Octr. 25th. 1775
A Method of collecting Salt Petre from the Air which is talked of here is this. Take of Lime and Ashes equal Quantities, and of horse dung a Quantity equal to both the Ashes and Lime, mix them together into a Mortar, with this Mortar and a Quantity of long Straw to keep it together build two Walls Eighteen Inches thick, and three feet high, about four feet asunder. Then make a Center and turn and [an?]
Arch over cemicircularly from the Top of one Wall to that of the other, and this Arch may be made Eighteen Inches thick too. These Walls
with the Arch over them, may be continued to any length you please. There must be a shed over it to keep off the Rain, and the Arch must be wett every Day with Urine. This, in summer, will collect so much salt Petre that an ounce may be extracted from every Pound of the Walls in three Months. In Winter it will make as fast provided you keep a Fire at one End of the Arch, that the Wind may blow the Fire and Smoke under the Arch, and keep it from freezing.
This is one Method as it is affirmed by Gentlemen here.
Sulphur, Nitre, and Lead We must have of our own. We must not depend upon Navigation for these. I wish the Committee of the General Court for Lead and Salt would transmit their Discoveries to me. I dont know whether you are one of that Committee or not.
Pray inform me if Obrian and Carghill1
were or were not commissioned by some Vote of the general Court, and whether they cant be put into the Continental service. An order is gone to Genl. Washington to that Purpose if it can be done.