into the shortest and most open path. He has devised some very ingenious experiments for proving this position, and has been able to show the sixty-thousandth part of a discharge passing by a very narrow and roundabout path, when a broad and direct one was open, and traversed by the larger proportions of the discharge. He brought this part of his subject under the notice of the Academy of Sciences of Belgium in a special note, which was printed in their 'Bulletins' in 1875.

In the year 1875 the Meteorological Society of London was moved to follow the lead of the French meteorologists in reference to lightning conductors, and to appoint a Lightningrod Committee. From the report made to the Society by the Council in the following year, it appears that the objects contemplated in this action were an investigation and record of accidents from lightning, an enquiry into the principles ' involved in the protection of buildings, the diffusion of exact information regarding the best form and arrangement for lightning conductors, and the consideration of all phenomena 'connected with atmospheric electricity.' It is obvious that in its first conception this committee was intended to be essentially one of investigation and enquiry, and it was for this reason appropriately designated a Permanent Committee.' The meteorologists concerned in its inauguration were actuated by the same consideration that was present to the Section of Physics of the Academy of Sciences in Paris when the following paragraph of the instruction of 1854 was drawn

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up :

'One knows, it is true, a very great number of examples of people being killed or of houses being set on fire; one knows, also, many and diverse instances of metals fused, of timber shattered, of stones and even of walls thrown far away, and many other analogous effects; but what is generally wanting is precise measurements relative to distance, dimensions, the position of the object--both that which is struck and that which escaped. For it is necessary to know what the lightning spares, as well as what it strikes. It is the work of all observers, but especially of officers in the navy and artillery, of engineers, of professors, inventors, and architects, to test these phenomena at the moment they are produced, and to describe them accurately for the benefit of science, as well as that of public economy. Such descriptions, when they refer to a stroke of lightning, should as much as possible point out the track of the lightning from its highest to its lowest point; also they should show by sufficiently numerous horizontal sections the relative positions of all objects in a circle wide enough to take in those which have been struck.'

See Quarterly Journal of the Meteorological Society, vol. iii. p. 75.

In this passage the instruction of the French Academy no doubt touches the one point which is necessary before all else to improve, if not to perfect, the practice of electrical engineering, so far as this is aimed against the destructive powers of lightning. The broad principles upon which the engineer prosecutes his work are happily such as can be referred to actual experiments carried out by the artificial apparatus of the electrician. But there still remain some incidental questions, such as the influence of surface, extent, and form in conductors, the relation of conductivity to tenacity, the area of protection, and the maximum effect of lightning, which cannot be settled in this way, and which require an appeal to the larger operations of nature. This, however, concerns opportunities which cannot be arranged at will. The method of the appeal must of necessity be observational rather than experimental. It proceeds upon the lines of close watching and systematic record. Observations where the great operations of nature are concerned are utterly worthless unless they are made with scientific insight and precision. The plan of investigation that has to be pursued is therefore to collect an exact account of all accidents that occur, and to arrange a system of organisation which enables all such chance opportunities to be seized upon and improved by an immediate investigation of concomitant conditions and circumstances. This method of study also must be followed up by patient persistence for a considerable length of time, seeing that accidents from lightning occur at uncertain intervals, and that they are scattered capriciously over the greater part of the surface of the earth. It is for this reason, essentially, that a Lightning-rod Committee needs to sit in permanence.

The Committee of the Meteorological Society, however, seems very soon to have lost sight of its own excellent design, and to have changed its plan into a mere Conference for the preparation of a report, which was drawn up under its auspices and printed and published in 1882, apparently by the Conference itself, and which assumes the form of a code of rules for the erection of lightning conductors, with numerous appendices referring to authorities which had been in some sense consulted. The report is published under the editorship of the Secretary, and simply as having been considered. and adopted by the delegates of the Conference, who seem indeed to have concentred their attention upon one subordinate object which had been proposed by the Meteorological Society, namely, the diffusion of exact information regarding the best 'form and arrangement of lightning conductors,' and to have

overlooked entirely the more important work of observation and record which had been contemplated by the Society in the first instance, and to which we have drawn attention.

The code of rules put forward by the Conference was obviously intended to possess the same kind of authority and position as the instructions' of the earlier French reports, and indeed its chief value seems to be the approval it accords to the practice of construction which had grown out of those instructions, and which is very generally in use at the present day. It virtually confirms most of the conclusions which had been arrived at by the French commissions.

The Rules' of the London Conference direct that the main stem of the conductor shall consist of a copper rod or tape, with an ascertained electrical conductivity amounting to ninety per cent. of that which pure copper would possess, and weighing six ounces per foot; or that it shall be an iron rod weighing two pounds and a quarter per foot; and that the earth connexion shall be made by a copper or iron plate presenting a superficial area of eighteen square feet, embedded in moist earth, and surrounded with coke. The terminal points are to be more prominent than those usually adopted in England, but they may be less so than the heavy tiges of thirty-three feet employed in France. The rod is not to be insulated from the building, but intimately connected with all large masses of metal used incidentally in the construction. All joints in its length are to be embedded in solder. Curves are not to be made too sharp, and ample provision is to be secured for free expansion and contraction by varying temperature. Water-mains and gas-mains are to be utilised as means of earth contact wherever practicable, and the conducting integrity of the rod is to be tested every year.

A careful perusal of the French instructions, or of Mr. Richard Anderson's very excellent manual upon lightning conductors, published in 1879, will show that this is substantially an authoritative acceptance of the measures already advised by the best authorities. It is, however, somewhat remarkable that in the report itself of the London Conference nothing whatever is said of the influence of length in reducing the efficacy of a conductor. This is the more strange, because, in speaking of the care required for the formation of joints in the final decision of the Conference on controverted points,' the report categorically remarks that bad joints have the same effect as lengthening a conductor,' and a reference is incidentally made to one instance, in which a bad joint was found to have had the same effect on a discharge of electricity that

the lengthening of a conductor to 1,900 miles would have had. This nevertheless was a point that was perfectly understood by the French investigators, and it is obviously one in which the London code is behind its predecessors. In the first French instructions, issued in 1823, there is a paragraph which says:

'Among the conducting bodies there are none, however, which do not oppose some resistance to the passage of the electric force; this resistance to the passage, being repeated in every portion of the conductor, increases with its length, and may exceed that which would be offered by a worse, but shorter, conductor. Conductors of small diameter also

conduct worse than those of larger diameter.'

It follows, as a matter of absolute certainty from this increase of resistance with augmented length, that a conductor which was of ample dimensions for the protection of a building eighty feet high would not be of the same efficacy for a building 400 feet high. It is for this reason that M. Melsens employed eight main conductors for the Hôtel de Ville at Brussels, and it is for this reason that eight half-inch copper ropes have been carried down from the lantern and cupola in St. Paul's. To use eight main conductors of a given size is obviously, in an electrical sense, the same thing as to use one conductor only of eight times the size. The practice of the French engineers has hitherto been to double the sectional capacity of the rod for each additional eighty feet of the length that is to be protected by its instrumentality. This practice is a sound one, and certainly should be observed.

There is one other particular in reference to the Conference report to which it seems desirable to draw attention on account of the erroneous doctrine to which it may possibly give a sanction. Amongst the appendices which have been added to the report there is a table, obviously prepared at the cost of some labour, which professes to give the sizes of lightning conductors recommended by various authorities. In order to facilitate the comparison of the several sizes, all have been reduced to what has been termed the equivalent dimensions of copper. But the oversight has been made, in preparing this table, of treating all cases of galvanised iron as if the zinc in the combination had no other function than the protection of the iron from rust. In reality, however, a

The solid copper tape which is chiefly used by Mr. Anderson is, to meet the circumstance here alluded to, manufactured of four different sizes, the smallest being inch wide and inch thick, and the largest 1 inch wide and inch thick.

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galvanised iron rod conducts as a combination of iron and zinc, in which the zinc possesses a much higher conducting power than the iron. Zinc surpasses iron in this particular at least three times. All the statements of conductivity that have been drawn from galvanised iron conductors have hence been given much too low. The influence of a too powerful electrical discharge upon a conductor of galvanised iron is, in the first instance, to strip off its coating of zinc by melting this more readily fusible metal. But until this is done the zinc assists very materially in the transmission of the discharge. Practically it is known that galvanised iron ropes effectually transmit discharges which could not be safely carried by ungalvanised ropes of the same diameter. The table is on this account worthless for the purpose for which it was avowedly prepared. It attributes to several of the authorities which are named views on the matter of the size of lightning conductors which they would certainly not endorse. For instance, Mr. Preece, the eminent electrician, is represented as holding that copper wire with a sectional area of only the onehundredth part of a square inch is sufficient to serve as a lightning-rod for any house.' The authority upon which this startling statement is made is a passage in the Journal of the "Society of Telegraph Engineers,' in which Mr. Preece says that he thinks galvanised iron wire one-quarter of an inch in 'diameter is sufficient for the protection of any house.' It needs no very large amount of acquaintance with electrical matters to enable the reader to understand that Mr. Preece would not himself have expressed the same confidence in a small copper bell-wire such as is given as the equivalent in the table of the report. Taken in connexion with the omission of all reference to the increased resistance in long conductors, it might be inferred from this estimate that Mr. Preece would hold a small copper bell-wire, carried from the golden cross of St. Paul's to the ground, to be a sufficient protection for the great metropolitan cathedral.

In his Notes et Commentaires sur la question des Paratonnerres,' printed in 1882, Professor Melsens complains that no notice of his system of numerous conductors of weak or small section has been taken in the code of laws of the Lightning-rod Conference of London, even as a possible alternative of construction, a silence which he interprets as equivalent to a formal condemnation. His own words are:'Quoi qu'il en soit, j'ai cru que le silence que la Conférence garde dans son code de loi sur l'application possible de mon système, équivalait à une condamnation; j'aurais été heureux de voir la Conférence