I have been honored with your letter of the 23rd. ulto.

Mr. Rumsey has been informed of the result of the inquiries you have been pleased to make on his account, and will I presume, take the necessary measures speedily. By my advice, he has enlarged the description to attend one of his discoveries.

I had no design that Col. Smith should settle for the magnet box; consequently did not mention the thing to him; and as it is a trifle, I hope you will have the goodness to let it rest where it does.

Mr. Nairne says the whalebone hygrometer has its degrees fixed, by water on the one side and unslaked lime on the other. It stands a month and upwards with unslaked lime, in a box. He never saw it vary so much as you state, in his house in London.

I used to keep my fillet of wood out of its groove, except when I wanted to measure it; consequently its whole surface was equably acted upon.

From all that I have seen, it will be difficult to find wood, the several pieces of which shall have exactly corresponding sensibilities. Age, depth within the stem, direction of the fibres, and other circumstances, will be likely to produce variations. Hence I have always had in view to have the hygrometer, when formed of wood, returned to the country whence it was first sent. This forms a proof of the observations; which, as far as relates to the question depending between our part of Europe and the United states of America, are only requisite to determine generally, where the difference lies. The degrees of the more and the less, I fear, must be determined in some future period, when we find ourselves provided with better instruments.

I shall be much obliged for one of your hygrometers, when finished, which is certainly ingenious, and liable to no objection that I foresee, except what depends upon the ingredient used, viz wood.

The fact you mention about the clouds in a certain part of western Europe, in a considerable degree corresponds with what I should have supposed. I have turned over some hundred volumes of accounts, antient and modern, and have fully satisfied myself of the inland and eastern climates in middle latitudes in the great continents in the Northern hemisphere agreeing with each other, and differing from Western climates in the same situation. I may some day have the honor to present you with my remarks and extracts; by which you will find that more than one great man has dreamed, or copied the errors of dreamers, in the very face of the authorities on which they all pretended to rest; and in this list I shall include Gibbon, an Englishman, Hume, besides Du Bos, and others. I find it only necessary for one grave man to commit a mistake uncontradicted, to mislead men of learning for ages.

The invention you allude to respecting the air pump, was long ago used by Mr. Nairne, as Mr. Cavendish has confirmed to me; but Mr. Nairne says he laid it aside, finding the great object was not so much to free the receiver from air, as the barrel in which the piston moves. A Mr. Heurter has moved the lower valve in that barrel by a power distinct from the air in the receiver, and it is said with great advantage.

Neither Mr. Cavendish, Mr. Elliot, nor Mr. Nairne are acquainted with an odometer of the kind you mention. We have them fixed to the wheel or carriage, upon other corresponding constructions, and they are said to answer within 1 or even ¼⅌ cent of true distances; the cost being from 7 to 10 guineas.

I have much pleasure in thinking that our mutual acquaintance Mr. Romilly will deliver this letter, as his worth, learning and ingenuity, are not often found equalled in this country, or indeed in any other. I have the honor to be with particular esteem, Dear sir, your respectful and faithful humble servt.