Having yesterday at your Request view’d the Magazines of Gunpowder at Purfleet, in order to consider of their Preservation from Danger by Lightning, I am of Opinion

1. That all the Bars of Iron passing down through the Arches of the Roofs towards the Powder, be taken away, as at present they do, with the Copper Hoops on the Barrels, form an imperfect Conductor, more or less compleat as the Piles of Barrels happen to be higher or lower, but such an one as may tend to draw down among the Powder any Lightning that shall happen to strike the Roof, and are therefore dangerous.

2. That [the] Building which has a Coping of Lead all along the Ridge from End to End, may be secured by a Pointed Iron Rod erected near each End, communicating with that Coping, and extending down thro’ the Chalk Rock on which the Buildings are founded, till it comes to Water. The Rod may be of at least an Inch Diameter (that it may last the longer as well as afford a freer Passage to the Lightning thro’ its Substance) and painted to prevent its Rusting. Its upper End should extend ten feet above the Top of the Roof, tapering gradually to a sharp Point; the last six Inches (the better to preserve Sharpness) to be of Copper as less liable to be blunted by Rust. If the Rod cannot conveniently be made all of one Piece, the Lengths of which it is compos’d should be strongly scrued together or into each other by a close Joint, with a thin Plate of Lead between the Shoulders to exclude Moisture (which might occasion Rust) and to make the Joining or Continuation of Metal more perfect.

From all the Electrical Experiments I have made with this View, and all the Instances I have yet known of the Effect of Lightning on these Conductors, it appears to me equally safe (when they are good and compleat down to Water or very moist Earth) whether they are apply’d close to the Wall, and steadied by Staples driven into it, or supported by a Pole or Mast fix’d in the Ground at some Distance from it. The first Method is most convenient, as the Rod may so be bent for avoiding Doors and Windows that are directly under the Pitch of the Roof. Yet as it may be more satisfactory to some Apprehensions, if the Rods are supported in the latter Manner, I should have no Objection to it, provided they can be conveniently posited so as to obstruct no Passage, and are so firmly fix’d as that the Wind may not, by shaking them, break the Iron or Leaden Communication between the Side of the Rod and the Lead that Covers the Ridge.

3. The other four Buildings being, as I understand, to be newroofed in the same manner with that just mentioned, the same Method will serve for them when so finished. but if it be required to secure them in the mean time, then, as their present Roofs are of a different Form, being hipped at the four Corners, and their Hip-joints as well as Ridges Cop’d with Lead extending down to the Eaves, I would advise the immediate boring or digging, near the Ends of each Building, the proposed Passages down to Water, and fixing in them that part of each Conductor which may reach upwards from the Water to the Height of the Eaves: From the Top of this I would extend two Iron Arms reaching to the Corners of the Eaves, where the Leaden Copings of the Hips must be connected with those Bars; and on the Joining of the Hips with the Ridge I would erect Rods nine or ten feet high, pointed as before directed, which, as any Roof is altered may be used as the upper Part of the more upright Conductor. I am, Sir, Your most obedient humble Servant