A graphometer is not always to be had. When approximate distances only are required, the following contrivance may be used. Trace on a cardboard of large size a semi-circumference which one divides first into 180 equal parts, then each of these is divided again in 2, 3, 4 divisions, etc., according to the size given to the circumference, which constitutes a large protractor.
To measure an angle place the cardboard upright in an horizontal position, supporting it by the center of the semi-circumference by means of a screw fixed on a stick. Then proceed as stated above.
From a pin stuck in the center mark the spot where the visual ray passes, go to A and to B, and you get approximately the desired result.
The meridian line of a place is an imaginary line passing through this place and the center of the sun, when the latter is at the highest point of the arc of the circle, which it daily describes. At that very moment it is noonday exactly at the place in question.
As the position of the earth changes from day to day, the sun does not every day touch the meridian line at noon; sometimes it is in advance, sometimes behind.
Various instruments have been invented to indicate in a practical manner the meridian of a place. We owe the following construction to Mr. E. Brunner of the longitudinal office.
On a window-sill in a southerly position, fix in a solid, permanent manner, a small cupful of quicksilver; cover it with a lid made of varnished metal, and pierced in its center by a small round hole about a quarter of an inch in diameter. This lid must fit well, but not too tightly, so as to permit its being lowered in close proximity to the surface of the quicksilver.
When the window is open the solitary ray reflected on the mercury will be projected on the ceiling of the room. At the exact noonday the center of the mirror and the center of the reflected image are in the meridian plane. It remains only to be traced.
At the moment of its passage one marks in B, for example, a spot corresponding to the center of the reflected image; one knocks a small nail there, and with a string connect this point with another outside the window, so that the string passes through the center of the diaphragm, M. The line, B M, is the meridian plane. From A, suspend a lead-line which meets the string, B M.
