Your favor of Nov. 24. is just now recieved, with the papers on the survey of the coast which you have been so kind as to send me. for these be pleased to accept my very particular thanks. in the last volume of the transactions of the A. Phil Society I had especially remarked these papers and the processes so peculiarly accurate and ingenious by which you had proposed to execute the survey of the coast. I regret much that it was not carried into execution, as, independantly of the permanent security it would have procured for the navigation of our coast, it would have been an honorable monument of the state of science at this early period of our history. Attentive, in every thing, to what I can find in it which may be useful to our University (now the only object and occupation of my life) I singled out your plan of an Observatory which for it’s simplicity, solidity, and every aptitude for it’s purposes and for it’s economy, presented at once what I had for some time thought of requesting some friend in Europe to procure for me. I therefore deposited the volume in the library of the University, with a note on a blank leaf at the beginning specially referring to your plan, as suiting us exactly whenever our Institution shall be sufficiently advanced to call for an Observatory. We have secured a site of peculiar advantages for one in future. There is a material part in your description which I did not understand and I was intending (with a request of your approbation of our adopting your plan) to have asked of you some more particular explanation of that part. your letter furnishes the occasion of doing it now. the passage is pa. 148. beginning with the words ‘the roof over the transit Etc to the words ‘cannot be described here.’ pa. 149. I do not understand this construction at all, and if you can aid me in it, you will oblige me.

Permit me, at the same time to suggest to you a construction of flat roofs different from yours. in Fig. 4. you have two strata of joists the one 5. or 6. feet above the other. I do not see the use you propose to make of the space between them. in the house in which I live, and it’s offices I have flat roofs of a different construction. they consist of rooflets 30. I. wide with gutturs between them. there are 2. strata of joists, the one about 9. I. higher than the other arranged thus. [GRAPHIC IN MANUSCRIPT] a single length of shingles reaches from the top of the upper joists to half way down of the guttur which is made in the lower joists, overlapping just enough to deliver the water safely into the guttur. these guttur joists, as well as those constituting the ridges of the rooflet have a descent of 6. I. from their middle point to each end, which over-jets the wall so as to deliver it’s water clearly over it. a floor is laid over the whole to walk on, the water first falling on that & passing thro’ it’s cracks drops into the gutturs. I have had upwards of 20 years experience of these roofs in this house, also in one I built at another residence, and more than half our buildings at the University are flat and so covered. they never have leaked, cost less than a rafter roof, as needing no rafters and admit repairs more easily than any other. I think it the best possible roof for an Observatory.

With every wish for your welfare and happiness accept the assurance of my great esteem and respect.

Th: Jefferson