which turn on a centre, so that they may be used separately or together, as the brightness of the object may require. The green glass may be used also alone, if the Sun be very faint; it is likewise used in taking observations of the Moon; when these glasses are used for the fore observation, they are set immediately before the fore Horizon glass, but in front of the other Horizon glass, when a back observation is made.

The Sight Vanes are pieces of brass, standing perpendicular to the plane of the instrument: that one which is opposite the fore horizon, is called the fore Sight Vane, the other the back Sight Vane. There are two holes in the fore Sight Vane, the lower of which, and the upper edge of the silvered part of the fore Horizon glass, are equidistant from the plane of the instrument, and the other is opposite to the middle of the transparent part of that glass; the back Sight Vane has only one hole, which is exactly opposite to the middle of the transparent slit in the Horizon glass to which it belongs: but as the back observations are liable to many inconveniences and errors, we shall not give any directions for their practice.

ADJUSTMENTS.

The several parts of the Quadrant being liable to be out of order from a variety of accidental circumstances, it is necessary to examine and adjust them, so that the instrument may be put into a proper state, previous to taking observations.

An instrument properly adjusted, must have the Index glass and Horizon glasses perpendicular to the plane of the Quadrant; the plane of the fore Horizon glass parallel, and that of the back Hori

zon glass perpendicular to the plane of the Index glass, when the 0 on the Nonius is at 0 on the Arch; hence the Quadrant requires five adjustments, the first three of which being once made, are not so liable as the last two to be out of order; however they should all be occasionally examin ed in case of an accident.

I. To set the Plane of the Index Glass perpendicular to that of the Instrument.

Place the Index near to the middle of the Arch, and holding the Quadrant in a horizontal position, with the Index glass close to the eye, look obliquely down the glass, in such a manner that you may see the Arch of the Quadrant by direct view, and by reflection at the same time; if they join in one direct line, and the Arch seen by reflection forms an exact plane, or strait line, with the Arch seen by direct view, the glass is perpendicular to the plane of the Quadrant; if not, it must be restored to its right position by loosening the screw, or tightening it, or vice versa, by a contrary operation.

II. To set the Fore Horizon Glass parallel to the Index Glass, the Index being at 0.

Set the 0 on the Nonius exactly against 0 on the Arch, and fix it there by the screw at the under side. Then, holding the Quadrant vertically, with the Arch lowermost, look through the Sight Vane, at the edge of the sea, or any other well defined and distant object. Now, if the Horizon in the silvered part exactly meets, and forms one continued line with that seen through the unsilvered part, the Horizon glass is parallel to the Index glass. But if the Horizons do not coincide,

then loosen 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 buttonThis 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 0 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 0 on the Nonius be to the right of the 0 on the Arch, but if to the left of the 0 on the limb, it is to be subtracted.

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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 0 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 towards 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 Ss

objects 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.

Having previously adjusted the instrument, place the 0 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 the eye to the upper hole in the fore Sight Wane, 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 fill 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