REDUCTION OF OBSERVATIONS 



place such large size balloons could be Inflated and 

 weighed was in the after companionway, where it was 

 tedious and difficult owing to drafts and the impossibili- 

 ty of installing permanent equipment for proper inflation. 



During the cruise from Panama to Callao the exper- 

 iment was tried of tying two balloons together to obtain 

 a large object in the field of the theodolite. The observ- 

 ers believed that the distance to which the balloon could 

 be followed was considerably increased. Owing to the 

 fear, however, that the ascensional rate of the two bal- 

 loons tied together would not be the same as for each 

 balloon separately, this practice was given up after a 

 few flights. 



Only a very few balloons burst during inflation, and 

 no inconvenience was experienced with oddly shapeid bal- 

 loons, nor was special care taken to inflate slowly. The 

 balloons showed no sign of deterioration in the tropics, 

 even after they were on board five months. The bal- 

 loons were stored in sealed tins in the instrument room, 

 where the temperature was approximately 30° C. 



Balloon-Sextant 



Since the observer had to steady the counterweight 

 with his hand, he had, at times of rapid motion of the 

 balloon, to make a choice between working the azimuth 

 or elevation micrometer. The procedure developed was 

 to keep the right hand adjusting the azimuth micrometer 

 head. An additional observer watched the balloon through 

 a sextant so that if the balloon were lost to the theodolite, 

 the sextant gave its height, and the direction of the sex- 

 tant pointing gave the approximate bearing of the bal- 

 loon. 



Captain Ault's report of March 14, 1929 describes 

 the following expedient: "in view of the length of time 

 required to hold up a sextant, and of the weight of the 

 new balloon-sextant, it became necessary to devise some 



mothod for supporting the instrument. One of the deck 

 chairs was provided with arms and two upright pieces 

 supporting an overhead bar. A fine spring was suspend- 

 ed from this bar, and the sextant is now used hanging 

 from this spring. The entire weight is supported at the 

 height of the observer's eye and the freedom of motion 

 is in no wise restricted. The chair can be moved to the 

 most advantageous position on deck for observing the 

 balloon; the ease of operation involves no strain on the 

 observer's arms and it serves its purpose with a high 

 degree of efficiency." 



When the balloon changed more than 1 or 2 points in 

 azimuth, the chair had to be shifted around so that the 

 observer would be able to look directly at the balloon. 

 The practical difficulties of following the balloon while 

 shifting the chair were considerable, and these, and the 

 hope of getting azimuths directly with the sextant, led to 

 the design of a sextant chair (fig. 3, p. 48) on a rotating 

 platform. This chair was rigidly attached to a small cir- 

 cular base about 80 cm in diameter, which rotated about 

 a central pin, and was supported by rollers near the out- 

 er edge. The chair and rotating bases were carried on a 

 small portable platform. The sextafit was suspended by 

 a coil spring from a crossarm carried above the observ- 

 er's head from the back of tho chair. A pointer attached 

 to the supporting platform showed the azimuth of the chair 

 on a scale of degrees marked on the rotating disc. When 

 set up on the ship's deck the platform was arranged so 

 that the chair reading was 0° when the observer looked 

 in a direction parallel to the mid-line of the ship. The 

 azimuth of the balloon was read directly from the scale 

 as an assistant moved the chair around to face directly 

 the balloon. Owing to the improvement in the new theod- 

 olite received at San Francisco, the sextant chair was 

 not required so often after leaving this port, but in the 

 earlier part of the cruise this expedient was of material 

 assistance. 



REDUCTION OF OBSERVATIONS 



The wind velocity and direction corresponding to the 

 balloon's height at various minutes after release from 

 the ship, were computed by graphical methods on a plot- 

 ting board. The procedure was along lines similar to 

 those adopted for pilot-balloon observations at a fixed 

 station on land. The plotting board permits the comput- 

 er to see and to correct, not only errors in reading an- 

 gles, but also those from the lurching and rolling of the 

 ship. 



The plotting board used was of United States Navy 

 design with a circular celluloid sheet of 87 cm diameter, 

 graduated in even degrees around the edge, and rotating 

 about a central pin. A set of parallel vertical lines 1 cm 

 apart was drawn on the rigid base beneath, but clearly 

 visible through the movable celluloid sheet. The verti- 

 cal line passing through the center of the board was sub- 

 divided for two scales--one for use with short flights 

 when the distance from center to edge of board repre- 

 sented 8000 m, and the other for longer flights when the 

 same distance represented 20,000 m. 



The ship's track was laid off by turning the cellu- 

 loid sheet so that the circle reading denoting the angle of 

 the ship's heading layover the azimuth arrow at the bot- 

 tom of the board and pointed off the ship's position from 



minute to minute along the appropriate distance scale. 



It was always assumed that the Carnegie maintained 

 constant speed throughout the time of observation, the 

 speed being determined from log readings at the begin- 

 ning and end of the observation. When the ship changed 

 her course during a flight, the celluloid sheet was ro- 

 tated to the new heading at the moment of change. The 

 ship's subsequent positions from minute to minute were 

 indicated along the vertical line following on from the 

 former course. 



The horizontal projection (d) of a line from the ship 

 to the balloon is 



d = h cot e 



where h = height of balloon and e = angle of elevation. 



For plotting the balloon's position at any particular 

 minute, the celluloid sheet was turned imtil the angle at 

 its edge read the same as the true azimuth of the bal- 

 loon's position. From the ship's position for this min- 

 ute and at the distance (d), a poini was located on the 

 vertical line toward the plotter. The horizontal projec- 

 tion of the balloon's course from minute to minute was 

 thus laid down on the celluloid sheet. 



