The telegraph manual: a complete history and description of the semaphoric, electric and magnetic telegraphs of Europe, Asia, Africa, and America, ancient and modern.

AMERICAN INSULATORS. 547
is anti-porous, is vitrified throughout, and is considered as perfect for insulating purposes as glass. It is very hard and difficult to be broken, and it has resisted bullets and other missles
thrown at it by mischievous persons. The form adopted has been
regarded as advantageous in preventing the gathering of lines
of water. The corrugations separate the watery accumulations.
Fig, 41.
Fig. 42. Fig. 43.
3q