A few pregnant sentences, which by themselves deserved the honour of permanently figuring in the ‘Instructions’ sent out by the War Office, completed the advice given by Mr. William Snow Harris in respect to the setting up of lightning conductors. ‘A building,’ he truly remarked, ‘may be struck and damaged by lightning without having a particle of metal in its construction. If there be metals in it, however, and they happen to be in such situations as will enable them to facilitate the progress of the electrical discharge, so far as they go, then the discharge will fall on them in preference to bodies offering more resistance, but not otherwise. If metallic substances be not present, or, if present, they happen to occupy places in which they cannot be of any use in helping on the discharge in the course it wants to go, then the electricity seizes upon other bodies, which lie in that course, or which can help it, however small their power of doing so, and in this attempt such bodies are commonly, but not always, shattered in pieces.’ He summed up as follows:—‘The great law of the discharge is, progress between the terminating planes of action--viz., the clouds and earth--and in such line or lines as, upon the whole, offer the least mechanical impediment or resistance to this operation, just as water, falling over the side of a hill in a rain storm, picks out, or selects as it were by the force of gravity, all the little furrows or channels which lie convenient to its course, and avoids those which do not. If in the case of lightning you provide, through the instrumentality of efficient conductors, a free and uninterrupted course for the electrical discharge, then it will follow that course without damage to the general structure; if you do not, then this irresistible agency will find a course for itself through the edifice in some line or lines of least resistance to it, and will shake all imperfect conducting matter in pieces in doing so. Moreover, it is to be especially remarked in this case, that the damage ensues, not where the metals are, but where they ceased to be continued; the more metal in a building, therefore, the better, more especially when connected by an uninterrupted circuit with any medium of communication with the earth.’