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 correspon dent ones, he may wish to have it in his power to deduce his 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 observations from confusion 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, in which to write down the observation belonging to each wire, whether that be time or azimuth. For if any cloud or accident should deprive him of any one or more of his observations, he will then at once see afterwards which of them is missing, when lie comes to compare the two sets together.
Leaving the instrument clamped for altitude, and screened entirely from the sun's rays, he must wait till it is at the same distance from noon in the evening to re sume his task. For that, he must hold himself ready against the time comes ; and, previous to it, he will do well to re-examine the adjustment of his instrument, to be certain that no change has happened in the stand or the central cone, so as to throw its axis out of a perpen dicular. Let him then observe the same method in this second set of observations as lie did in those of the fore noon ; considering those wires as first, at which the sun's limbs touch first ; and setting down the times of their appulse to each respective horizontal wire, and bringing the preceding or subsequent limb to the corresponding perpendicular one, and reading off the azimuths just as lie did before.
When all are passed, he may release all the clamps, and replacing his shade, leave the instrument till he has reduced his observations.
Of corresponding altitudes, if he has observed then; all, he will have obtained ten pair; and of azimuths six pair ; which he must now select from each other.
In respect of col responding altitudes, the practice is so common, and the method of deducing the time from them is so well known, that nothing need be said on that head. Indeed, if a person tt ish for instruction in that, he inav find it in tile late Alr Wales's Tract on the ?llethod of finding the Longitude at Sea by 7'inte-keepers ; which contains new tables of equations for equal altitudes, and precepts for the use of them : a valuable tre,,tise, which may be had at a small expellee, and ought to be in the hands of every one who pays attention to these subjects.
But the idea of observing corresponding azimuths be ing perhaps new, or at least not vent common, and seem ingly peculiar to this instrument, it may call for a little explanation how to reduce them : and a very little will suffice.
They must be opposed to each other in pairs, just in the same manner as corresponding altitudes ; the first in the morning to the last in the evening, and so of the rest. Then, deducting the one from the other, and applying half the difference between the two to the smallest num ber in each pair, it will give a number of degrees, mi nutes, and seconds, in which, if all the observations were perfect, the whole six pair would coincide ; and, if they do not, the fair mean deduced from anion than will approach neatly to the truth, i. e. the error of on the azimuth circle from the true meridian.
To that mean point, deduced from these observations, the instrument must now be turned, and fixed there till the proper correction can be applied to it. Upon the telescope being turned down to the horizon each way, it may he observed what distinct object there may lie either to the north or south that coincides with one of the per pendicular wires ; or if no such object should occur, a anark may be placed each way, or either way, to which to keep the instrument till the correction can be investi gated, which is requisite on account of the change of the •in's declination dui ing the interval between the morning and evening observations; for any alteration in his decli riation will allcct the azimuth deduced in this way, as it does the hour; and neither of those corrections can be ascertained till the latitude of the station be known.