THE TELEGRAPH 
118 
inferior to the latter, on account of its being liable to the causes 
of damage we have just indicated. 
The iron stirrup does not, as has been asserted, endanger 
the insulation. The losses of current to the earth occur 
almost exclusively along the surface of the insulator on which 
atmospheric moisture has been deposited. The insulating pro¬ 
perty depends then on the length of that surface, and it may 
be increased either by increasing the height of the bell, or, as 
we shall presently see, by the addition of a double inside bell. 
The conditions arising from the use of an iron stirrup are 
sensibly the same as if the wire rested on the head of the 
insulator, and this rested on an iron bracket fixed inside, like 
that used in the English systems (fig. 53). 
3rd. The hook beiug sealed into the part of the insulator 
that is protected by the stirrup, it is very difficult to completely 
break the insulator, even on purpose ; when the lower part of 
the bell is broken, the hook continues to support the wire. In 
this case, the insulation is endangered in part ; but the wire, 
not being entangled with those below it, may still be used. The 
interruptions that necessarily occur under such circumstances, 
where other insulators are used, are almost always entirely 
avoided. 
The fixing of the hook in the insulator is effected by means 
of a cement made by mixing finely ground Plaster of Paris 
with water containing 7 per cent, of strong liquid glue. The 
great hardness acquired by this cement renders it possible to 
fix the hook without any hole having to be made for it in the 
porcelain. It is free also from the defect of the cement, made 
of sulphur and iron filings, that was formerly used, which was 
apt to burst the porcelain by its expansion. It is, besides, 
cheaper than the old cement. 
A proper degree of curving is allowed to the wires when they 
are stretched by apparatus placed at every thousand yards on 
insulating supports of a particular shape. 
Until lately the tension support was the only one used, 
either with double stretchers as in France, or with single 
stretchers. At the present time the mushroom insulator is 
generally used ; it is traversed by a bolt carried by two stays 
screwed to the post. It is, on account of the iron work, some¬ 
what higher in price, and requires more time in fitting than 
