Thomas Jefferson Papers

From Thomas Jefferson to Charles Bonnycastle, 30 September 1825

Sep. 30. 25.

Ideas on the subject of a Meridian for the University.

The small mountain a little to the West of the South from the University was purchased for the purpose of an Observatory, whenever the establishment of one may become desirable. it is proposed that wherever the meridian from that may cross the South West ridge of mountains, to cut a vista through the woods across the ridge in the direction of the meridian, and in that Vista erect an Obelisk with an iron needle in the top. this will give a meridian of probably 7. or 8. miles in length.

In the mean time however one will be wanting at the University. for this I propose to use the Portico of the Rotunda in this way. in the center between the two middle columns, make fast to the Soffite a strap of strong hoop iron in this form [GRAPHIC IN MANUSCRIPT] securing the limb a.b. across the bottom of the Soffite by screws, and making an eyelet hole in the limb b.c. for a ray of the sun to pass through: perpendicularly under the eyelet hole, drill in the pavement as small a hole as can be drilled, and plug it with an iron pin, filed down to the level of the surface of the pavement. on this, as a center, draw several concentric portions of circles, tracing them very slightly on the pavement, and, by repeated observations, before and after noon, for several days mark the points at which the solar ray crosses them, and the middle point between these crossings of every circle, and average the whole for a true meridian line. draw it strongly on the pavement, from the iron plug to the wall of the house, and perpendicularly up the wall, as far as the meridian altitude of the sun at the winter solstice shall require. but this cannot be done until the pavement of the Portico shall be laid.

Until it is done, the rising or setting sun will give time by the clock, with sufficient accuracy for meals and meetings of the different schools.

For the plan of an observatory see a late Vol. of the A.P. Transactions which I send to mr Keane for the Library.

Th: Jefferson


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