It is not ascertained, at what time Mr. Rittenhouse first conceived the plan of that extensive, complicated and inestimable Orrery, which he afterwards executed. Probably, he had long thought on the subject, before he publicly announced his design. It is certain, however, that before the beginning of the year 1767, there was some correspondence and some understanding, respecting it, between himself and the Rev. Mr. Barton. It appears in fact, that, prior to that period, Mr. Barton had been fully apprized of his brother-in-law’s desire to carry into effect his meditated design of constructing a complete Orrery, on a plan entirely new; and that some arrangement was previously made, between these gentlemen; by which Mr. Barton undertook to indemnify Mr. Rittenhouse, for such actual expenditures as he should incur in making the machine and his loss of time while employed in the work, not exceeding a stipulated sum; provided he should not be able to dispose of it, when finished, at a price then fixed on. The prudential caution of our young Philosopher (then about thirty-four years of age,) and the public spirit of his friend, grounded on the confidence he had in the artist’s talents and abilities, were alike evinced on this occasion. The first written communication made by Mr. Rittenhouse to Mr. Barton, on the subject of the Orrery, is contained in a letter under the date of Jan. 28th, 1767: it is in these words:—“I am glad you took the pains to transcribe, and send me, Martin’s _Account of Orreries_.”[137] “Two forms (he says) have principally obtained, the Hemispherical Orrery and the Whole Sphere. But the idea given us by the former, is very unnatural and imperfect. An Orrery, then, adapted to an Armillary Sphere is the only machine that can exhibit a just idea of the true System of the World.”—“But in my opinion,” says Mr. Rittenhouse, “the latter is likewise very unnatural; for, what has a Sphere, consisting of a great number of metaline Circles, to do with the true System of the World? Is there one real, or so much as apparent Circle, in it? (the bodies of the Sun and Planets excepted.) Are they not all merely imaginary lines, contrived for the purpose of calculation? I did not intend to let one of them have a place in my Orrery, except the Zodiac, on which I would have the true latitude and longitude of each planet pointed out by its proper Index.” ----- Footnote 137: See Note 131. ----- “I did not design a Machine, which should give the ignorant in astronomy a just view of the Solar System: but would rather astonish the skilful and curious examiner, by a most accurate correspondence between the situations and motions of our little representatives of the heavenly bodies, and the situations and motions of those bodies, themselves. I would have my Orrery really useful, by making it capable of informing us, truly, of the astronomical phænomena for any particular point of time; which, I do not find that any Orrery yet made, can do.”