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CH. VI] HELIOSTATS FOR THE MAGIC LANTERN 147
G F Column supporting the clock-work.
N P L° The clock-shaft. It is parallel with the earth's axis and hence points toward the celestial north pole. The angle L° at the lower end is equal to the latitude of the place where the heliostat is used.
T R Fork where the movement of the clock-hand (N 0) is transferred to the shaft actuating the mirror (D E S).
f g Plate bearing the clock-work. It must be elevated sufficiently to make the angle of the clock-shaft equal to the latitude of the place (see fig. 85).
It answers very well for all the work required by the photographer and the projectionist.
§ 240. How to make the clock-shaft parallel with the earth's axis at any given place. — For this it is necessary to know two things :
(1) One must know the north and south direction.
(2) One must know the latitude of the place.
The first information can be gained by referring to the pole star. Buildings are often set due north and south, and thus serve as guides; or one might use a compass. If a magnetic needle is used it must not be forgotten that there is a certain variation from the true north and south line assumed by the compass needle, and for accurate observations it is necessary to know the magnetic variation at any given place and to correct for it.
For the latitude, a good map like that issued by the U. S. geological survey will give the information. The geological survey maps also give the magnetic variation.
Making the clock-shaft parallel with the earth's axis is easily accomplished if one knows the latitude and the north and south direction. As a general statement all that is necessary is to make the clock-shaft point toward the north star or more accurately, toward the celestial north pole.
By referring to fig. 85 it is evident that this is brought about by putting the instrument due north and south and then elevating the clock-shaft above the level or horizontal line an amount equal to the latitude of the place.
For example, if an experiment with the heliostat is to be made in one of the buildings of Cornell University at Ithaca with, in round numbers, a latitude of 42.5 degrees, the instrument is set on a level place and due north and south, then the free end of the