating supports are various. Tubes, 

 rings, collars, and double cones, are severally used. The material 

 used most commonly in England, a sort of brown stoneware, has 

 the advantage, besides being a good insulator, of throwing off wet, 

 as water falls from a duck's wing, leaving the surface dry. A pitcher 

 of this ware, plunged in water, scarcely retains any moisture upon it. 



52. The posts vary generally from 15 to 30 feet in height, the 

 lowest wire being about ten feet above the ground, except in 

 cases where greater height is required to allow vehicles to pass 

 under it, as when the wires cross a common road, or pass from 

 one side of the railway to the other. The poles are about 6 

 inches square at the top, and increase to 8 inches at the bottom. 

 In some cases they are impregnated with certain chemical solutions, 

 to preserve them from rotting, and are generally painted, the parts 



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