seconds of deviation in time, which multiplied by 15 will give the number of seconds of arc.

When this problem is performed with the altitude and azimuth instrument, the azimuth circle will enable the observer to correct the deviation, as it is divided into degrees, minutes, and seconds.

299. The telescope of the portable transit instrument is generally of sufficient power to shew stars, at least of the first magnitude, in the day-time. To identify stars, or to fix upon that portion of the meridian at which they will arrive, we call in to our aid the circle attached to the axis, which need not, in general, be divided lower than minutes. When the bubble of the spirit-level is in the centre, the verniers having been set to the meridian altitude, the telescope will be adjusted.

To find the meridian altitude of a heavenly body, or the zenith distance at the time it culminates, which is the difference between its meridian altitude and 90°, we adopt the following method:

Rule. Write down the latitude of the place, marked N. or S.; underneath this write the declination of the object, marked N. or S.: if the names are unlike, the sum of these two quantities, or if they are like, the difference will be the zenith-distance; 90-Z.D. = meridian altitude. If the circle be set to this quantity, the star will appear in the telescope some time before the calculated instant of meridian passage. If great correctness is required, apply the correction for refraction in altitude, taken from suitable tables, with the sign +.

300. The meridian having been determined by either or all of the above methods, the true time may be ascertained any day, at noon, by a meridian observation of

the sun. Observe the exact time when the sun's western limb approaches each vertical wire, which, as the telescope inverts, it will do from the right-hand side of the field of view. Do the same when the other limb transits each wire, add these times together, and divide the sum by 10, which will be the instant that the sun's centre was on the meridian. As it will often happen that, in cloudy weather, only one limb can be observed, the time of the sun's semi-diameter passing the meridian is given in the Nautical Almanac and all other ephemerides, which must be added to the time of passage of the western limb, or subtracted from that of the eastern, to give the time of the passage of the sun's centre.

The instant the sun's centre is on the meridian will be apparent noon-apply the equation of time, and the result will be mean noon. With this correction, the time shewn by a chronometer or mean-time clock should be 0 h. 0 m. 0 s.; if it is not, the difference is its error, fast or slow, + or as the case may be.

The error of a timekeeper may also be determined by observation of the transit of any one of the standard Greenwich stars. Find the time of the star's transit, if the clock or chronometer shew mean time, by the rule given in § 278; the comparison of this with the observed time will give the error. Observations either of the sun or star, continued from day to day, and compared with the time shewn by any timekeeper, will give its rate, gaining or losing; which, if the workmanship be good, will be uniform, and of trifling amount.

SECTION XI.

ON THE ALTITUDE AND AZIMUTH CIRCLE.

301. WE now proceed to describe an instrument which is of very extensive use in Practical Astronomy-the altitude and azimuth circle; or altazimuth circle, as it has lately been designated. A drawing of a small one of modern construction is given in Plate VII. fig. 55.

A, B, C are the foot-screws by which the horizontal plate D E is levelled. The telescope has an axis similar to that of the transit instrument; but the vertical circle carried by this axis is much more minutely divided than that of the transit need be for the mere identification of stars. The drawing is a representation of an astronomical circle made by Adie of Edinburgh. The vertical circle is eight inches in diameter, divided on silver, and read off to 10" by three verniers; the horizontal circle is six inches in diameter, also divided on silver, and read off to 10" by two verniers. The advantage of more than one vernier is, that the opposite verniers correct one another, in case of bad centering, or errors of division. By this instrument altitudes of a heavenly body may be taken when it is not in the meridian; while by clamping the instrument in the meridian, it will admit of all the adjustments, and answer the purpose of a transit instrument.

302. The horizontal circle has a motion in azimuth, produced by turning the screw at h, by which the whole of the instrument may be turned so as to bring 0°, or zero, into the plane of the meridian, or upon any par

ticular object. When the zero is in the meridian, and the telescope is turned to a star east or west of that circle, the vernier at E, and another exactly opposite, will point out its azimuth.

Underneath the telescope is a spirit-level, by which it may be placed in a horizontal position: by means of this, the zero of the vertical circle may be determined by the following methods:

1. When the telescope is horizontal, or rather, when on turning the instrument completely round, the bubble of the level underneath remains in the centre, mark the degrees, minutes, and seconds shewn by the three verniers, and take the mean by dividing the sum of them by three. This will give the zero approximately. Bring the cross wires on a terrestrial object, and take its angle of elevation. Turn the instrument half-way round in azimuth, reverse the telescope, and bring the cross wires on the same object; the measure of the angle in this position ought to differ 180° from the former reading; if it does not, an index error will be the result, which will be half the excess above 180°; which error must be regularly applied to the angles measured with the instrument in the same position as that which gave the least elevation; to those indicated by the instrument in the reversed position.

2. Observe the meridian altitude of a star on a certain night, by direct observation; find its depression on the following night by remarking its image reflected from the surface of mercury, which forms the artificial horizon the point midway between these readings-off will give the zero, or horizontal point of the instrument, which, if the observations are well performed, will

agree with that part of the arc determined by the former method, after the index correction was applied.

3. In cases where expedition is required, and the latitude of the place is well known, find the meridian altitude of a standard Greenwich star; then from the Nautical Almanac, determine what its meridian altitude ought to be, in the manner shewn in § 299; the difference, if any, between the observed and computed altitudes will be the index correction, + or case may be.

as the

303. A tripod-stand accompanies the instrument; but for astronomical purposes it will be better to fix it on a stone pillar, and, in the absence of a fixed observatory, to provide a waterproof covering strained over hoops, which may be lifted off without touching the instrument this will be a protection against damp, and, which is of more consequence in correct observations, from the effect of the sun's rays, which might cause an unequal expansion of the parts of the instrument, and produce a deviation, which the sensitive levels will immediately point out.

304. After the horizontal plate is rectified by means of the foot-screws and the levels mn, the same adjustments for collimation and causing the telescope to describe a vertical plane must be gone through as with the transit (see § 289). For this latter purpose there is a striding level, with a scale noting the seconds of deviation of the bubble, which must be applied to the axis. A reflecting eye-piece is also provided, for viewing objects near the zenith: this consists of a small mirror within the usual eye-piece, placed at an angle of 45° with the axis of the telescope. By this the image is reflected