THE USE OF THE MAGNETIC NEEDLE IN SURVEYING. l6/ 



are not, on the whole, so convenient as the use of the pole star. One 

 objection to them is, that, as the centre ot so large a body cannot be 

 accurately observed, it is necessary to observe the limb or edge ; and thus 

 a computation is necessary to reduce to the centre, or a second observa- 

 tion must be made on the opposite limb to eliminate the error. Hence, 

 in no way can a meridian be directly found by solar observations without 

 computation and correction for azimuth, except by the rough and unreli- 

 able method of guessing at the sun's centre when on the meridian. These 

 observations require, also, the use of a telescope with a darkened glass, 

 which is not always at hand. The most convenient way of locating a 

 meridian by the sun is to take its altitude in the morning or evening, 

 when the altitude is rapidly changing, and measuring the angle between 

 it and some fixed mark. The azimuth of the sun may be computed by 

 data found in the nautical almanac, whence the azimuth of the mark 

 becomes known, and the meridian is determined. By this method, 

 double observations must be made to eliminate the error of taking the 

 limb instead of the centre, and, also, by reversing the telescope, to 

 eliminate error in collimation and height of Y's, and in the position of 

 the zero of the vertical circle. In one observation, bring the sun into one 

 angle of the spider lines and tangent to each ; then read the horizontal 

 and vertical circles; point to the mark, and read the horizontal circle; 

 reverse the telescope, and take the sun in the same manner as before, 

 but in the opposite angle, that is, upon the opposite side of both lines ; 

 read the circles again, and observe the mark as before. It would make 

 the work still more sure to take a second set in the other angles of the 

 lines, but this is not essential. The mean azimuth of the mark, as obtained 

 by the different observations, will be its true angle with the meridian. 



If the local time is known and noted with each observation, the azimuth 

 of the sun may be computed without observing its altitude; but it is 

 easier to observe the altitude than to find the time. To take the sun, 

 when on the meridian, will also require the time with a correction for the 

 difference between apparent and mean time. Much better than these 

 solar methods, will be found the following: 



3. By observations upon any one of the stars. Select some bright star, 

 as Sirius or the planet Jupiter, that, if possible, the spider lines may be 

 seen without artificial illumination. If this can be done, it will save the 



