From Robert Erskine9
ALS: American Philosophical Society; AL (draft1), Ringwood Manor Museum, Ringwood State Park, Ringwood, N.J.
Newark August 16th. 1776.
I beg leave to enclose you a drawing of a new Contrivance for stopping Channels and Rivers, which I call Marine Chevaux de Frise; a model of which was exhibited at Headquarters in New York, the 20th of last month.
After the ships passed the Batteries with such facility, I considered with regret, that the Channel was not obstructed; and since the exegence required something both speedily executed and effectual, the Enclosed Construction occurred, which I have reason to beleive is now putting in practice.2
In addition to what is set forth on the Drawing, I beg leave to observe, that it may be described as a Tetrahedron with four horned Corners; having three Horns to each Corner; that the Consequence of a Ships running against it, must either be that she will stake upon it, or overset it, in which case the other horns will rise and take her in the bottom, and either overset her, go through her; or else she must break it with her weight; but here it is to be Considered, what force it will require to break a Beam 12 or 15 Inches square, standing only ⅓d from the perpendicular; which seems too great for any ship to apply, without injuring her so effectually, as to render her unfit for further service. Such Chevaux dropt here and there in Anchoring grounds and Harbours, would render them very unsafe. Supposing the Channel to be obstructed Seven fathoms deep, if it is made of Beams 32 feet long, its perpendicular height would be near 28 feet; the Horns would be within 14 feet of the surface, Consequently obstruct any vessel which drew more than that depth of water. Two Chevaux of these dimensions would stop 10 Fathoms, as the Horns of the one would be within 20 feet of those of the other. 20 would stop an hundred Fathoms, and require only 120 pieces of Timber. As the Current has full liberty to flow under, they Cannot injure Channels or sensibly obstruct the tide: indeed they need not be put in till an alarm, for if they are ready prepared, a number of them may be rigged up and thrown into a Channel in a few hours-but I need not enlarge or add a number of Circumstances which will naturally occur to any one Conversant in Mechanics.
I shall think my self happy if any endeavours of mine tend to serve the Cause of Freedom, Humanity and the States of America and am Dear Sir with the Greatest respect Your most Obedient humble servant
Honble. Dr. Benjamin Franklin
Addressed: To / The Honble. / Dr. Benjamin Franklin / Philadelphia
9. Erskine (1735–80) was a Scot who moved to London as an engineer and inventor. In 1771 he was elected to the Royal Society with BF’s recommendation (above, VIII, 359), and the same year emigrated to Bergen Co., N.J., where he was an ironmaster; in 1775 he organized a militia company from his employees and became its captain. In 1777 he was surveyor general and cartographer of the continental army and produced a series of maps for Washington. DAB. BF’s answer to this letter is below, Oct. 16.
1. Dated Aug. 15.
2. In early July Washington had ordered the channel obstructed, but nothing effective had been achieved by the 12th, when H.M.S. Phoenix and Rose and three tenders sailed up the Hudson and anchored in the Tappan Zee, without receiving any damage worth mention from the shore batteries. Fitzpatrick, Writings of Washington, V, 129; Naval Docs., V, 511, 1040.