=The Outer Shell only of a Lightning Rod Acts as a Conductor.=--As a consequence of self-induction or electrical inertia, Professor Lodge contends that a lightning discharge in a conductor consists of a series of oscillations. These oscillations follow one another with extraordinary rapidity--there may be a hundred thousand in a second, there may be a million. Now it has been shown that, when a current starts in a conductor, it does not start at once all through its section; it begins on the outside, and then gradually, but rapidly, penetrates to the interior. From this he infers that the extremely rapid oscillations of a lightning discharge have not time to penetrate to the interior of a conductor. The electricity keeps surging to and fro in the superficial layer or outer shell, while the interior substance of the rod remains inert and takes no part in the action. A conductor, therefore, will be most efficient for carrying off a flash of lightning if it present the greatest possible amount of surface; a thin, flat tape will be more efficient than a rod of the same mass; and a number of detached wires more efficient than a solid cylinder. As for existing lightning conductors, the greater part of their mass would, in many cases, have no efficacy whatever in carrying off a flash of lightning. =The Discussion.=--The discussion at the meeting of the British Association was opened by Mr. William H. Preece, F.R.S., Electrician to the Post Office, who claimed to have 500,000 lightning conductors under his control. He expressed his conviction that a lightning rod, properly erected and duly maintained, was a perfect protection against injury from lightning; and in support of this conviction he urged very strongly the report of the Lightning Rod Conference. This report represented the mature judgment of the most eminent scientific men, who had devoted years to the study of the question; and he wished particularly to bring before the meeting their clear and decisive assertion--an assertion he was there to defend--that “there is no authentic case on record where a properly constructed conductor failed to do its duty.”