60 A MANUAL OP TOPOGRAPHIC METHODS. 



The notes should always state explicitly where the instrument and 

 signals are and give their coordinates (preferably polar coordinates) if they 

 are not centered. 



Objects seen through the atmosphere appear almost always unsteady, 

 and sometimes this unsteadiness is so great as to render the identity of the 

 object doubtful. The unsteadiness is usually greatest during the middle of 

 the day. It generally subsides or ceases for a considerable period between 

 2 p. m. and sundown. There is also frequently a short interval of quietude 

 about sunrise, and on cloudy days many consecutive hours of steady 

 atmosphere may occur. For the best work, observations should be made 

 only when the air causes small or imperceptible displacements of signals. 

 In applying this rule, however, the observer must use his discretion. Errors 

 of pointing increase rapidly with increase of unsteadiness, but it will fre- 

 quently happen that time may be saved by counterbalancing errors from this 

 source by making a greater number of observations. Thus, if signals are 

 fairly steady it may be economical to make double the number of observa- 

 tions rather than wait for better conditions. 



The best results in a triangulation are to be obtained by measuring the 

 angles separately and independently. Thus, if the signals in sight around 

 the horizon are in order A, B, C, etc., the angles A to B, B to C, etc., are by 

 this method observed separately; and whenever there is sufficient time at 

 the observer's disposal this method should be followed. 



Besides measuring single angles, it is desirable to measure independ- 

 ently combined angles — i. e., angles which consist of the sum of two or more 

 single angles. Thus, supposing ( ) to be the observing station and A, B, and 

 ( ' stations sighted on, the observer should measure not only the angles AOB 

 and BOC, but the combined angle AOC. This is necessary in it only because 

 this angle may be used directly in the triangulation, but it will be needed in 

 forming conditions for adjusting the angles about the observing station, or 

 the station adjustment, as it is called. 



In order to secure the elimination of the errors mentioned above, the 

 following programme must be strictly adhered to: 



Pointing on A and readings of both microscopes. 



Pointing on B and readings of both microscopes. 



