659.—For Using this Artificial Horizon, when the mercury is poured out in the tray M, it is levelled by the three screws AA′A″ so that it covers the bottom of the tray and presents a clear, level surface. A separate disc of parallel glass, which fits the tray M very loosely, is provided with the instrument. This floats on the surface of the mercury and keeps it quite still, even when the covering glass is removed. This arrangement is useful also in case of an accident to either of the glass covers. The disc is kept when out of use in a soft leather bag which fits in the tray M. This artificial horizon is generally carried in a solid leather case with sling to go over the shoulder. Its weight complete is about 4½ lbs.; size, 9½ inches by 4 inches by 1½ inches. The surface of mercury is a circle of 3 inches diameter.
660.—Improved Captain George's Artificial Horizon.—Mr. S. A. Ionides, C.E., has devised an improved form of the foregoing instrument shown at Fig. 295.
Fig. 295.—Ionides's artificial horizon.
Larger image
In this the container is formed beneath the horizon box with a plug tap fitted in the thickness of metal between the two; this form makes the whole much lighter and less than half the size of the usual Captain George's pattern.
661.—In Using the Artificial Horizon with the Sextant it is generally placed on the ground at such a distance in front of the observer that he can conveniently see the required reflection of the star or sun, the observer moving about until the reflection is obtained. This is a tedious process and requires some practice. It is much more easily effected if the sextant be mounted on a tripod or other stand. When a stand is used it has generally a universal joint, so as to be able to take surface angles also from the fixed position. When the altitude of objects on the earth is taken, the observation requires reduction for refraction, which becomes an important factor, although this is variable with atmospheric conditions; but upon the whole it always tends to make the object appear higher than it really is. Commonly one-seventh of curvature is used as an approximate correction. For solar and stellar refraction, works on astronomy should be consulted.
The index error of the sextant is corrected before refraction, when the natural horizon is employed. When the artificial horizon is used the index error is allowed before taking its half as a single measure. The artificial horizon is used also with the theodolite. It forms the most perfect means of adjusting the transverse axis by taking an observation of the pole star with the telescope, first directly and then by its reflection from the artificial horizon. If the images cut the centre of the webs in the two positions by the movement of the transverse axis only from the one to the other, this axis is proved perfectly level.
662.—Various schemes for obtaining the horizon by some system of levelling apparatus attached to the sextant have been devised, none of which are very practical, as they all depend upon a pendulum or a gravitation surface of a liquid or a gyroscope, and are all unstable as hand instruments. There have been numerous patents taken out with this object from that of Winter (1760) downwards, which anyone interested in the subject may consult.[49] The matter is mentioned here as the recurrence of the idea appears to be frequent.
