Your Favour of the 14th of March6 came to my Hands the 15th of May last, and gave me great pleasure. The first Experiment of Mr. Kinnersley’s which you mention, is, as you observe, a beautiful one to see; and I think, fully proves that the Fusion of Metals by Lightning is not a cold Fusion. I have myself, several times, melted small brass Wire by a Stroke from your Case of Bottles, which left a Mark where it lay upon the Table, and some Balls of twice or three times its Diameter near the Mark but no part of the Wire could be found. At the time of the Stroke, a great Number of Sparks, like those from a Flint and Steel, fly upward and laterally from the place where the Wire was laid, and lose their Light in the daytime, at the Distance of about two or three Inches. The Diameter of a piece of Mr. Kinnersley’s Wire, which you was so kind as to send me with the Balls, I found to be one part in 182 of an Inch; mine was but one part in 330.

The second of Mr. Kinnersley’s Experiments which you relate, and which seems to be a very extraordinary one, I have several times endeavour’d to make, but without Success. The Air with you must certainly be much drier than in England: For I have never observ’d the inclos’d pith Balls to separate by the electris’d Air of a Room, without having first heated the Phial; notwithstanding which, they always came together in the Phial, before the outward Air had lost its Electricity, as appears by their separating again when taken out of it. I once electrified the Air of my largest Room to a considerable degree, and by opening the Windows and Doors suffer’d the Wind to blow through for about five Minutes; I then shut them, and examin’d the Air in the Room, but found no Sign of Electricity remaining. This Air I electrified to about the same degree as before, and leaving it confin’d, it retain’d a sensible degree of its Electricity for more than three quarters of an hour. Hence I entirely agree with you, that the Glass in Mr. Kinnersley’s Experiment receiv’d some degree of Electricity from the electrised Air, and so kept the Balls separated after that Air was blown away.

I have put your ingenious Friend Mr. Bowdoin’s Telescope into Mr. Nairn’s Hands, who is making a Pedestal for it, which I think will be an Improvement of that which Mr. Bowdoin has describ’d in his last Letter to me, which you saw. You may depend on my taking all possible Care to get it well executed, and soon. I find the fitting Dollond’s Micrometer to the Telescope is impracticable.7

Since the publication of a short Paper in the Transactions, which contains an account of Experiments to prove that Water is not incompressible,8 I have discover’d a remarkable property belonging to that Fluid, which is new to me, though perhaps it may not be so to you. The Property I mean is, It’s being less compressible in Summer than in Winter.9 This is contrary to what I find in Spirit of Wine, and Oil of Olives; which are (as one would expect Water to be) more compressible when expanded by Heat, and less so when contracted by Cold. For when Fahrenheit’s Thermometer is at 34 Degrees, and the Barometer at 29½ I [nches], Water is compress’d by the Weight of the Atmosphere 49 parts in a Million of its whole Bulk, and Spirit of Wine 60 of the same parts; When the Thermometer is at 50 degrees, Water is compressed 46 parts, and Spirit of Wine 66 parts in a Million, by the same Weight; and when the Thermometer is at 64 degrees, this Weight will compress Water no more than 44 parts in a Million, but it will compress Spirit of Wine 71 of these parts.

As I am not able, at present, to account for this Difference in the Compressibility of Water myself, I should be very glad to have your Thoughts upon it.

The Compression by the Weight of the Atmosphere, and the specific Gravity of the following Fluids; (which are all that I have yet try’d) are set down as they were found in a temperate Degree of Heat, and when the Barometer was at a mean Height.

You will easily perceive that the Compressions of these Fluids by the same Weight, are not in the inverse Ratio of their Densities, or specific Gravities, as might be expected. The Compression of Spirit of Wine, for instance, being compar’d with that of Rain-Water, is greater than in this proportion it ought to be, and the Compression of Sea-Water is less.

Mr. Price, Mr. Rose, Mr. Cooper and the rest of the Club1 desire their most respectful Compliments to you, and very much regret, as I do myself, your leaving England. I am, with the most sincere Regard, Dear Sir Your most obliged and most humble Servant