the button-screw in the middle of the lever, on the under side of the Quadrant, and move the Horizon glass on its axis, by turning the nut at the end of the adjusting lever, till you have made them perfectly coincide; then fix the lever firmly in this situation by tightening the button-screw. This adjustment ought to be repeated before and after every observation. Some observers adopt the following method, which is called finding the Index error. Let the Horizon glass remain fixed, and move the Index till the image and object coincide; then observe whether O on the Nonius agrees with 0 on the Arch, if it does not, the number of minutes by which they differ is to be added to the observed altitude or angle, if the O on the Nonius be to the right of the O on the Arch, but if to the left of the O on the limb, it is to be subtracted.

It has already been observed, that that part of the Arch beyond 0, towards the right hand, is called the Arch of excess: the Nonius, when the O on it is at that part, must be read the contrary way, or which is the same thing, you may read off the minutes in the usual way, and then their complement to 20 minutes will be the real number, to be added to the degrees and minutes pointed out by the O on the Nonius.

III. To set the Fore Horizon Glass perpendicular to the Plane of the Quadrant.

Having previously made the above adjustment, incline the Quadrant on one side as much as possible, provided the Horizon continues to be seen in both parts of the glass; if when the instrument is thus inclined, the edge of the sea seen through the lower hole of the Sight Vane continues to form

one unbroken line, the Horizon glass is perfectly adjusted; but if the reflected Horizon be separated from that seen by direct vision, the speculum is not perpendicular to the plane of the Quadrant: then if the limb of the Quadrant is inclined to wards the Horizon, with the face of the instrument upwards, and the reflected sea appears higher than the real sea, you must slacken the screw before the Horizon glass, and tighten that which is behind it; but if the reflected sea appears lower, the contrary must be performed. Care must be always taken in this adjustment to loosen one screw before the other is screwed up, and to leave the adjusting screws tight, or so as to draw with a moderate force against each other.

This adjustment may be also made by the Sun, Moon, or a Star; in this case the Quadrant is to be held in a vertical position; if the image seen by reflection appears to the right or left of the object seen directly, then the glass must be adjusted as before by the two screws.

It will be necessary, after having made this adjustment, to examine if the Horizon glass still continues to be parallel to the Index glass, as sometimes by turning the sunk screws the plane of the Horizon glass will have its position altered.

USE OF HADLEY'S QUADRANT.

The use of the Quadrant is to ascertain the Angle subtended by two distant objects at the eye of the observer; but principally to observe the altitude of a celestial object above the Horizon: this is pointed out by the Index when one of the ob

Ss

jects seen by reflection is made to coincide with the other, seen through the transparent part of the Horizon glass.

To take an Altitude of the Sun, Moon, or a Star, by a Fore Observation.

the eye

Having previously adjusted the instrument, place the O on the Nonius opposite to 0 on the Arch, and turn down one or more of the screens, according to the brightness of the Sun; then apply to the upper hole in the fore Sight Vane, if the Sun's image be very bright, otherwise to the lower, and holding the Quadrant vertically, look directly towards the Sun so as to let it be behind the silvered part of the Horizon glass, then the coloured Sun's image will appear on the speculum; move the Index forward till the Sun's image, which will appear to descend, just touches the Horizon with its lower or upper limb; if the upper hole be looked through, the Sun's image must be made to appear in the middle of the transparent part of the Horizon, but if it be the lower hole, hold the Quadrant so that the Sun's image may be bisected by the line joining the silvered and transparent parts of the Horizon glass.

The Sun's limb ought to touch that part of the Horizon immediately under the Sun, but as this point cannot be exactly ascertained, it will be therefore necessary for the observer to give the Quadrant a slow motion from side to side, turning at the same time upon his heel, by which motion the Sun will appear to sweep the Horizon, and must be made just to touch it at the lowest part of the Arch; the degrees and minutes then pointed out by the Index on the Limb of the Quadrant will be the observed altitude of that limb which is brought in contact with the Horizon.

When the meridian or greatest altitude is requir ed, the observation should be commenced a short time before the object comes to the meridian; being brought down to the Horizon, it will appear for a few minutes to rise slowly; when it is again to be made to coincide with the Horizon by moving the Index forward; this must be repeated until the object begins to descend, when the Index is to be sccured, and the observation to be read off.

From this description of the Quadrant and its use, the manner of adjusting and using the Sextant will be readily apprehended. Our limits will not allow a particular description of this excellent instrument.

The Artificial Horizon.

In many cases it happens that altitudes are to he taken on land by the Quadrant or Sextant; which, for want of a natural horizon, can only be obtained by an artificial one. There have been a variety of these sorts of instruments made, but the kind now described is allowed to be the only one that can be depended upon. It consists of a wood or metal framed roof, containing two true parallel glasses of about 5 by 24 inches, fixed not too tight in the frames of the roof. This serves to shelter from the air a wooden trough filled with quicksilver. In making an observation by it with the Quadrant, or Sextant, the reflected image of the sun, moon, or other object, is brought to coincide with the same object reflected from the glasses of the Quadrant or Sextant: half the angle shown upon the limb is the altitude above the horizon or level required. It is necessary in a set of observations that the roof be always placed the same way. When done with, the roof folds up flat-ways, and, with the quicksilver in a bottle, &c. is packed into a portable flat case.

SECTION III.

To find the Latitude by the Meridian Altitude of the Sun.

The Latitude of a place is its distance from the equator, either North or South; and is measured by an arch of a Meridian contained between the Zenith and the equinoctial. Hence, if the distance of any heavenly body from the Zenith, when on the Meridian, and its declination, or the number of degrees and minutes it is to the Northward, or Southward of the equinoctial, be given, the Latitude may thence be found

The Altitude of the Sun, observed by a Quadrant, or Sextant, requires four corrections in order to obtain the true altitude; these are the Semidiameter, Dip, Refraction, and Parallax.

By the Semidiameter of the Sun is meant the angle subtended by the distance from its centre to its apparent circumference. The quantity of this angle is given for every sixth day in the year in table 10.

The Dip of the Horizon is a vertical angle contained between a Horizontal plane passing through the eye of an observer, and a line drawn from his eye to the visible Horizon. This Dip is found in Table 8, when the visible horizon is formed by the apparent junction of the water and sky; but in Table 9, when land intervenes. In this case, the line that separates the land and water is used as the Horizon, and its distance from the observer must be duly estimated.

The Refraction of any celestial body is the difference between its apparent place, and that wherein it would be seen, if the space between the observer and object, was either a void, or of a uniform desity. Table 6 contains this Refraction.

That part of the heavens, in which an object appears, when viewed from the surface of the earth, is called its apparent place; and the point, wherein it would be seen, at the same instant, if viewed from the centre of the earth, is called its true place; the difference between the true and apparent places, is called the Parallax. Sun's Parallax in Altitude is found in Table 7.

RULE

The

For finding the Latitude from the Sun's Meridian Allitude.

Having observed with the Quadrant, or Sextant, the altitude of the Sun's lower limb above the visible horizon, or the line of separation of the land from the water, when that horizon is obstructed by land-add thereto the semidiameter, taken from table 10 at the given day of the month, or the one nearest to it, and from this sum subtract the