1 V') 



CIRCLE. 



r!nl. once found will remain permanently the tame, as long at 

 ** "V^* the micrometer wires remain undisturbed. A long te- 

 net of stars, therefore, may be obwrved with the face of 

 the instrument in one position ; and at a very considera- 

 ble interval, if it ihould so happen, a corresponding sc- 

 ries may be obtained in the contrary position, and this 

 permanency of the error of collimation, and the increas- 

 ing degree of accuracy with which it becomes known, 

 ia an advantage peculiar to these instruments. 



In Troughton's circles, no previous adjustment of the 

 microscopes is neceisary ; but as the result supposes that 

 erery part of the instrument remains strictly the same, 

 they cannot be too often reversed ; and we would advise 

 the observer to change their position every 24 hours, 

 which is no objection to an instrument moving freely in 

 azimuth. 



But in whichever of these modes the instrument be 

 constructed, it will always possess the beautiful proper- 

 ty of pointing out to the observer, with considerable 

 accuracy, the maximum of its own probable error, which 

 will be shewn by the discordance of the error of collima- 

 tion, as deduced from different stars, and it may be ta- 

 ken as a general rule, that half the extreme difference of 

 a great number of observations, will be nearly the maxi- 

 mum of error to which any one determination is liable. 

 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 Greenwich. Catalogue, 

 just published (1813) for the determination of the posi- 

 tion of his instrument, which method we shall again 

 hive occasion to recommend in describing different me- 

 thods 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- 

 tun is so universal an instrument, and so proper to be 

 placed in the hands of undents who are only commen- 

 cing the study of practical astronomy, that we are in- 

 duced, to extend the subject by adding the directions for 

 its me. which have been given by Mr Wollaston in his 

 Fasciculus ; and this has been done in so clear and per- 

 spicuous a manner, that we shall offer no apology for 

 t ranscnbing the author's own words. 



" Let us imagine," says he, " an observer cast upon an 

 unknown shore, with no other astronomkal instrument : 

 having indeed a chronometer; but we will suppose that to 

 be gone down : and having likewise a Nautical Almanack, 

 and the necessary tables for calculation. We will consider 

 him as knowing neither the longitude nor the latitude of 

 the place, nor the hour of the day, within 1.5 or '20 de- 

 grees either way. The tracing of things thus from the 

 beginning, will best explain what may be done by an in- 

 strument 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 ((certain the spot over which he means to place his 

 rattniment. Let him also provide some screen or shade 

 to keep off the sun's rays when he is not in actual ob- 

 servation : for the levels are so sensible, that any shining 

 cf the suti unequally upon the instrument, will immedi- 

 ately be detected by them } and he will never satisfy 



himself with the adjustments. Were the levels less sen. 

 sible he might not perceive the difference ; but that 

 difference would 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 ad 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 the 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 ho- 

 rizontal 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 ar- 

 rives at the second horizontal wire. After setting that 

 down as before, let him prepare for the third or centra! 

 wire ; by now clamping the instrument in azimuth like- 

 wise, 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 setting 

 it down, let him 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 perpendicular wire, at the same instant that the 

 upper limb arrives at the fourth horizonal one ; setting 

 that time down again, and reading off the azimuth again, 

 and setting it down, let him do the same by the fifth 

 wire each way, and record them as before. 



He will now find the lower limb of the sun, and its 

 second 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 corre- 

 spondent horizontal wire ; and setting down the time, 

 and after 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 obser- 

 ved, as it quits the fourth and fifth horizontal wires re- 

 spectively. 



As soon as the sun has thus passed all his wires, he 

 should read off at both the microscopes the zenith dis- 

 tance and altitude at which he had clamped the vertical 

 circle ; and, if he has a barometer and thermometer, he 

 should set down their station at the same time : for, 

 though he probably will have no occasion to regard the 

 precise altitude at which he made these observations, yet, 

 if any thing should deprive him of the correspondent 

 ones, he may wish to have it in his power to deduce hit 

 time or his azimuth from these : and the reading off the 

 microscopes after all is over is attended with very little 

 trouble. 



These things at first will appear hurrying ; and till a 

 person becomes a little accustomed to it they certainly 

 will be so. But after a little practice, there will be found 

 time enough to go through the whole with ease ; for the 

 vertical circle remains clamped the whole time ; and all 

 the six azimuths lie much within the limits of their ad- 

 justing screw. 



The easiest method of keeping so many observation n 

 from confuuion is, to have a slate, or a sheet of paper, 

 ready ruled into five columns, to correspond with the 

 five wires in the telescope as they occur in succession, ia 

 which to write down the observation belonging to each 

 wire, whether that be time or azimuth. !'<>" if any cloud 

 or accident should deprive him of any one or more of his 



Circle. 



