iisrsTKUCTioisrs for making pilot balloon observations 5 



These plates have a row of half-inch holes down the center for receiv- 

 ing the tips of the theodolite legs and for adjusting the height of the 

 theodolite for different observers. The size and shape of the platform 

 may be arranged to suit the needs of the station force, but the general 

 scheme above outlines should be followed and the construction made 

 as rigid as possible. Any plan wherein the observation platform is 

 well insulated from the theodolite stand and rigidly constructed will 

 answer. A suitable windbreak should also be built around the plat- 

 form to a height of 5 feet, with a door on one side (preferably the 

 leeward) for entrance to the platform. 



II. THEODOLITES 



10. Description of theodolites in use. — The theodolite, one type of 

 which is illustrated in figure 2, is a specially designed and constructed 

 instrument, similar in many respects to the transit, yet possessing 

 distinctive features which make it far superior for balloon work. A 

 small telescope is mounted in such manner that it turns on a hori- 

 zontal axis, passing through the center of a vertical circle, and revolves 

 about a vertical axis, passing through the center of a horizontal circle. 



11. The telescope is bent through an angle of 90°. The eyepiece 

 is produced through the angle of the bend to act as the horizontal axis 

 of the telescope, while the object end turns freely in the vertical plane 

 about this axis. In a cubical chamber about the right-angle bend of 

 the telescope a prism, acting as a mirror, is rigidly fixed in such a 

 position as to turn the line of sight with the bend of the telescope and 

 give a clear, well-defined image. The eyepiece is further provided 

 with cross hairs, stretched over a reticle for centering the objective. 

 Focusing of the adjustable-focus type of telescope is accomplished by 

 means of a rack and pinion gear. The fixed-focus type of telescope 

 is automatically focused by means of the cross-hair focusing device 

 on the eyepiece. The objective end terminates in a cylindrical sleeve, 

 which acts as a sunshade to protect the object lens. The mass of 

 both eyepiece and object end of telescope are compensated by counter- 

 weights, thus providing a free, even movement of little resistance. 



12. The telescope is supported over the center of the horizontal 

 plate by a yoke standard. A vertical circle for elevation and a hori- 

 zontal circle for direction are provided for determining the relative 

 movement of the telescope. Both vertical and horizontal circles are 

 graduated in whole degrees. More accurate readings may be made 

 by using verniers or micrometers which will be fully described in a 

 later paragraph. 



13. The levels are arranged on the horizontal plate, one parallel 

 to the horizontal axis called the plate level, PL, figure 2, while the 

 other, perpendicular to the first, is known as the standard level, SL, 

 figure 2. 



The instrument thus far assembled revolves about a vertical axis, 

 whose bearing is a sleeve and spindle, at the center of a graduated 

 horizontal circle loiown as the base plate. An extension of the vertical 

 axis, or the sleeve and spindle, passes through the shifting center and 

 terminates in a spring and knurled nut to form the shifting center 

 tension. The base plate is capaable of revolution about this center 

 but is ordinarily held in a rigid position by plate clamp screw, P, 

 figure 2. The shifting center, S, assembled with and encircled by a 



