DESCRIPTION OF THE QUADRANT.

The quadrant contains an arc of 45°, but owing to its double reflection, it measures 90°, reading from right to left. The arc is divided into degrees, and these are subdivided into 3 parts of 20 minutes each, andthe vernier on the sliding limb of the instrument is divided into single minutes. The sliding limb is moved from right to left in measuring altitudes, and the screw on the back is used for clamping it against the arc when the altitude is noted. The screw on the forward part of the limb is called the “ tangent screw," and it is used for making a perfect contact after the sliding limb has been clamped. The colored glasses are for shading the eye when obtaining an altitude.

TO READ OFF AN ALTITUDE.

Ascertain by the zero on the vernier (sometimes marked thus, (0), and sometimes as an arrow) how many degrees and thirds of a degree it has passed on the arc, and then look along the vernier until one of its lines coincides with one of the lines on the arc, and the number of minutes given will be added to the last 20 minutes division the vernier zero has passed over, and the whole answer will be the required altitude.

MANNER OF ADJUSTING A QUADRANT.

FIRST.

Place the index at about 45° on the arc, and look into the mirror so as to see both the arc and its reflection. If they show in one line, the glass is perpendicular to the plane of the instrument; but if they show on a broken line, move the screws in the frame upon which the glass stands, slackening one and tightening another, until the desired effect is realized.

SECOND.

Now make the zero on the vernier coincide with zero on the arc, and hold the instrument with its face upwards and look at the horizon. If the reflected part and the horizon itself (shown through the clear part of the horizon glass) show in one line, this adjustment is perfect; but if not, they must be brought in contact by moving the screw at the back of the glass.

With the two zeros meeting, hold the instrument vertically and look again at the horizon, and if the reflected part and the horizon itself show in an unbroken line, the instrument has a correct adjustment; but if otherwise, move the screw at the back of the horizon glass until one line shows straight across the glass.

TO FIND THE INDEX ERROR

When it is impossible to secure a correct adjustment, proceed as follows:

By the aid of the tangent screw on the vernier, gently move the limb of the instrument until the image of, and the horizon itself, coincide; then the difference between the zero on the vernier and zero on the arc will be the "index error," and the amount will be added to the observed altitude if zero on the vernier is to the right hand of zero on the arc, but it will be subtracted if to the left hand.

ate.

The sextant is used for the same purposes as the quadrant, but being an instrument of more delicate mechanism the results obtained are more accurWhere the quadrant is only cut to minutes, the sextant is cut to ten seconds, or the sixth of a minute. The sextant frame is of metal and the arc and vernier of silver these preventing the warping of the parts, so common an occurrence in the wooden and ivory quadrant. The sextant is also

fitted with telescopes so that the observer can make a more perfect contact with the horizon. The adjustment of the sextant is performed in the same manner as has been explained for the quadrant.

An instrument between the quadrant and the sextant-cut to 15"-is called an octant.

DIRECTIONS FOR OBSERVING ALTITUDES.

The extreme limits of the sun's declination are 23° 27′ 80", north and south.

In the latitude of New York the sun always bears south at 12 o'clock (meridian). So it is that the observer, in any place north of the sun, must always observe the meridian altitude to the south. If the observer is south of the sun's declination, the situations become reversed, and the altitude will be observed to the north.

When taking an altitude of the sun, cant the instrument from side to side (that is, oscillate it), without removing the eye from the sight-vane or telescope, and the sun's image will describe the lower part of a circle on the horizon. It is from the lowest point of this circle that the observer must make a contact of the sun with the horizon, for that is the point directly below the centre of the sun. Should the sun be very bright, regulate the shade-glasses of the instrument to accommodate the eye.

To find the altitude of the sun from the horizon, let the two zeros, on the arc and vernier of the instrument, coincide; then look through the sight, directing it toward the sun's place in the heavens. As soon as the sun's image is seen in the small (horizon) glass, slide the limb of the instrument, and the image will move downward toward the horizon. When it is almost in contact, fasten the clamp-screw on the back of the instrument, and then make the sun's lower limb just "kiss " the horizon. If you are observing a meridian altitude, you must follow up the sun as it rises, by gently fingering the tangent screw; that is, you must preserve a delicate contact with the horizon by advancing the sliding limb equal to the sun's rise.

NOTE.-When observing an altitude for finding longitude by equal altitudes, or for a chronometer sight, simply throw the sun's image to that part of the horizon which is directly beneath the sun, and, at the instant of making the contact, call "Time," and the person stationed at the chronometer will note the hour, minute, and second, shown.

When it is almost 12 o'clock (noon) the sun will be verging on the highest point of its meridian circle, and it is necessary to watch it carefully, for the instant it crosses the highest point the image in the glass will drop below the line of the horizon seen through the clear (unsilvered) part of the horizon glass. At this instant, call "eight bells" and read the altitude.

An altitude will always be some part of 90°-an arc measured from the horizon to the point directly overhead of the observer, called the "zenith." This arc is always a quadrant, 90°, and what an altitude lacks of 90° is always the zenith distance.

EXPLANATION OF SEMI-DIAMETER, DIP OF THE HORIZON, PARALLAX, AND REFRACTION.

SEMI-DIAMETER.

The diameter of the sun, from its upper to its lower limb, is equal, on a quadrant, to 32'; that is, if you should observe an altitude of the sun's lower limb, and note it, and then sink its upper limb to the horizon line, it would be found that the first altitude had been increased 32', proving that the length across the face of that body equaled that amount; and, as we must always observe an altitude to the center of a body in working navigation, it is seen that the sun's semi-diameter (half way) is equal to 16', and must be applied to all observations of the sun.

If the lower limb is taken, we must add 16' to the observed altitude, because that will allow for the 16' we miss by the sun's center, being that amount above the horizon.

If the upper limb is taken, we must subtract 16' from the observed altitude, because the center is 16' below the horizon.

The sun's upper limb is very little practiced, and it is recommended that only the lower limb be used, thus always adding 16' to the altitude observed.

DIP OF THE HORIZON.

Dip of the horizon means the depression of the true horizon, caused by the observer's eye being elevated above the surface of the sea. Owing to this elevation an altitude is always greater than it should be, consequently we must subtract the correction from the altitude shown on the quadrant.