HISTORY AND PROGRESS. 



191 



being cemented in ; sometimes the porcelain cup is replaced 

 by a cup of vulcanite. These insulators are a little heavy, 

 but their superior solidity and insulation is ample compen- 

 sation, the iron cap forming at once a perfect protection 

 against injury and a screen against the deposit of dew on 

 the porcelain. 



In 1856 Mr. Clark patented an insulator in which he 

 increased the length of surface of the porcelain over which 

 the current escapes, without increasing its section. He 



attained this by a double bell 

 insulator is supported by a 

 stalk, D, Fig. 101, cemented 

 into the interior of the inner 

 cavity ; the line- wire is car- 

 ried through a deep groove 

 on the top, and is tied to the 

 bell by a binding wire. 



Lieut.-Col. Chauvin, Direc- 

 tor of Prussian Telegraphs, 

 has adopted this style of insu- 

 lation for many of the lines 

 under his charge. He has 

 also made numerous experi- 

 ments on the most favourable 

 proportions between the length 



formed in one piece. The 



and section of the 



cups, 



and 



has given them a form differ- 

 ing slightly from Mr. Clark's 

 only in an increased depth 

 and narrowness of the inner Flg> lolt 



cavity, by which the deposit of dampness from the atmo- 

 sphere is still further guarded against, as well as the sudden 

 cooling of the insulator bell. 



Lieut.-Col. Chauvin has also constructed an insulator for 

 attaching to the stems of living trees when these are used in- 

 stead of posts for the support of telegraph lines. The insulator 

 is hung upon a hook, free to swing about ; and the stalk, or 

 wire-carrier, bent in a curve away from the stem of the tree, 



