652.—To Use the Sextant the right foot should be placed nearly 2 feet in advance of the left and directed at right angles to it. In this position the body is firm. The instrument is supported by the right hand, the elbow being brought down firmly upon the body. The clamp screw first and then the tangent screw are moved by the thumb and finger of the left hand. Some practice is required to make a steady observation. To bring two objects into apparent juxtaposition, methods of observation for terrestrial objects will be reconsidered in discussing the box sextant further on. As regards celestial observations reference should be made to works on practical astronomy, as the subject would take too much space to be entered upon here. The whole subject, with many refinements of correction of parallax, etc., which fall beyond the limits of practical surveying with the sextant, is ably discussed in Chauvenet's Spherical and Practical Astronomy.
653.—Artificial Horizon.—For ascertaining the latitude of a place from the observation of a celestial body by means of a sextant, it is necessary to have some means of estimating the position of the horizon. A method of doing this, originally proposed by the elder George Adams, optician, 1748,[46] was to float a parallel disc of glass upon a basin of mercury, and to receive the reflected image of a star from the mercury by the sextant simultaneously with its direct image. The angle then given by the reading of the arc is double the angle at which the true horizon is placed relatively at the same time. This idea, carried out in a practical form in an instrument henceforth called the artificial horizon[47] is due to Wm. Jones, a well-known optician at the end of the 18th and beginning of the last centuries, who arranged convenient means of making the instrument portable, and to keep the mercury from disturbance of the air by covering it with a glass roof. The form of artificial horizon that he invented has been in common use ever since. He also invented another simpler form, which was that of taking the reflection from a piece of silvered, or of black, glass. The performance of the artificial horizon depends in any case entirely upon means of obtaining a reflection from a perfectly horizontal surface.
Fig. 290.—Diagram of artificial horizon.
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654.—Theory of the Artificial Horizon.—A ray M′ Fig. 290, from a luminous body, at infinite distance will have its image reflected from a level reflecting surface SS′ at an angle equal and opposite to the incident ray, the angles M′AS and EAS′ being equal. Let E be the place of the eye or the sextant: this will receive a ray from the same distant body in direction ME, which is sensibly parallel with M′A. The angle MEA being double the angle of incidence M′AS, the half of this angle will therefore produce the horizontal line EH at the height of the observer's eye if the plane of reflection SS′ be level. Therefore if we take half this angle MEA as it appears in the sextant, it will give an angular position of the object in relation to the horizon at the height of the eye, or be tangential to the surface of the earth. If M′AS be 30°, the angle AEM will be 60°, showing the elevation of object half this or 30°. The sextant takes 120° with certainty; therefore 60° will be the limit of meridian altitude the artificial horizon will measure.
Fig. 291.—Artificial horizon of black glass.
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655.—Artificial Horizon in Black Glass.—This instrument is the most portable, packing in a close pocket case. It is made of both circular and square form in plan. Fig. 291 is the Admiralty pattern. The black glass should have a truly plane surface. It is fixed over a brass tray by being floated on plaster of Paris to avoid strain. A light rim of brass is screwed down over the glass to keep it in position. There are three adjusting screws AA′A″. It is adjusted to level by a loose level tube ground on its under face P. The level tube shown in detail [Fig. 51], p. 94, is placed on the surface lineally with the two screws, Fig. 291 AA′, and afterwards at a right angle to its first position with one end of the tube towards A″. It is finally tested by traversing at the position shown in the Fig. 291, and at right angles to this direction. There is a great risk of getting a strain on the glass in fixing it in its frame. The author therefore prefers the circular form that leaves the glass quite free except at its fixings at three equidistant points only. In this kind of artificial horizon there is only one surface of glass to be worked true; therefore, there is perhaps less risk of error on this account than in other forms. On the other hand the mercury presents a more perfectly level plane. The circular artificial horizons are commonly made 3¼ inches diameter; weight, ¾ lb.; the oblong, Fig. 291, 4 inches by 3 inches; weight 2 lbs.