In instruments of Mr Troughton's construction, great care should be taken to examine the level, or bisect the plumb line point at the moment of observation ; and all those little precautions which we had occasion to recom mend in our general treatise on Practical ASTRONOMY, should carefully be attended to.
When the observer, however, is not desirous of dedu cing every thing from his own instrument alone, he had much better avail himself of the Green-with Catalogue, just published (1813) for the determination of the position of his instrument, which method we shall again have oc casion to recommend in describing different methods of using the repeating circle.
The above observations relate to astronomical circles generally, and are not confined to any particular instru ment; but the altitude and azimuth circle of Trough ton is so universal an instrument, and so proper to be placed in the hands of students who are only commenc ing- the study of practical astronomy, that we are induc ed to extend the subject by adding the directions for its use, which have been given by Alt- Wollaston in his Fas ciculus ; and this has been done in so clear and perspi cuous a manner, that we shall offer no apology for tran scribing the author's own words.
4. Let us imagine," says he, " an observer cast upon an unknown shore, with no other astronomical instru ment: having indeed a chronometer ; but we will sup pose that to be gone down : and having likewise a Nau tical Almanack, and the necessary tables for calculation. We will consider him as knowing neither the longitude Mr the latitude of the place, nor the hour of the clay, within 15 or 20 degrees either way. The tracing of things thus from the beginning, will best explain what may be done by an instrument of this sort ; any part of which process may be omitted, according to the data he may have on which to work.
Let him first drive a small stake into the ground, or make any other mark in the pavement or floor, whereby to ascertain the spot over which he means to place his instrument. Let him also provide some screen or shade to keep off the sun's rays when he is not in actual obser vation : for the levels are so sensible, that any shining of the sun unequally upon the instrument, will immediately lie detected by them ; and lie will never satisfy himself with the adjustments. Were the levels less sensible, he
might not perceive the difference ; but that difference really exist, whether he could discern it or not.
Let him next set his stand over the mark, and the in strument upon it, as described already ; and let him ad just it in every particular as truly as he can.
Let him also wind up his chronometer, and set that to the supposed time of day.
In the morning, two, three, or more hours before noon, let him point tile telescope toward the sun, and a little above it ; and, clamping the vertical circle, let him fol low the sun till its upper limb just touches the first hori zontal wire. Then, noting down the exact second of time, as shewn by his chronometer, when that happened, let him follow the sun again till its upper limb just arrives at the second horizontal wire. After setting that down as before, let him prepare for the third or central wire; by now clamping the instrument in azimuth likewise, and holding its adjusting screw between his finger and thumb, let him bring the preceding limb of the sun just to touch the third or central perpendicular wire, at the same instant that the upper limb just touches the third or central horizontal one. Noting that instant, and set _ ting it down, let hint now read off the azimuth marked on the azimuth circle, and set it down under the other ; and then prepare for making the preceding limb to touch the fourth pei pendieular wire, at the same instant that the upper limb arrives at the fourth horizontal one ; set ting that time clown again, and reaching off the azimuth again, and setting. it down, let hint do the same by the fifth wire each way, and record them as before.
Ile will now find the lower limb of the sun, and its se cond or following limb, ready for observing in the same way at the first, second, and third wires : making each perpendicular wire a tangent to the sun's last limb, at the instant that its lower limb just leaves the correspon dent horizontal wire ; and setting down the time, and af ter reading off the azimuth, setting that down too under the other. After these, the instrument may be released in azimuth, and the lower limb alone be observed, as it quits the fourth and fifth horizontal wires respectively.