28 INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 



it there until the second signal of "read" is given, when the motion 

 will be stopped to allow the reading of the angles. The recorder, 

 at the "warning" signal, will post himself just behind the observer 

 and a little to the right, so that he can easily see both elevation and 

 azimuth verniers, and make a mental note of the degrees of each. 

 Then at the signal "read," as quickly as possible after the motion of 

 tangent screws has been stopped, the angles of elevation and azimuth 

 will be read and recorded on Form No. lllOA-Aer. Always read the 

 azimuth angle from the same horizontal vernier by which the theod- 

 olite has been oriented. 



81 . The observer will find that much relief is obtained, and much eye- 

 strain eliminated, by observing with both eyes open. Do not squint 

 or close the unused eye. A httle practice will enable the observer 

 to keep one eye at the instrument and read one angle with the other 

 eye without difficulty. When he becomes proficient, even though 

 the gaze of the one eye may be directed upon one or the other of the 

 verniers, the other eye will gain and register an impression of the 

 movement of the balloon, and should the balloon pass a little from 

 the field during the reading, it may be regained readily by aid of the 

 movement just mentally registered. 



82. Instances will arise, however, wherein the inexperienced 

 observer will be unable to read either angle, due to the fact that the 

 balloon movement is so rapid as to require his full attention. In 

 such cases, the recorder mil read both angles, reading that first which 

 the observer indicates is changing the more rapidly. A reading 

 should be missed rather than be the means of losing the balloon. 

 During the first 2 to 4 minutes the balloon can generally be seen by 

 the naked eye, and thus easily placed in the field again. In a few 

 instances, principally when the balloon turns and comes back directly 

 over the station, the balloon movement will be more rapid than can 

 be followed by turning of the tangent screws. In this case the 

 tangent screw should be disengaged and the theodolite revolved 

 directly. Care should be taken that the motion is steady and uniform 

 so that the balloon is not turned out of the field. When the move- 

 ment of the balloon has diminished sufficiently, the tangent screw is 

 thrown in again and the ordinary procedure followed. 



83. Reading and recording angles. — In single-theodolite observations 

 the angles will be read to the nearest tenth of a degree (see descrip- 

 tion of vernier and figs. 3 and 4), and only at the completion of the 

 minute as signaled by the time-interval clock or the recorder. How- 

 ever, when the angles are comparatively low, say 20 or below, they 

 should be read to five-hundredths of a degree whenever possible. At 

 the signal "warning" it is well to read the angles to the point of ascer- 

 taining the whole degrees, then the final reading or the reading to 

 tenths of degrees can be made in much less time and directly at the 

 "read" signal. It is practically necessary that the angles be read 

 quickly and accurately. Comparatively small errors in reading 

 angles can be detected when the run is carefully plotted. Thus the 

 necessity for quick and accurate work. 



84. As soon as the angles are read, the observer will bring the 

 balloon near the center of the field by means of the tangent screws, 

 where he will keep it until the following "warning" signal is given. 



