Surveying and Levelling Instruments, Theoretically and Practically Described. / For construction, qualities, selection, preservation, adjustments, and uses; with other apparatus and appliances used by civil engineers and surveyors in the field.

by William Ford Stanley

408.—When the instrument is made perfectly horizontal, the equatorial lines and the opposite lenses being accurately adjusted to each other by a previous operation, the sun's position in the heavens with reference to the horizon will be defined with precision. Suppose the observation to be made at the time of one of the equinoxes; the arm R set at zero on the declination arc V; and the polar axis is placed exactly parallel to the axis of the earth. Then the motion of the arm R, if revolved on the polar axis around the hour circle H, will exactly correspond with the motion of the sun in the heavens on the given day and at the place of observation; so that if the sun's image be brought between the lines cc on the image plate in the morning it will continue in the same position, passing neither above nor below the lines as the arm is made to revolve in following the motion of the sun about the earth.

409.—In the morning as the sun rises from the horizon, the arm R will be in a position nearly at right angles to that shown in the illustration, the lens being turned towards the sun and the silver plate, on which his image is thrown, directly opposite. As the sun ascends, the arm must be moved around, until when he has reached the meridian, the graduated side of the declination arc will indicate XII on the hour circle; and the arm R, the declination arc V, and the latitude arc, that is the vertical arc of the theodolite, will be in the same plane.

As the sun declines from the meridian the arm R must be moved in the same direction, until at sunset its position will be the exact reverse of that it occupied in the morning.

410.—Allowance for Declination.—Let us now suppose the observation made when the sun has passed the equinoctial point, and when his position is affected by declination. Then, by referring to the Nautical Almanac and setting off on the arc his declination for the given day and hour, we are still able to determine his position with the same certainty as if he remained on the equator.

When the sun's declination is south, that is, from the 22nd of September to the 20th of March in each year, the arc R is turned towards the plates of the instrument in the opposite position to that shown in the engraving, using the solar lens at I, with the silver plate opposite at L.

The remainder of the year the arc is turned from the plates, and the lens at L and plates at I are employed in the position shown in the figure.

411.—When the solar compass is accurately adjusted and its plates made perfectly horizontal, the latitudes of the place and the declination of the sun for the given day and hour being also set off on their respective arcs, the image of the sun cannot be brought between the equatorial lines until the polar axis is placed in the plane of the meridian of the place, or in a position parallel to the axis of the earth. The slightest deviation from this position will cause the image to pass above or below the lines and thus discover the error.

412.—We thus, from the position of the sun in the solar system, obtain a certain direction absolutely unchangeable from which to run our lines and measure the horizontal angles required.

The transit theodolite will, without the solar compass, perform the same functions; but by means of this instrument the calculation for position is much more simple.