In its essence there cannot be anything more elementary than the theory of protection against lightning. It is simply to lay a metallic line from the top of a building, or other object to be protected, into moist ground, so as to make a path for the electric force, along which, not finding impediments, it will travel freely, without causing the least damage. But, like many other simple theories, their practical execution is not without perplexities. The first of these, in regard to conductors, arises from the existence of more or less considerable quantities of metals, to be found in almost every building which requires protection against lightning. As the use of metals, especially iron, in the construction of dwellings, both exterior and interior, is rapidly extending, this becomes a very important consideration in planning the design of lightning conductors. Of equal moment is a second point--that of the existence of water or great moisture under the buildings, or part of them. This must decide invariably the direction of the conductor towards the earth, and its depth underground. There are many minor matters to be taken into account, but these two may be laid down as the chief questions to be kept in view in settling the best mode of application of any conductors under given circumstances. It happens often enough that a proper solution as to what is best is not a little difficult. Still, it can always be arrived at by careful study, which must, however, be aided by experience.