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Observations by means of the artificial horizon should be made at a spot sheltered from the wind and as quiet as possible. The mercury should be clean and pure; and for a clear, brilliant surface, before pouring the mercury from the bottle,“ place the finger over the orifice, and give the bottle a shake in an inverted position, holding it over the trough previously cleaned : ease the finger, and allow the mercury to flow gently, keeping the bottle inverted, and taking care to stop the opening of the bottle before the last portion, with the dross, flows out.”

When the mercury is pure, its surface clean, and the glass of the roof without flaw, trustworthy observations may be made. Errors arising from the glass roof may be partly eliminated by reversing the cover between each pair of observations.

When one of these instruments is used, the observer is to place himself at a convenient distance, in such direction that he may see the object reflected from the artificial horizon as well as the real object; then, having screwed on the telescope of the sextant (using no shades, but only a dark glass at the eye-piece), the upper or lower limb of the sun's image, reflected from the index-glass, is to be brought into contact with the opposite limb of the image reflected from the artificial horizon, observing that when the inverted telescope is used the upper limb will appear as the lower, and vice versa ; the angle on the instrument being then read off, and the index error applied to it, will give double the altitude of the limb above the horizontal plane.

NOTE.-It will perhaps be more easy to the observer if he first brings the images of the sun nearly into contact by the naked eye, and afterwards screws on the telescope, and makes the contact perfect by the tangent-screw.

It is usual to observe three, five, or seven angles, and take the mean by dividing by 3, 5, or 7, as the case may be. Then, to the mean of the instrumental measurements apply the index error, if any, and the result will be double the apparent altitude : therefore dividing by 2 will give the correct apparent altitude of the limb observed.

When observing the moon, place a green shade in front of the horizonglass; you require no dark glasses.

It is always difficult to observe a star by the artificial horizon.

No altitudes less than 15°, or much beyond 60°, can be observed by the artificial horizon.

The following diagram will illustrate the method of observing altitudes with an artificial horizon.

Let A B represent the horizontal surface of the mercury contained in a shallow trough, whose plane is continued to C;DEF is the roof, in which are fixed two plates of glass, D E and EF; and O is the sun at S, whose altitude is required. Now the ray SH, proceeding from the sun's lower limb to the surface of the mercury, will be reflected thence to the eye, in the direction H G, and the upper limb of the sun's image, reflected from the mercury, will appear in the line H G, as if it proceeded from a point R, whose

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angular depression A HR, below the horizontal plane, is equal to the altitude, A HS, of the object above that plane. If, then, I G is a direct ray from the object parallel to S H, an observer at G can measure with the sextant the angle I G R=S H R=twice S H C (or 2 S H A) by bringing the image of the object reflected by the index-glass into coincidence with the image R reflected by the mercury and seen through the horizon-glass. The instrumental measure corrected for index error will be double the apparent altitude. Thus, if we suppose the angle S HR, measured by a sextant, to be 70°, the altitude of the sun's lower limb will be 35o

Example.—Suppose the observed angles between the lower limb of the sun, reflected from the index-glass of the sextant, and the upper limb reflected from the artificial horizon on shore, to be as follow; the index error of the sextant being 2' 20" to add ; required the apparent altitude of the sun's lower limb.

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No correction is required for the height of the eye (dip), as in observations made by the sea horizon; but the other corrections, as usual (see “Correction of Altitudes").

Example.—Suppose the following angles of the sun's lower limb were observed by means of an artificial horizon ; the index error of the sextant being 1' 50" to subtract ; find the apparent altitude of the sun's lower limb.

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An artificial horizon fitted to the sextant, if it gave accurate results, would have inestimable value ; it has been the hope of many an observer and inventor to produce such an instrument; but all the attempts hitherto made in that direction, if they have not been total failures, have yielded no satisfactory results, inasmuch as strict accuracy under all circumstances was not a certainty.

When the altitude of a body is more than 60°, it may be observed from the opposite point of the horizon as well as from the point of the horizon immediately beneath it.

See that the tangent screw has plenty of run onward or backward, as the case may be.

When two screws work against each other, be sure when tightening one to loosen the other, if necessary.

When reading off the sextant, whether at night or by day, do not hold the instrument sideways to the light, but take care that the light comes directly along the index-bar to the vernier ; neglect of this may cause an error of one or two minutes of arc.

When using the telescope, close the eye not required for vision ; with the tube it is sometimes preferable to keep both eyes open, because the ima ng seen by both eyes under the same magnitude one assists the other.

The shades, if they are of doubtful character, are not always necessary. For the sun, by using only a dark glass at the eye-piece of the telescope, correction for shade error is avoided.

What is called a star telescope is now very often fitted to a sextant, and is strongly recommended by those who take altitudes of stars to determine the ship's position ; by its aid the horizon on a dark night can generally be discerned, though otherwise not to be distinguished.

In the tropics the great heat of the sun will certainly affect the instrument if it be kept too long exposed.

Before putting your sextant away in its box, wipe the glasses with a piece of soft chamois leather : do not use a pocket-handkerchief or rag for this purpose. Be careful not to use much pressure, otherwise the adjustments may be disturbed. Moisture allowed to remain on the mirrors will soon impair the silvering.

The vernier was invented by Pierre Vernier in 1631 ; but it differed slightly from that now in use : the term nonius for this invention is quite a mistake, as it is only applicable to another kind of subdivision.

A good book specially devoted to the sextant, quadrant, and other reflecting instruments is not at present to be procured; but a small work, viz., “The Sextant,” by Major H. Wilberforce Clarke, Royal (late Bengal) Engineers, and officer charged with the duty of taking observations for latitude and longitude in the Upper Nile Valley, should be in the hands of all navigators; the instructions given on the use and management of the sextant are terse, concise, and very instructive.

If the navigator is desirous of possessing a good and reliable instrument he should send it to Kew Observatory, where all its defects will be ascertained, and errors given, for a small fee; when purchasing a new, first-class sextant, ask the optician for the Kew certificate.

THE BAROMETER The barometer is an instrument with which to measure the variations in the weight or pressure of the atmosphere. The principle of the mercury barometer was discovered by Torricelli, a pupil of Galileo, in 1643.

A mercury barometer consists of a glass tube (see Fig. I) about 33 inches long, closed at one end and filled with pure mercury, which has been boiled to get rid of all air bubbles; the tube is then inverted and its open end immersed in a cistern of mercury (D). This cistern has a small hole (H) in it covered with a leather washer on the inside to prevent the mercury escaping from the cistern, and it is through this small hole that the atmosphere exerts its pressure on the mercury in the cistern and causes the mercury in the tube to rise and fall as the atmospheric pressure gets greater

О Е or less. The pressure of the atmosphere is about 14.7 pounds per square inch at sea level at the temperature of 32° F.

The latest pattern barometers are graduated in inches and decimals of an inch on one side and in centibars and decimals of a centibar on the other side. A vernier, the principle of which is thoroughly explained in the chapter on the sextant, is fitted to the scale to facilitate the reading of small variations of pressure. The vernier is moved up and

F down by a rack and pinion, which is operated by a milled screw as shown in Fig. I (E). It is only A necessary to graduate a sufficient length of the tube at the top to measure all the variations in the atmospheric pressure, which varies from about 27.5 inches to a little over 31 inches.

In a marine barometer the tube is very much contracted for the greater part of its length, and at one portion (C) it is. of capillary dimensions, in order to prevent what is technically known as "pumping,” which is caused by the labouring of a vessel when in a heavy sea, and also to lessen

Fig. 1. the weight the tube has to support.

The tube is also fitted with an air trap (A) which consists of a small funnel, or pipette, whose function is to prevent air from getting into the tube above the mercury, which space should be a perfect vacuum. If any air should find its way into the tube above the mercury, the higher the reading the greater the error from this cause, because the pressure of a gas is inversely proportional to its volume. A thermometer (Fig. I, F) is also attached to the barometer and should

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always be noted when recording readings for the purpose of reducing them to standard temperature. This attached thermometer shows absolute temperature and the graduations are centigrade.

Fig. 5 is an enlarged drawing of the attached thermometer shown at F in Fig. 1. It is graduated on the left-hand side in Centigrade divisions, and reckoned from the absolute zero; on the right-hand side it is graduated in accordance with the Fahrenheit scale, whose freezing-point is + 32o. It will be observed that 273° A. are coincident with + 32° F., hence it follows that the freezing-point on the absolute scale is 273o. As 5o C. are equal to 9° F., it follows that every 5° on the absolute thermometer above and below the freezing-point must be coincident with every 9° on the Fahrenheit thermometer. Thus, it will be found that 278°, 283°, 288°, and 293° A. are coincident with + 41°, + 50°, + 59°, and + 68° F. respectively, and 268°, 263, 258°, and 253° A. are coincident with + 23°, + 14', + 5°, and — 4° F. The spaces between the divisions on any thermometer are equal, because equal increments of heat give equal increments in the length of the thread of mercury.

The marine barometer is so constructed as to obviate any correction for capillarity which tends to depress the mercury in the tube, or for the ever varying height of the mercury in the cistern caused by the mercury rising and falling in the tube as the pressure gets greater or less. There are, however, other corrections which cannot be eliminated in the construction, such as changes of temperature, changes of level, and change of latitude, each of which will now be explained.

Temperature correction.-As a column of mercury lengthens when heated and shortens when cooled it is necessary to apply a correction for temperature to show what the readings would have been at 32° F., which is the standard temperature to which all barometer readings are reduced for purposes of comparison.

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The correction is to be added when the sign t, and subtracted when the sign is at the head of the column.

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