VII. ARTIFICIAL HORIZON.

An Artificial Horizon is a means of obtaining the angular elevation of any object lying above the plane of the Horizontal Plane of the Earth.

This Horizontal or Tangential Plane is a plane touching the Earth's spherical surface at the point where the observer stands.

If an object be elevated above the Horizontal Plane of the Earth, and a line be drawn from the observer's eye to such object, the angle that is made between that line and a line on the Tangential Plane drawn towards a point perpendicularly under the object, is the Angle of Elevation of that object, terrestrial or celestial; as a hill-summit, a Tower, the Head of Flag-Staff, a Star, the Moon, or the Sun.

In fact, to measure this angle the means is a Tangential Line, i. e. a line in the Tangential plane lying in the vertical plane which passes through the object and your eye.

If you are at sea, the most ready means of obtaining this Tangential Line is to use the sea's horizon; if on land, a small quantity of quicksilver poured out into a saucer secures you a fair mirror in which to see an object reflected. But whether you use the sea's horizon, or an artificial horizon, as this quicksilver mirror is called, the true means of observation is actually, or assumedly, a Tangential Line; in the case of the Sea Horizon, evidently so, the line going from your eye to that point in the horizon which is in the Vertical Plane in which the Heavenly Body is, being the medium of observation; and in the case of the artificial horizon of quicksilver it is in reality the same, inasmuch as the whole angle of reflection is divided by a line lying in the

plane of the Mirror, and that plane does the same duty as the Horizontal or Tangential plane at the point where the liquid surface stands, if it be quicksilver, or the metallic plate, if it be of speculum or quicksilvered glass.

A substitute for that metallic liquid has been found in a metallic plate levelled by means of a Spirit-Level, and three adjusting screws.

The present Instrument gives the Tangential Line in four ways:

i. A Horizontal Line at right angles to a Plumb Line, by which angles of elevation can be taken.

ii. One, in one leg of the Sector made tangential to the Earth's surface by means of the Spirit-Level.

iii. One, used in the hands, by which levels can be taken in the distance; and lastly by

iv. One, a Metallic Mirror, the substitute, with the aid of the Spirit-Level, for the Metallic Liquid, the Quicksilver Artificial Horizon.

Each of these expedients has its peculiar use; the first in taking elevations of heights, buildings, &c., the second in finding the Sun's elevation, the third for noting the ranges of the contours of a survey, and the fourth as a substitute for the Artificial Horizon generally.

That which is described under this (VII.) head of the subject is the last, the Metallic Plate, to be adjusted to true tangency to the earth's surface by means and aid of the Spirit-Level.

Should the observer not have the ball-and-socket tripod with him, which is the most convenient way of levelling the metallic plate, that plate must be embedded on the top of a small clay or earth heap, and be levelled by applying the edge of the Sector to its face in two directions, crossing one another.

It is to be observed in favour of a Metallic Plate or Artificial Horizon that, unlike the expedients of liquids, Quicksilver, Treacle or Ink, it requires no glass cover, nor glass penthouse roof, in order to keep the wind from ruffling its surface. Hence, when well managed, it is the best sort of Horizontal Plane.

To take an observation by means of this or any other sort of reflecting Artificial Horizon, observe the angle between the true object and its reflection.

The reading-off is the same as already described in the former case; but now it will not be necessary to double it, because the angle observed is itself double of the Angle of Elevation required.

The use of the other characters of Artificial Horizon, that is, of the Tangential Line, will be described under the respective heads to which the subject belongs-namely, in Articles X. and XVII.

VIII. 1. THE QUINTANT COMPASS.

Under Art. IV. notice was made of the Magnetic Needlebox, with the sight-vane, which constitutes the lid of the box. In order to observe the bearing of any object from yourself, with the Magnetic Meridian, you must, having opened the Sector to an angle of 90°,

1. Turn over the Needle-box, and erect the sight-vane and the Mirror in the joint.

2. Turn yourself about with the instrument in your right hand, until the Needle vibrates freely and rests in the middle of the Needle-box, i.e. with its north end in correspondence with the Lubber's Point' painted on the end of the Needlebox.

3. Raise the instrument always level to your eye, maintaining, as nearly as you can, its exact horizontality.

4. If the object whose bearing you wish to obtain is west of you, open the left leg of the Sector until you see through the slit in the sight-vane the object you seek on the scratch of the Mirror.

5. Having found the object thus, verify again the direction of the Needle by seeing that its contact with the Lubber's Point is good, as viewed through the slit in the sight-vane.

6. When such is the case, the bearing will be read off by the indication of the half-arrow, on the right-hand Quadrant. Thus (the Needle lying exactly magnetic North), if the object be seen to the West, and the half-arrow fall against the graduation 30°, that will indicate that the object bears from the Magnetic Meridian 60° west. Had the halfarrow pointed at 70° it would have indicated that the object bore 140° westward of North.

In the same manner, should the object lie East of the Meridian, the left leg of the Sector must be closed, until the object sought is seen on the scratch of the Mirror; the readingoff then will be on the left-hand Quadrant, and if, say, the half-arrow shall coincide with the degree 51, that will indicate that the bearing of the object is 102° eastward of North.

Magnetic observations taken by the Quintant Compass are only strictly true when the object whose bearing is sought is in the true horizon. Objects whose elevation or depression is so great as to involve such an inclination of the plane of the instrument as to prevent the oscillations of the Needle, cannot be taken at all by direct observation. In this case mark with the level or plumb some point in the same vertical line with your object, nearer the level of your eye, and its bearing will be the same as that required. Should however a tripod or plane

table be used, the inequalities of the horizon do not affect the observation.

There will be no practical disadvantage in this disability to take the whole circle of magnetic bearings, as it will rarely occur but that the bearing of some one object in the whole circular horizon can be realized exactly, and the bearing of the others may be obtained very approximately by their angular distance from this one, or from one another; large angles being the best. Any one bearing thus obtained at one station will suffice perfectly for the whole survey, so that the compass will be rarely required; and it may be observed as the rule, that when the Quintant is furnished with an Annex, with the help of which it takes all angles up to 180°, angles are taken much more rapidly, as well as with greater accuracy, than bearings with the always unsteady compass.

N.B. 1. That that order of reading of the Vernier Scale is in every case to be used, which accords with the direction of the reading on the Quadrants of the Disc, which the observer is using. If the numbers read from left to right, the left-toright reading of the Vernier Scale must be used. If the numbers run from right to left on the Quadrants, the right-toleft reading of the Vernier must be used.

N.B. 2. That the reading by the Magnetic compass is, it is to be expressly observed, not from 0 or North to 360°, but from 0 to 179° East, and from 0 or North to 179° West; 180° being due South. In most Prismatic Compasses the reading is arranged in accordance with the former plan: in this instrument it cannot be so. Some considerable authorities, Colonel Jackson for example, find this the preferable method.

7. As the acuteness of the angle at which the visual ray falls upon the Mirror at the north end of the instrument, viz. the angles from 0 to about 10°, which is practically 20°, pre