A very common, and, it may be added, a very mischievous opinion is prevalent, that lightning conductors should be carefully insulated from the buildings to which they are attached, and consequently many conductors are made to pass through insulators of glass and other materials of low conductivity. This practice of separating the building from the lightning conductor is not only utterly useless but positively dangerous. It is not unusually thought that by insulating the conductor the electric discharge will be prevented from entering the building. Such an idea is _ipso facto_ absurd, for it is preposterous to suppose that a flash of lightning which can travel through thousands of feet of air--itself a very bad conductor of electricity--and then shatter to pieces the most compact bodies, would be stopped in its course by means of a few inches of glass, or a few feet of air. It may therefore be confidently asserted that no insulator can possibly be made that would be capable of preventing the electric discharge leaving the lightning conductor provided it could find an easier path leading to the earth. Mr. Phin, in his work on ‘Lightning-Rods’ says very pertinently:--But not only are insulators worthless--they are positively dangerous if the principle upon which they are adopted is fully carried out, which, however, is but rarely done. A little consideration will show this. Thus, if a house be furnished with a carefully-insulated lightning-rod, and should also have any large surface of metal, such as a tin roof, an extensive system of gutters, or such like, connected with it, it is easy to see that the house must resemble a large Leyden jar, of which the tin roof, or other mass of metal, constitutes one coating, and the lightning-rod and the earth constitute the other, while the insulators and the dry material of the house represent the glass of the jar. If both the outside and the inside of this jar (the tin roof and the earth) had been connected together, it would have been impossible to have brought one coating into a condition opposite to that of the other. But the rod being carefully insulated from the roof, it is obvious that the inductive action of the cloud will bring the roof and the earth into opposite conditions; and if a man were to form the path of least resistance between them, the discharge would take place through his body, and he would probably be destroyed. It is obvious, then, in the first place, that lightning-rods should be connected with all large masses of metal which may exist in or upon the house, such as metallic roofs, tin or iron gutters, or pipes, iron railings, &c. In the second place, the rod should be attached to the house in the neatest and least obtrusive manner possible.’ [Illustration: Fig. 33.]