Instructions for Setting a Sundial
[ca. 6 Mar. 1817]
To set the Dial.
The first and all-important object is to have the top of the dial post perfectly horizontal. without this it never can be true one moment. to this end, after the post is immoveably fixed in the ground, the top should be tried with a level and planed to the true horizontal level in every direction. it will take a butt of a tree 28. or 29.I. diameter. when planed, place the dial on it concentrically with the post. fix the gnomon of the dial in it’s groove truly, the toe at the 6. aclock mark, the heel on the 12. aclock or meridian line.1 test the perpendicularity of the gnomon by the walnut square sent; (which square had better be kept to rectify the perpendicularity occasionally when, by any accident it gets bent.) set the dial then as nearly right as you can, by a watch, or by guess. about 9. aclock in the forenoon stick a bit of wax, of the size & form of a Bristol-drop shot on the upper edge of the gnomon, sliding it up or down on the edge until2 it’s shadow shall fall exactly on the outer circle of the dial. mark that point in the circle slightly with the point of a pin. let the wax stay on exactly in the same place,3 and, in the afternoon, watch when it’s shadow shall be crossing the same circle on the other side, and mark the circle there again with the point of a pin. if these two points are equidistant from the 12. aclock, or meridian line, the dial is right: if not, take half the difference between their two distances from the meridian line, and twist the dial plate round on the block exactly that much from the nearest point. do the same the next day, & so the 3d 4th Etc. days until the shadow of the wax pellet shall cross the circle exactly equidistantly from the meridian. your dial will then be accurately true. have 2. nails ready entered in the North and South nail holes in the margin and tap them alternately that they may fix the dial firmly4 to the block without jostling it out of place, and when driven home secure it by screws thro’ the other holes. the gnomon being of sheet lead and liable to be bent is made to take out easily to be straitened again between two boards.
MS (MHi); written entirely in TJ’s hand on verso of portion of a reused address cover to him; undated. Enclosed in TJ to John Wayles Eppes, 6 Mar. 1817.
In 1782 William Watts of Bristol, England, patented a method of dropping hot lead from different heights to produce small round shot of varying sizes (Repertory of Arts and Manufactures 3 : 313–5).
1. Reworked from “the heel where the 12. aclock or meridian line intersects the inner circle.”
2. Preceding nine words interlined in place of “so that.”
3. Preceding five words interlined.
4. Word interlined.
- Jefferson, Thomas; Writings; Instructions for Setting a Sundial search
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- Watts, William (of Bristol, England); and shot manufacture search