The Qualities hitherto sought in a Hygrometer, or Instrument to discover the Degrees of Moisture & Dryness in the Air, seem to have been, an Aptitude to receive Humidity readily from a moist Air, and to part with it as readily to a dry Air. Different Substances have been found to possess more or less of this Quality; but when we shall have found the Substance that has it in the greatest Perfection, there will still remain some Uncertainty in the Conclusions to be drawn from the Degree shown by the Instrument, arising from the actual State of the Instrument itself as to Heat & Cold. Thus if two Bottles or vessels of Glass or Metal being filled, the one with cold & the other with hot-Water, are brought into a Room; the Moisture of the Air in the Room will attach itself in Quantities to the Surface of the cold Vessel, while if you actually wet the surface of the hot Vessel, the Moisture will immediately quit it, and be absorbed by the same Air. And thus in a sudden Change of the Air from cold to warm, the Instrument remaining longer Cold may condense & absorb more Moisture, and mark the Air as having become more humid than it is in Reality; and the contrary in a Change from warm to cold.

But if such a suddenly changing Instrument could be freed from these Imperfections, yet when the Design is to discover the different Degrees of Humidity in the Air of different Countries; I apprehend the quick Sensibility of the Instrument to be rather a Disadvantage; since to draw the desired Conclusions from it, a constant & frequent Observation Day & Night in each Country will be necessary for a Year or Years, and the mean of each different Set of Observations is to be found & determined. After all which some Uncertainty will remain respecting the different Degrees of Exactitude with which different Persons may have made and taken Notes of their Observations.

For these Reasons I apprehend that a Substance which tho' capable of being distended by Moisture & contracted by Dryness, is so slow in receiving and parting with its Humidity that the frequent Changes in the Atmosphere have not time to affect it sensibly, and which therefore should gradually take nearly the Medium of all those Changes & preserve it constantly, would be the most proper Substance of which to make Such an Hygrometer.

Such an Instrument, you, my dear Sir, tho' without intending it, have made for me; and I without desiring or expecting it have received from you. It is therefore with Propriety that I address to you the following Account of it, and the more as you have both a Head to contrive and a hand to execute the Means of perfecting it. And I do this with greater Pleasure as it affords me the Opportunity of renewing that antient Correspondence & Acquaintance with you, which to me was always so pleasing & so instructive.

You may possibly remember that in or about the Year 1758, you made for me a Set of artificial Magnets, Six in Number, each 5 ½ Inches Long, ½ an Inch broad, & of an Inch thick. These with two Pieces of Soft Iron which together equalled one of the Magnets were inclos'd in a little Box of Mahogony Wood, the Grain of which ran with, & not across, the Length of the Box; and the Box was clos'd by a Little Shutter of the same Wood, the Grain of which ran across the Box; and the Ends of this shutting Piece were bevel'd so as to fit & slide in a kind of Dovetail Groove when the Box was to be shut or open'd.

I had been of Opinion that good Mahogony Wood was not affected by Moisture so as to change its Dimensions, & that it was always to be found as the Tools of the Workman left it. Indeed the Difference at different Times in the same Country is so small as to be scarcely in a common Way observable. Hence the Box which was made so as to allow sufficient Room for the Magnets to Slide out and in freely, and when in, afforded them so much Play that by shaking the Box one could make them strike the opposite Sides alternately, continued in the same State all the Time I remain'd in England, which was four Years, without any apparent Alteration. I left England in August 1762 and arriv'd at Philadelphia in October the same Year. In a few Weeks after my Arrival, being desirous of showing your Magnets to a Philosophical Friend, I found them so tight in the Box, that it was with Difficulty I got them out; and constantly during the two Years I remain'd there, Viz till November 1764, this Difficulty of getting them out and in continued. The little Shutter too, as Wood does not shrink lengthways of the Grain, was found too long to enter its Grooves, & not being us'd was mislaid and Lost, and I afterwards had another made that fitted.

In December 1764 I returned to England and after some time I observed that my Box was become full big enough for my Magnets, and too wide for my new Shutter; which was so much too Short for its Grooves, that it was apt to fall out; and to make it keep in, I lengthen'd it by adding to each End a little Coat of Sealing Wax.

I continued in England more than 10 Years, and During all that time after the first Change, I perceived no Alteration. The Magnets had the same Freedom in their Box, and the little Shutter continued with the added Sealing Wax to fit its Grooves, till some Weeks after my second Return to America.

As I could not imagine any other Cause for this Change of Dimensions in the Box, when in the different Countries, I concluded first generally that the Air of England was moister than that of America. And this I supposed an Effect of its being an Island, where every Wind that blew must necessarily pass over some Sea before it arrived, and of Course lick up some Vapour. I afterwards indeed doubted whether I had not been too general in my Conclusion; and whether it might not be just only so far as related to the City of London, where I resided; because there are many Causes of Moisture in the City Air, which do not exist to the same Degree in the Country; such as the Brewers' & Dyers' boiling Cauldrons, and the great Number of Pots and Teakettles continually on the Fire, sending forth abundance of Vapour; and also the Number of Animals who by their Breath constantly increase it, to which may be added that even the vast Quantity of Sea Coals burnt there, in kindling discharge a great deal of Moisture.

When I was in England the last time, you also made for me a little Achromatic Pocket Telescope. The Body was Brass, and it had a round Case (I think of thin Wood) covered with Shagrin. All the while I remained in England, tho' possibly there might be some small Changes in the Dimensions of this Case, I neither perceived nor suspected any. There was always comfortable Room for the Telescope to slip in and out. But soon after I arrived in America, which was in May 1775, the Case became too small for the Instrument, it was with much Difficulty & various Contrivances that I got it out, and I could never after get it in again; during my Stay there, which was 18 Months. I brought it with me to Europe, but left the Case as useless, imagining that I should find the continental Air of France as dry as that of Pensilvania where my Magnet Box had also returned a second time to its Narrowness, & pinched the Pieces as heretofore, obliging me, too, to scrape the Sealing Wax off the Ends of the Shutter.

I had not been long in France, before I was surprized to find, that my Box was become as large as it had always been in England, the Magnets enter'd and came out with the same Freedom, and when in, I could rattle them against its Sides; this has continued to be the Case without sensible Variation. My Habitation is out of Paris, distant almost a league, so that the moist Air of the City cannot be supposed to have much Effect upon the Box: & I am on a high dry Hill in a free Air, as likely to be dry as any Air in France. Whence it seems probable, that the Air of England in general may, as well as that of London, be moister than the Air of America, since that of France is so, and in a Part so distant from the Sea.

The greater Dryness of the Air in America appears from some other observations. The Cabinet Work formerly sent us from London, which consisted in thin Plates of fine Wood glu'd upon Fir, never would stand with us, the Vaneering, as those Plates are call'd, would get loose & come off; both Woods shrinking, and their Grains often crossing, they were forever cracking and flying. And in my Electrical Experiments there, it was remarkable, that a Mahogony Table on which my Jars stood under the Prime Conductor to be charged, would often be so dry particularly when the Wind had been some time at N. W. which with us is a very drying Wind, as to isolate the Jars, and prevent their being charged till I had formed a Communication between their Coatings and the Earth. I had a like Table in London which I us'd for the same Purpose all the time I resided there; but it was never so dry as to refuse conducting the Electricity.

Now what I would beg leave to recommend to you, is, that you would recollect if you can, the Species of Mahogony of which you made my Box, for you know there is a good deal of Difference in Woods that go under that Name; or if that cannot be, that you would take a Number of Pieces of the closest and finest grain'd Mahogony that you can meet with, plane them to the thinness of about a Line, and the Width of about two Inches across the Grain; and fix each of the Pieces in some Instrument that you can contrive, which will permit them to contract & dilate, and will show in sensible Degrees, by a moveable Hand upon a marked Scale, the otherwise less sensible Quantities of such Contraction and Dilatation. If these Instruments are all kept in the same Place while making, and are graduated together while subject to the same Degrees of Moisture or Dryness, I apprehend you will have so many comparable Hygrometors which being sent into different Countries, and continued there for some time will find and show there the Mean of the different Dryness & Moisture of the Air of those Countries; and that with much less Trouble than by any other Hygrometer hitherto in use. With great Esteem I am, dear Sir, Your most obedient & most humble Servant