Mr. Snow Harris then went on philosophising. ‘Whenever,’ he said, ‘the peculiar agency—whatever it be—active in this operation of nature, and characterised by the general term electricity or electric fluid, is confined to substances which are found to resist its progress, such, for example, as air, glass, resinous bodies, dry wood, stones, &c., then an explosive form of action is the result, attended by such an evolution of light and heat, and by such an enormous expansive force, that the most compact and massive bodies are rent in pieces, and inflammable matter ignited. Nothing appears to stand against it: granite rocks are split open, oak and other trees of enormous size rent in shivers, and masonry of every kind frequently laid in ruins. The lower masts of ships of the line, 3 feet in diameter and 110 feet long, bound with hoops of iron half an inch thick and five inches wide, the whole weighing about 18 tons, have been in many instances torn asunder, and the hoops of iron burst open and scattered on the decks. It is, in fact, this terrible expansive power which we have to dread in cases of buildings struck by lightning, rather than the actual heat attendant on the discharge itself.’
He continued: ‘When, however, the electrical agency is confined to bodies, such as the metals, and which are found to oppose but small resistance to its progress, then this violent expansive or disruptive action is either greatly reduced or avoided altogether; the explosive form of action we term lightning vanishes, and becomes, as it were, transformed into a sort of continuous current action of a comparatively quiescent kind, which, if the metallic substance it traverses be of certain known dimensions, will not be productive of any damage to the metal; if, however, it be of small capacity—as in the case of a small wire—it may become heated and fused; in this case the electrical agency, as before, is so resisted in its course as to admit of its taking on a greater or less degree of explosive and heating effect, as in the former case. It is to be here observed, that all kinds of matter oppose some resistance to the progress of what is termed the electrical discharge, but the resistance through capacious metallic bodies is comparatively so small as to admit of being neglected under ordinary circumstances; hence it is, that such bodies have been termed conductors of electricity, whilst bodies such as air, glass, &c., which are found to oppose very considerable resistance to electrical action, are placed at the opposite extremity of the scale, and termed non-conductors or insulators. The resistance of a metallic copper wire to an ordinary electrical discharge from a battery was found so small, that the shock traversed the wire at the rate of 576,000 miles in a second. The resistance, however, through a metallic line of conduction, small as it be, increases with the length, and diminishes with the area of the section of the conductor, or as the quantity of metal increases.’
After these theoretical explanations, Mr. Snow Harris went into the practical part of the business of protecting buildings, and, more especially, powder magazines and others containing explosive materials, against the effects of lightning. ‘It follows,’ he remarked,’from these established facts, that if a building were metallic in all its parts, an iron magazine for example, then no damage could possibly arise to it from any stroke of lightning which has come within the experience of mankind. A man in armour is safe from damage by lightning. In fact, from the instant the electrical discharge, in breaking with disruptive and explosive violence through the resisting air, seizes upon the mass in any point of it, from that instant the explosive action vanishes, and the forces in operation are neutralised upon the terminating planes of action—viz., the surface of the earth and opposed clouds. All this plainly teaches us that, in order to guard a building effectually against damage by lightning, we must endeavour to bring the general structure, as nearly as may be, into that passive or non-resisting state it would assume, supposing the whole were a mass of metal. To this end, one or more conducting channels of copper, depending upon the magnitude and extent of the building, should be systematically applied to the walls. These conducting channels should consist either of double copper plates, united in series one over the other, as in the method of fixing such conductors to the masts of her Majesty’s ships, the plates being not less than 3½ inches wide, and of 1/16th and ⅛th of an inch in thickness; or the conductors may with advantage be constructed of stout copper pipe, not less than 1/16th of an inch thick, and 1½ to 2 inches in diameter; in either case the conductors should be securely fixed to the walls of the building, either by braces, or copper nails, or clamps. They should terminate in solid metal rods above, projecting freely into the air, at a moderate and convenient height above the point to which they are fixed, and below they should terminate in one or two branches leading outward about a foot under the surface of the earth; if possible, they should be connected with a spring of water or other moist ground. It would be proper, in certain dry situations, to lead out, in several directions under the ground, old iron or other metallic chains, so as to expose a large extent of metallic contact in the surface of the earth.’
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.’
‘Such is, in fact,’ he concluded, ‘the great condition to be satisfied in the application of lightning conductors, which is virtually nothing more than the perfecting a line or lines of small resistance in given directions, less than the resistance in any other lines in the building, which can be assigned in any other direction, and in which, by a law of nature, the electrical agency will move in preference to any others. The popular objections to lightning conductors on the ground that they invite lightning to the building, that we do not know the quantity of electricity in the clouds, and that hence they may cause destruction, are now quite untenable, and have only arisen out of a want of knowledge of the nature of electrical action. What should we think of a person objecting to the use of gutters and rain-pipes for a house, on the ground of their attracting or inviting a flow of water upon the building; and since we do not know the amount of rain in the clouds, it is possible that the building may be thereby inundated,—yet such is virtually the argument against lightning conductors.’
Mr. Snow Harris, as already mentioned, received the honour of knighthood in 1847; and after this date lived in comparative retirement for twenty years at his residence, Windsor Villas, Plymouth. However, he was called upon, in 1855, to undertake one more important work in designing a perfect system of lightning conductors for the new Houses of Parliament at Westminster. It was on the initiative of Sir Charles Barry, the architect, that the proposal was made by the Board of Works to Sir William Snow Harris, who accepted it with all his old eagerness for serving the cause of lightning protection. Accordingly, he drew up a plan, which he himself characterised, in a letter to the President of the Board of Works, dated February 14, 1855, as ‘somewhat costly,’ but which he felt sure would be absolutely certain ‘for insuring the safety of the buildings against one of the most terribly destructive elements of nature.’ In its essence, the plan consisted in protecting all the most elevated parts of the Houses of Parliament, including the towers, by ‘a capacious metallic conductor of copper tube, two inches in diameter, and not less than one-eighth of an inch in thickness,’ to be fastened together ‘by solid screw plugs and coupling pieces,’ and ‘secured to the masonry by efficient metallic staples.’ To do this, Sir William Snow Harris calculated, would involve an expenditure of somewhat over 2,000l., but nothing less would accomplish it. ‘What I have recommended,’ he wound up his letter, ‘has been the result of very serious and attentive deliberation, and I conscientiously think that what I have proposed is absolutely requisite to a permanent and satisfactory security of the buildings against the destructive agency of lightning.’ The Board of Works entirely adopted all the recommendations of Sir William Snow Harris, and, in accordance with them, there was included in the Civil Service Estimates laid before the House of Commons in the session of 1855 a vote of 2,314l., on account of ‘works necessary for securing the new Houses of Parliament against danger from lightning.’
The vote passed without demur. It was in the height of the Crimean War fever, political questions absorbing all others. Perhaps in a time of less excitement some voice might have been raised in the House of Commons asking whether it was wise to spend over 2,000l. in putting up lightning conductors, without previously ascertaining, from the best scientific authorities, that the system adopted was the best, and absolutely efficacious. The strongly recommended ‘copper tubes,’ with their ‘screw plugs and coupling pieces,’ were at least a novelty, not having stood the test of experience, and there were practical men who shook their heads when they heard of them. However, with war discussions raging fiercely, and reports of battles and sieges absorbing all attention, the House of Commons had no time to bestow upon such trifling matters as that involved in the plans of Sir William Snow Harris; and thus the vote passed unchallenged. Perhaps silent repentance came afterwards to the official mind. At any rate, as it was the first, so it was the last time of Parliament granting money for lightning conductors.
