Your favor of April 5. 1788. covering that also of Jan. 26.–87 did not miscarry; but I have been prevented answering it by a long journey through Holland and Germany, and by a necessary attention to some pressing matters in the first moments after my return. In fact, I was occupied on the subject of Hygrometers and preparing to write to you, when I received your favor of the 11th. inst. This requiring the first attention, I went immediately to the Marquis de Condorcet, Secretary to the Academy of arts and sciences, and spoke to him on the subject of the Caveat for Mr. Rumsey. He informed me that this was not their method of acting in such cases: that the best thing Mr. Rumsey could do would be to commit to paper a perfect description of his invention, expressed without any mystery, to seal this up with his own seal, and indorse on it the general title of his invention, to inclose this in a letter addressed ‘à Monsieur Monsieur le Marquis de Condorcet, Secretaire de l’Academie des Sciences, à l’hotel de la Monnoie,’ in which letter he will say that he means to come to France to ask an exclusive privilege for his invention as soon as his affairs will permit him, and praying that in the mean time the paper he incloses may be deposited in the office of the academy, unopened, but with an endorsement of the day on which it is received. The Marquis de Condorcet will comply with this request and Mr. Rumsey may rest most perfectly assured that his seal will remain unviolated. He will be at liberty also to withdraw it when he pleases. He will of course take care to find a safe conveiance from London to Paris for his letter. If, before the receipt of his letter, any application is made to the academy on the same head, the Marquis de Condorcet will give me notice of it.

I will now recur to the subjects of your letter of Jan. 1787. and first of all accept my thanks for Dr. Crawfurd’s book which I had long desired, for the 7. Nos. of the Repository, which I like so well that I am become a subscriber for it, for De Luc’s Hygrometer, and that of Mahogany on Dr. Franklin’s principle and your plan. All these came while I was in Germany. The case of Magnets sent a twelvemonth sooner, I had desired Messrs. Nairne & Blount might ask paiment for of Colo. Smith, and I had desired Colo. Smith to pay for them, and supposed it done. But turning now to his account I do not find he has charged me with that paiment, so that if he made it, he has forgot to charge it, and consequently I am his debtor for it; if he did not pay it, then I am indebted to Messrs. Nairne & Blount, and will ask the favor of them to apply for it to Mr. John Trumbul No. 2. North street, Rathbone place, who will pay them.  I have been a little at a loss with the Hygrometer of De Luc you were so kind as to send me. It is graduated from zero to 100, and I had understood these were his extremes. Those of De Saussure are the same. Yet, while this of De Luc, exposed to the open air has never fallen below 26. nor risen above 55. since it was in my possession, those of De Saussure have been generally, during the wet spell we have had, at about 90. Do you suppose any thing may have lessened the sensibility of the whale bone, or to what other cause must I ascribe the smallness of it’s range?  The manner in which Mr. Nairne has carried Dr. Franklin’s idea into execution is estimable for it’s simplicity, and simplicity in the hygrometer is peculiarly necessary. But it is liable to the objection you justly make that equal extensions of the wood are not equally indicated on the dial plate. The fillet of wood expanding in the direction of the tangent of the circle, the circle should be divided into portions corresponding to equal parts of the tangent, that is, the index should point out, not the angle it has moved over, but it’s tangent. I think too that Mr. Nairne’s fillet moving in grooves of wood, will be considerably resisted in it’s motion, by the grooves which hold it. The fillet placed in a single groove in your manner is less resisted in it’s motion, but the fillet being half buried in the groove is less exposed in that half to the influence of the atmosphere; consequently it dilates and contracts more slowly than the outer half, and necessarily becomes curved; it’s indications become very false sometimes from this cause. I think too that it’s dilatations must be expressed in numbers in order to give a result which shall be the just average of all our observations. For it is the result of all the observations to be made in Europe which is to express the ordinary humidity of it’s atmosphere, and the result of all the observations to be made in America, which is to express the humidity of that atmosphere, and thus place the two in comparison. I have thought of the following plan, and a workman is now employed in executing it. a.b.c.d. is the stock of mahogany, suppose 20. Inches long, and 2½ I. wide and thick. e.f.g.h. is the fillet of wood 1/10I. thick, 2I. lengthwise of the grain from e. to f. and 18.I. across the grain from e. towards g. I prefer beach because more sensible than mahogany. i.i.i. &c. are pullies fixed on the stock, in the grooves of which the edges of the fillet move, that it may meet less resistance in it’s expansion and contraction. k.l. is a rack made fast to the end of the fillet e.f. which is moveable, while the end g. h. is made fast to the stock. m. is a toothed wheel the axis of which passes through a dial plate and carries an index n. o. to point out the degrees from 0. to 100. marked on the dial plate, these extremes of 0. and 100. to indicate the same state of dryness and moisture which the same numbers do on De Saussure’s hygrometer. If, on experiment, I find this construction to answer, I will have four fillets cut of the same peice of wood, and adapted to so many instruments for comparative observations here, in England, and two different parts of America, and will take the liberty of presenting to you the one for England. I verily believe it will turn out in event that the atmosphere of our part of America is less humid than that of this part of Europe: and that this furnishes an instance the more wherein philosophers, as you justly observe, hasten to general conclusions from too few observations, and on false testimony of these observations. I must do the justice to those of your country to say they have given less than any others into the lies of Paw, the dreams of Buffon and Raynal, and the well-rounded periods of their echo Robertson.

From what I have seen of the climate of Europe, and what I have been able to learn of it from others, it seems to me that it’s middle parts are covered by an almost perpetual bank of clouds, extending Northwardly beyond Copenhagen, but not so far as Stockholm, Eastwardly to Switzerland, Southwardly to a little beyond Lyons, and Westwardly to the Western coast of Ireland, and perhaps it is the same which is always hovering over the banks of Newfoundland. Should further enquiry confirm the fact it will be a curious question to examine why the middle parts of Europe should be subject to this general cloudiness while it’s Northern and Southern parts enjoy a clear sky, as we do also in America?  A considerable improvement in the Air pump has taken place in America. You know that the valves of that machine are it’s most embarrassing parts. A clergyman in Boston has got rid of them in the simplest manner possible. The alteration he has made is described in the philosophical transactions of Boston. I put this description into the hands of a Mr. Jones, instrument maker in London, when I was there. But he has not tried it. I have since wished I had made an acquaintance there with Mr. Nairne, and communicated it to him. If he should wish to see the description, it is in the hands of Mr. Paine (Common sense) now in London who has withdrawn it for me from Jones. I have heard of an improvement made in this instrument at Amsterdam, but do not know what it is, and very much doubt whether it be as great as the one I have spoken of.—I have heard that they make in London an Odometer, which may be made fast between two spokes of any wheel, and will indicate the revolutions of the wheel by means of a pendulum which always keeps it’s vertical position while the wheel is turning round and round. Thus [see Fig. 1.] I will thank you to inform me whether it’s indications can be depended on, and how much the instrument costs. Should you sometimes see Lord Wycombe, and Dr. Price, I will trouble you to present my respects to them, begging you at the same time to accept assurances of the esteem & attachment with which I am Dear Sir Your most obedt. humble servt,