That I have so long delayed acknowledging your favours has been owing to my absence from home. Two Summers past I have been employed in determining the Boundary of Pennsylvania. In 1784. Observations were continued for near 3 months on the Eclipses of Jupiters Satellites, on the Banks of the Delaware and near the Ohio. The result gave 53,225 Miles to a degree of Longitude in Latitude 39.° 43.’ 18.”, something less than we had reason to Expect from former measurements of Degrees of Latitude.

The Autumn of 84 and Summer of 85 I have spent in assisting to carry on the remainder of the Southern and the Western Boundary of this State. This last being one Straight Meridian Line is certainly the most grand and beautiful Vista in the World. In these Excursions I have not been inattentive, so far as my Situation wou’d permit, to the natural Curiosities of that Country, and I wish to give you some account of them when I can find leisure for the purpose.

You have greatly obliged me by the Books you have sent. Each of the Connoissance Des Temps contains many things Curious and useful. ’Tis a pity they are not more correctly printed. But the Notes on Virginia are an inestimable Treasure. I have just received the Book and have not yet read it with that attention I mean to bestow on it. Nothing has hitherto occurred to me in it contrary to my own Philosophical notions, except that of Stones growing in imitation of Shells without real animal Shells to give them that form. The Petrifications I have collected on the Allegany Mountains and beyond them seem to me sufficient to induce you to give up that opinion. But I may be mistaken. What has with me the greatest weight is that abundance of the Shells and Bones thus found lodged in Solid Stone are still real Shells and Bones, as not only their appearance but their taste and smell when heated sufficiently evince. I must beg a continuance of your favours and that you will inform me how I may reimburse the Expence you have been at on my account. Mrs. Rittenhouse’s Compliments and mine to Miss Jefferson. I hope she is very happy, nay I am sure she must be so under the care of such a parent. Dear Sir, your most sincere friend and Humbl Servt.,