I have this Day received your Favour of the 2d Instant. Every Information in my Power respecting the Balloons I sent you just before Christmas, contain’d in Copies of my Letters to Sir Joseph Banks. There is no Secret in the Affair, and I make no doubt that a Person coming from you would easily obtain a Sight of the different Balloons of Mongolfier and Charles, with all the Instructions wanted; and if you undertake to make one, I think it extreamly proper and necessary to send an ingenious Man here for that purpose. otherwise for want of Attention to some particular Circumstance, or of being acquainted with it, the Experiment might miscarry which being in an Affair of so much public Expectation, would have bad Consequences, draw upon you a great deal of Censure, and affect your Reputation. It is a serious thing to draw out from their Affairs all the Inhabitants of a great City and its Environs, and a Disappointment makes them angry. At Bourdeaux lately, a Person who pretended to send up a Balloon and had received Money of many People, not being able to make it rise, the Populace were so exasperated that they pull’d down his House, and had like to have kill’d him.

It appears as you observe, to be a Discovery of great Importance; and what may possibly give a new Turn to human Affairs. Convincing Sovereigns of the Folly of Wars, may perhaps be one Effect of it: since it will be impracticable for the most potent of them to guard his Dominions. Five thousand Balloons capable of raising two Men each, would not cost more than Five Ships of the Line: And where is the Prince who can afford so to cover his Country with Troops for its Defense, as that Ten thousand Men defending from the Clouds, might not in many Places do an infinite deal of Mischief, before a Force could be brought together to repel them? It is a pity that any national Jealousy, should, as you imagine it may, have prevented the English from prosecuting the Experiment, since they are such ingenious Mechanicians, that in their Hands it might have made a more rapid Progress towards Perfection, and all the Utility it is capable of affording. The Balloon of Messrs. Charles and Robert, was really filled with inflammable Air. The Quantity being great it was expensive, and tedious in filling, requiring two or three Days and Nights constant Labour. It had a Soupape near the Top, which they could open by pulling a String and thereby let out some Air when they had a mind to descend; and they discharg’d some of their Ballast of Sand, when they would rise again. A great deal of Air must have been let out when they landed, so that the loose Part might envelope one of them; yet the Car being lightned by that one getting out of it, there was enough left to carry up the other rapidly. They had no Fire with them. That is us’d only in Messr. Mongolfier’s Globe, which is open at Bottom, and Straw constantly burnt to keep ut up. This kind is sooner and cheaper fill’d; but must be much bigger to carry up the same Weight; since Air rarified by Heat is only twice as light as common Air, and inflammable Air ten times lighter. M. de Morveau a famous Chemist at Dijon, has found an inflammable Air that will cost only a 25th part of the Price of what is made by Oil of Vitriol pour’d on Iron Filings. They say it is made from Sea Coal. Its comparative Weight is not mentioned.

I dispatch’d your Letters by the petit Poste to day as soon as I receiv’d them.

As the Pacquet of German Papers and Almanacs were 18 Months in getting to your hands, I began to fear for my last Packet, since it was trusted to the same Conveyance, being sent to the Ambassadors, who promis’d to take care of it. I therefore send a Copy of my Letter with this per Post, but I cannot copy the other Papers (they being too long) so as to go by this Courier. Nor can I now add farther but that I am, as ever, my dear Friend Yours most affectionately