The concave ceiling of the Rotunda is proposed to be painted sky–blue and spangled with gilt stars in their position and magnitude copied exactly from any selected hemisphere of our latitude. A seat for the Operator movable and fixable at any point in the concave, will be necessary, and means of giving to every star it’s exact position.
[GRAPHIC IN MANUSCRIPT]
Machinery for moving the Operator.
a. b. c. d. e. f. g. is the inner surface of 90° of the dome. o. p. is a boom, a white oak sapling of proper strength, it’s heal working in the center of the sphere, by a compound joint admitting motion in any direction like a ball and socket.
p. q. r. is a rope suspending the small end of the boom, passing over a pully in the zenith at q. and hanging down to the floor, by which it may be raised or lowered to any altitude.
at p. a common saddle with stirrups is fixed for the seat of the operator and seated on that, he may by the rope be presented to any point of the concave.
Machinery for locating the stars.
a. s. is the horizontal plane passing thro the center of the sphere o. an annular ream of wood, of the radius of the sphere must be laid on this plane and graduated to degrees and minutes, the graduation beginning in the North rhomb of the place. call this the circle of amplitude. a movable meridian of 90° must then be provided, it’s upper end moving on a pivot in the zenith, it’s lower end resting on the circle of amplitude. this must be made on thin flexible white oak like the ream of a cotton spinning wheel; and fixed in it’s curvature, in a true quadrant by a similar lath of white oak as it’s chord a. n. their ends made fast together by clamps. this flexible meridian may be of 6.I. breadth, and graduated to degrees and minutes.
the zenith distance and amplitude of every star must then be obtained from the astronomical tables. place the foot of the moveable meridian in that of the North rhomb of the place, and the polar star at it’s zenith distance, and so of every star of that meridian; then move the foot to another meridian at a convenient interval, mark it’s stars by that zenith distance, and so go around the circle.
bh. ci. dk. el. fm. are braces of window cord for keeping the meridian in it’s true curve.
perhaps the rope had better be attached to the boom at s. instead of p. to be out of the way of the operator. perhaps also the chord board an had better present it’s edge to the meridian than it’s side.
if the meridian ark and it’s chord be 6. I. wide & ½ I. thick they will weigh about 135℔ and consequently be easily manageable.
if the boom op. be 35. f. long, 6.I. at the but & 3.I. at the small end, it will weigh about 100℔ and be manageable also
ViU: Papers of Thomas Jefferson.