As the sun ascends, its image will move below the lines, and the arc must be moved to follow it. Continue thus, keeping it between the two sets of lines until its image begins to pass above the equatorial lines, which is also the moment of its passing the meridian. Now read off the vernier of the arc, and we have the latitude of the place, which is always to be set off on the arc when the compass is used at the given place. It is the practice of surveyors using the solar compass to set off, in the manner just described, the latitude of the point where the survey begins, and to repeat the observation and correction of the latitude arc every day when the weather is favorable, there being also nearly au hour at mid-day when the sun is so near the meridian as not to give the direction of lines with the certainty required. 162. To Run Lines with the Solar Compass.-Having set off in the manner just given, the latitude and declination upon their respective arcs, the instrument being also in adjustment, the surveyor is ready to run lines by the sun. To do this, the instrument is set over the station and carefully leveled, the plates clamped at zero on the horizontal limb, and the sights directed north and south, the direction being given, when unknown, approximately by the needle. The solar lens is then turned to the sun, and with one hand on the instrument, and the other on the revolving arm, both are moved from side to side, until the sun's image is made to appear on the silver plate; when by carefully continuing the operation, it may be brought precisely between the equatorial lines. Allowance being now made for refraction, the line of sights will indicate the true meridian; the observation may now be made, and the flag-man put in position. When a due east and west line is to be run, the verniers of the horizontal limb are set at 90°, and the sun's image kept between the lines as before. The solar compass being so constructed that when the sun's image is in position the limb must be clamped at ( in order to run a true meridian line, it will be evident that the bearing of any line from the meridian may be read by the verniers of the limb precisely as in the ordinary magnetic compass, the bearings of lines are read from the ends of the needle. 163. Use of the Needle.-In running lines, the magnetic needle is always kept with the sun; that is, the point of the needle is made to indicate 0 on the arc of the compass box, by turning the tangent-screw connected with its arm on the opposite side of the plate. By this means, the lines can be run by the needle alone in case of the temporary disappearance of the sun; but, of course, in such cases the surveyor must be sure that no local attraction is exerted. The variation of the needle, which is noted at every station, is read off in degrees and minutes on the arc, by the edge of which the vernier of the needle-box moves. 164. Allowance for the Earth's Curvature -When long lines are run by the solar compass, either by the true meridian, or due east and west, allowance must be made for the curvature of the earth. Thus, in running north or south, the latitude changes about one minute for every distance of 92 chains 30 links, and the side of a township requires a change on the latitude arc of 5' 12'', the township, of course, being six miles square. This allowance is of constant use where the surveyor fails to get an observation on the sun at noon, and is a very close approximation to the truth. In running due east and west, as in tracing the stand ard parallels of latitude, the sights are set at 90° on the limb, and the line is run at right angles to the meridian. If no allowance were made for the earth's curvature, these lines would, if sufficiently produced, reach the equator, to which they are constantly tending. Of course, in running short lines either east or west, the variation from the parallel would be so small as to be of no practical importance; but when long sights are taken, the correction should be made by taking fore and back sights at every station, noting the error on the back sight, and setting off one-half of it on the fore sight on the side toward the pole. 165. Time of Day by the Sun.-The time of day is best ascertained by the solar compass when the sun is on the meridian, as at the time of making the observation for latitude. The time thus given is that of apparent noon, and can be reduced to mean time by merely applying the equation of time as directed in the Almanac, and adding or subtracting as the sun is slow or fast. The time, of course, can also be taken before or after noon, by bringing the sun's image between the hour lines, and noticing the position of the divided edge of the revolving arm, with reference to the graduations of the hour circle, allowing four minutes of time for each degree of the arc, and thus obtaining apparent time, which must be corrected by the equation of time as just dedescribed. 166. Caution as to the False Image.-In using the compass upon the sun, if the revolving arm be turned a little one side of its proper position, a false or reflected image of the sun will appear on the silver plate in nearly the same place as that occupied by the true one. It is caused by the reflection of the true image from the surface of the arm, and is a fruitful source of error to the inexperienced surveyor. It can, however, be readily distinguished from the real image by being much less bright, and not so clearly defined. 167. Approximate Bearings.-When the bearings of lines, such as the course of a stream, or the boundaries of a forest, are not desired with the certainty given by the verniers and horizontal limb, a rough approximation of the angle they make with the true meridian is obtained by the divisions on the outside of the circular plate. In this operation, a pencil, or thin straight edge of any sort, is held perpendicularly against the circular edge of the plate, and moved around until it is in range with the eye, the brass center-pin, and the object observed. The bearing of the line is then read off at the point where the pencil is placed. Time for Using the Solar Compass.-The solar compass, like the ordinary instrument, can be used at all seasons of the year, the most favorable time being, of course, in the summer, when the declination is north, and the days are long, and more generally fair. It is best not to take the sun at morning and evening, when it is within half an hour of the horizon, nor, for about the same interval, before and after it passes the meridian. II. THE SOLAR ATTACHMENT. 168. The Solar Attachment is essentially the solar apparatus of Burt placed upon the cross-bar of the ordinary transit, the polar axis only being directed above instead of below, as in the solar compass. A little circular disk of an inch and a half diameter, and having a short round pivot projecting above its upper surface, is first screwed firmly to the axis ofthe telescope. Upon this pivot rests the enlarged base of the polar axis, which is also firmly connected with the disk by four |