, 



HISTORY AND PROGRESS. 189 



bent in a dish form. The buckled plate a a is secured by 

 four bolts to the socket b b a cast-iron cylindrical tube 7 

 feet long and 4 inches outside diameter. Near the top, 

 inside, the socket is furnished with a flange, upon which the 

 bottom of the upper or main-post, as it is called, rests. This 

 upper post, c c, is of wrought iron with welded joints ; it stands 

 12 feet high out of the socket, and is somewhat conical. 

 At its upper end an iron ring is welded in to carry an iron 

 rod, d, 20 inches long, forming the lightning guard. The 

 stretching posts are of the same height, but of larger dia- 

 meter and stronger than the ordinary ones. 



Mr. Siemens' post derives much of its merit from the role 

 played by the buckled plate at the bottom. These buckled 

 plates are things of great engineering utility. They are 

 squares of sheet-iron, which Mr. Mallet by a simple process 

 presses into a form very slightly different, but endows them 

 with a strength immensely superior \ so strong indeed are they 

 that if one of these posts were pulled up bodily out of the 

 ground, it would bring up the superincumbent ground with 

 it, and the buckled plate would not be deranged unless the 

 bolts gave way ; while the same piece of iron, as a simple 

 sheet, would bend' under a much less weight, offering no 

 resistance worth speaking of against the strain. 



There can be little doubt that, in course of time, only metal 

 posts will be employed, on account of their superior durability, 

 solidity, and freedom from damage by accidents. In some 

 climates wooden posts require to be renewed every two or 

 three years, and, in the most favourable, rarely last over six 

 years ; while an iron telegraph post is as durable as a lamp- 

 post, and would certainly last ten times as long, and not cost 

 five times as much as a wooden one ; so that in the end an 

 immense saving would be effected by their employment, 

 although the first cost is so much greater. 



103. Line Insulators. Cook's insulator was the first used 

 in England. It consisted of a body of earthenware the size 

 and form of an egg, slightly flattened at the ends : the wire 

 was passed through a hole in its longer axis. 



Bright's insulator, used by the Magnetic Company, consists 



