Surveyors Office 15th. Octr 1804
Being requested by Mr Briggs to assist him in running a true Meridian line, which should pass through the center of the Presidents house; and to perpetuate the same, as also the point of intersection by a due West line drawn from the center of the Capitol, by fixing permanent marks thereon; and as the pressure of other Official engagements prevented his attention to more than ascertaining the Meridian line, and marking it temporarily, it devolves on me to describe the mode pursued in ascertaining the line and the required intersections, that others may know what dependence is to be placed on their accuracy.
In running the Meridian line, I acted only in the capacity of assistant, conforming entirely to the instructions of Mr. Briggs; and executing with all the care in my power the marking of his temporary line, agreeably to his signals and instructions, in giving which I believe he used all the accuracy of which the instrument was capable.
With a new Transit instrument, executed by Voigt of Philadelphia, he ascertained the place of the Star in the tail of the constellation Ursa Minor, on its greatest eastern elongation, and continuing the Vertical circle to the surface of the ground by the instrument, he determined the bearing; in the line of which, on a very low stand was placed one of Argands lamps covered by a tin cylinder in which a small slit was made, for a sight; and the line from the light towards the instrument was drawn upon the stand. Knowing the radius of the circle described by the Star, or half the angle formed by its greatest elongations, east and west, and the altitude of the pole; he by calculation deduced the horizontal angle made by two Vertical circles, one of which is the Meridian, passing through the pole, the other through the Stars place when farthest east. This horizontal angle being found, and the base line measured from the place of the instrument (the North door of the Presidents house) to where it is intersected by an east and west line from the place of the light or sight used (the north side of north I street) afforded the necessary data, for calculating the distance to be measured west, from the place of the sight to the true meridian line: which offset was very carefully made and the line marked on the head of a post firmly driven into the ground. The meridian being thus fixed and a point found due north from the place of observation the line was continued by the instrument at one sight, and tested by reversing the telescope at an intermediate station in the line, to the top of a hill near two miles north of the presidents house, on the lands of Mr Robert Peter; where temporary posts were fixed, and the line marked upon them according to the instructions by signal from Mr Briggs at the instrument. From the presidents house the line was reversed, by the instrument, and continued south across the Tyber creek, and marked in the same manner on the head of posts driven in the Public appropriation called the mall.
Having obtained the permission of Mr Peter, early in september I planted a small obelisk of free stone, prepared by Mr Blagden, in the Meridian line north of the presidents house on the hight where the stakes had been fixed under the direction of Mr Briggs. The apex is in the true meridian from the center of the north door.
In perpetuating the south line it was deemed best to place the stone where the Meridian line should be intersected by a West line from the Capitol: the surface of the ground however being unfavorable,—the capitol not being visible at the point of intersection,—it suggested the planting an obelisk similar to that on the north line, at a point on the meridian; west from the South end of the capitol, and where the building was distinctly seen, and from thence find the intersection of the center line, by measuring northwards half the length of the Capitol. Although the body of the building was in full view, yet intervening trees prevented my seeing with the necessary distinctness the south end of the capitol; and I had to find the required point on the Meridian by setting off the angle included between the Northwest corner of the building and the center of the presidents house. This angle I had previously calculated, from the distance 7696 8/10 feet, and the length of the Capitol 351 feet 5 Inches, to be 87°. 23’. 06". 8.—For the greater accuracy I measured this angle from different parts of the circle of a Theodolite, made by Adams; going several times round the instrument, and taking the mean as the correct angle. In one instance however I found a variation of nearly two minutes in the angular distance of the buildings, in others they would coincide for several observations. I thus ascertained my possition, and the point on the meridian line, from which a line drawn at right angles, due East, would touch the south end of the capitol, with all the precision the instrument is capable of. Here I planted the obelisk, and measured from the center of it, north 175 feet 8½ inches, half the length of the building for the point of intersection on the meridian drawn through the center of the Presidents house, by a West line from the center of the Capitol. It is on the south bank of tyber creek, and marked by the erection of a small pier, covered by a flat free stone on which the lines are drawn.
I am sir, respectfully yours.
Nichs King s.C.W
DLC: Papers of Thomas Jefferson.