The explosion of the electric fluid in the atmosphere Lightning Conductors, are pointed metallic rods fixed to the upper parts of buildings, to secure them from strokes of lightning. They were invented and proposed by Dr Franklin for this purpose, soon after the identity of electricity and lightning was ascertained; and they exhibit a very important and useful application of modern discoveries in this science. This ingenious philosopher having found that pointed bodies are better fitted for receiving and throwing off the electric fire than such as are terminated by blunt ends or flat surfaces, and that metals are the readiest and best conductors, soon discovered that lightning and electricity resembled each other in this and other distinguishing properties; he therefore recommended a pointed metallic rod to be raised some feet above the highest part of a building, and to be continued 'down into the ground, or the nearest water. The lightning, should it ever come within a certain distance of this rod or wire, would be attracted by it, and pass through it in preference to any other part of the building, and be conveyed into the earth or water, and there dissipated without doing any damage to the building. Many facts have occurred to prove the utility of this seemingly trifling apparatus.

Some electricians have objected to the pointed termination of this conductor, preferring rather a blunt end, on the supposition that a point invites the electricity from the clouds, and attracts it a greater distance than a blunt conductor.

Although the application of lightning conductors to buildings on shore is always judicious, and their advantages very apparent, yet on ship-board, where the effects of lightning are most to be dreaded, the introduction of this means of defence has been slow and imperfect. The conductor hitherto employed at sea consists of long flexible chains or links of metal, about a quarter of an inch thick, sometimes of iron; those employed in the British navy are however of copper; they are usually packed in a box, and are intended to be set up from the mast-head to the sea when occasions require, so that, as observed by Mr. Linger in his excellent work on electricity, partly from inattention, and partly from prejudice, they frequently remain in the ship's hold during long and hazardous voyages quite unemployed; a remark, the truth of which is but too frequently verified in the damage so constantly happening at sea during lightning storms. The necessity of providing the best possible security against the effects of lightning on ship board has been long admitted; but continuous and fixed metallic rods have been deemed inapplicable to ships in consequence of their masts (the only parts to which they can be attached) being exposed to chances of injury, to motion in a variety of ways, to frequent elongation and contraction, and to the necessity which frequently arises for removing the higher masts altogether and placing them on deck.

It was probably from these causes that the small flexible chains or links above mentioned were employed. Such conductors, however, will probably, on examination, be found less applicable than fixed continuous lines of metal, and, in every point of view, inefficient substitutes for them. Their great want of continuity, as well as their want of mass and surface, is very unfavourable to the transmission of severe explosions, the electric matter becoming sensible at the points of junction, as is evident by the sparks which appear upon them at the time of the discharge, so that in some instances they have been actually disunited; they are likewise objectionable as being liable to every species of injury incident to a ship's rigging and much difficulty is experienced in keeping them in their position and unbroken, more especially during gales of wind, and at night, when the ship is under sail, and when it is perhaps required, as is already observed, to remove some portion of the higher masts. It has therefore been long considered desirable to apply, if possible, a permanent conductor, which should be always in its place and ready for action; and various attempts have been made, and suggestions advanced at different times, to apply fixed lightning conductors in ships, as the subject, from time to time, has demanded further consideration.

To protect a ship effectually from damage by lightning, it is essential that the conductor be as continuous and as direct as possible, from the highest point to the sea, that it be permanently fixed in the masts throughout their whole extent, so as to admit of the motion of one portion of the mast upon another; and in case of the removal of any part of the mast, together with the conductor attached to it, either from accident or design, the remaining portion should still be perfect and equivalent to transmit an electrical discharge. To fulfil these conditions, Mr. W. J. Harris, of the Plymouth Institution (to whose valuable paper on this subject, contained in a recent number of Jameson's Edinburgh Journal, we are indebted to the previous remarks), has recommended pieces of sheet-copper, from one-eighth to one-sixteenth of an inch thick, and about two feet long, and varying from six inches to one inch and a half in breadth, be inserted into the masts in two laminae, one over the other; the butts or joints of the one being covered by the central portions of the other.

The laminae should be rivetted together at the butts, so as to form a long elastic continuous line; the whole conductor to be inserted under the edges of a neat groove, ploughed longitudinally in the aft side of the different masts, and secured in its position by wrought copper nails, so as to present a fair surface. The metallic line thus constructed will then pass downward from the copper spindle at the mast-head, along the aft sides of the royal mast and top-gallant mast, being connected in its course with the copper about the sheeve holes. A copper lining in the aft side of the cap, through which the top-mast slides, now takes up the connexion, and continues it over the cap to the aft side of the top-mast, and so on as before, to the step of the mast; here it meets a thick wide copper lining, turned round the step under the heel of the mast, and resting on a similar layer of copper fixed to the keelson. This last is connected with some of the keels on bolts, and with three perpendicular bolts of copper, of two inches diameter, which are driven into the main keel upon three transverse or horizontal bolts, brought into immediate contact with the copper expanded over the bottom.

The laminae of copper are turned over the respective mast heads, and secured about an inch or more down on the opposite side; the cap which corresponds is prepared in a somewhat similar way, the copper being continued from the lining in the aft part of the round hole over the cap, into the fore part of the square one, where it is turned down and secured as before, so that when the cap is in its place the contact is complete. In this way we have, under all circumstances, a continuous metallic line from the highest point to, the sea, which will transmit the electric matter directly through the keel, being the line of least resistance. But since the main mast does not step on the keelson, it will be necessary to have a metallic communication at the step of the mast, with the perpendicular stancheon immediately under it, and so on to the keelson as before, or otherwise carry the conductor out at the sides of the vessel.

From what has been already observed, it will be apparent that in whatever position we suppose the sliding masts to be placed, whether in a state of elongation or contraction, still the line of conduction will remain perfect; for that part of the conductor which necessarily remains below the cap and top when the sliding masts are struck, is no longer in the line of action; consequently its influence need not be considered.