I did not receive your Favour of Octr. 1785 till last Month, or so long a Time should not have elapsed, before I made you, at least, the Return of my sincere Thanks for your valuable Communications. The Trunk with the Books arrived safe, and its precious Contents were disposed of as you directed. As to those you were so kind as to favour me with, Nothing could have been more acceptable. Impatient as we are here to be informed of the Progress of Philosophy, and great as that Progress seems to be in the European World, we certainly ought, and do, hold your Favour in the highest Estimation. We rejoice that an American Ambassador bestows such Attention to the literary Improvement of his Countrymen, and that we have in him a Friend, who knows so well the Importance of that Improvement. We must endeavour to avail ourselves of the Lights he transmits, and thus prove that we are not unworthy of his generous Attention.

The Circumstance you mentioned relative to the Planet Herschel is indeed a curious one, and was perhaps also a fortunate one for the Calculation of that valuable Work the Conn. des Temps. I had not before heard of the Observations of Mr. Pigott. They seem to be more precise than any before made upon that remarkable Phænomenon of a periodical Variation of Light in some of the Stars. As early as 1596, one in the Neck of the Whale was observed to increase and decrease regularly, it’s Period being about 334 Days. But I have seen no Account of any Stars whose periods were so short, or so well ascertained as that you mention. Such Phænomena undoubtedly afford ample Room for Philosophical Speculation. Perhaps, like our Sun, those stars have a Rotation upon their Axes, and also their Maculæ, the larger and more constant, which, during their rotatory Motion, may cause a regular variation in their Light, in proportion as those Maculæ are more or less turned to the Observer. Or, perhaps as Maupertuis supposes, that Appearance may arise from their Form. They may be considerably flattened, so that they will appear more or less luminous, as the broad or narrow Side is turned towards us. Your Idea that they may be Suns, which have their orbits of Revolution is more extensive than either of those Suppositions; and it is not the Part of a Philosopher to deny the Possibility of certain Dispositions in Nature, when the Phænomena seem to indicate them, merely because he cannot fully comprehend the manner in which they act. It seems to be doubtful whether Men are more slow in collecting the Laws of Nature, or in applying them when collected and known. Who would have supposed, a few years past, that, by Means of a Screw acting only in the Air, a Vessel should be fairly drawn across a River, Or indeed that the bold Aeronaut should dare to attempt Excursions in so rare a Medium, and would be able to direct his Course nearer to the Wind than the best sailing Vessels. Is it not probable that these aeorostatic Machines will in Time be applied to other purposes than as mere Philosophical Experiments tho’ in that Respect alone they are certainly very valuable. Yet I have seen no Result of Observations made by them, relating to several Matters, for which they seem particularly adapted, Such as the Rate of Decrease in the Density of the Atmosphere at different Elevations, also the Rate in which it’s Temperature varies. Meteors in general, Propagation of Sounds, Descent of Bodies &c. are all proper Subjects of Investigation and which no Doubt will be investigated as those Machines are more perfected. We raise here the small Balloons filled with inflammable Air. I have once made a Trial of Pit Coal, which I find Cavallo also mentions as affording an air of the same Levity as that you wrote me an account of. We raise those filled with rarified Air, tolerable large (about 20 Feet in Diam). But I beleive no one in America has yet ventured to mount with a Balloon.

I have been since sorry that I proposed those Queries respecting the Chrystal. It was giving you a Trouble which I ought to have spared. I have lately seen some Account of the Icelandic Chrystal, as long known to Opticians for its singular refractive Powers. It is described as a Kind of Talc, found in the form of an oblique Parallelopiped, and composed of Lamina which will cleave parallel to either of it’s Sides; from which Constitution, its Property may probably be derived. Dr. Franklin brought a Peice of this Chrystal to Phila. which he gave to a Friend of mine who informed me of it’s Property of giving a double Image. They have also in Phila. one of the Telescopes made of it, by which the Suns apparent Diameter is readily measured, and small Distances accurately determined.

In the Continuance of the meridian Line which bounds the western Extremity of Pennsylvania, marine shells were found on the highest Ground between the Ohio and Lake Erie. I have written to Mr. Ellicott, who was concerned in running the Line, in Order to procure some of them, which I mean to forward to you; I shall be happy to send them to you, as they will afford you some useful Data, and would no Doubt be a particular Gratification to those who are capable, like Buffon, of penetrating into Ages past.

I must not forget to thank you for your kind offer with Respect to the Encyclopedie. We have however, contracted with a Person who sent over last Fall 20 Vol. of the Encyclop. Method. and who is to supply us with them as they come out.

Being in Richmond, at the Time of the last Session of our Assembly our mutual Friend Col. Madison, shewed me a work, which, tho I had an opportunity of viewing but too transiently, yet I wish its Author could be induced to render more public. It would be an Ornament to our Library, and highly profitable to our Youth. They will gain from it a Knowledge relative to their Country, not elsewhere to be obtained, and will be impressed with the great Advantage of treasuring up, and methodically arranging Facts of Importance, whether they belong to the moral or the physical world. Such a Work should not be kept in private. Let it have the broad Light of an American Sun. It will assist greatly in dispelling some Misty Fogs, which still hover about us.

I have no literary Intelligence worth communicating from this Side of the Atlantic. A Dr. Ramsay of So. Carolina has attempted a History of the late War as far as it related to that State, but I beleive most of us would regret to have an Event so fortunate for Mankind, handed down to Posterity in no better a Dress, yet the Author has some Merit, particularly that his Dress is altogether Americain, of which he makes no small Boast.

Mr. Wythe informs me that he wrote to you some Time past. Your Friends here, whom you mentioned, all request to be affectionately remembered. Be assured, Dr. Sir, of the great Esteem & Respect with which I am, Yr. Friend & Servt.,