INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 11 



vertical and horizontal circles. A little light clock oil, in limited 

 amounts and applied properly, will eliminate much friction and 

 reduce the wear on the base plate. Close examination of the instru- 

 ment from time to time will reveal small parts and screws which have 

 become loosened. These should be attended to immediately, so far 

 as possible without interference with proper adjustment; that is, if 

 the parts which have become loosened materially affect the adjust- 

 ment, it will be necessary to readjust and check the instrument after 

 such parts have been tightened. 



21. The instrument is not to be taken apart more than is necessary 

 for packing and shipment. Further, taking apart for the purpose of 

 cleaning or repairs should be done only by one experienced with the 

 construction of the instrument, or by a competent person, and upon 

 the receipt of authoritative instructions from the central office. 



Care should be taken that the hands do not come in contact with 

 silvered surfaces of the circles or the verniers, for the moisture and 

 oil thereby deposited tend to oxidize the surfaces, making the gradua- 

 tions indistinct and difficult to read. If these parts do become 

 tarnished thej^ may be brightened to some extent with a soft rubber 

 pencil eraser, or better, with a good grade of silver polish. 



22. Adjustments of the theodolite. — Before the new theodolite is used 

 it must be thoroughly adjusted and checked. This is done at the 



Figure 5.— CoUimation adjustment. 



central office before the instrument is assigned to any station. How- 

 ever, due to rough handling in shipment, it becomes necessary to 

 recheck and sometimes to readjust the theodolite at the field station. 

 An instrument in daily use should be checked occasionally — at least 

 once every 3 or 4 months. If the initial adjustment is carried aut 

 carefuUj^ and accurately, these periodic corrections will be slight if at 

 all noticeable, yet they should not be neglected. Each tim.e the 

 theodolite is adjusted a statement to that effect should be entered under 

 ''Notes" on Form No. lllOA-Aer. for the succeeding observation. 

 When the theodolite adjustment has been completed, the entire 

 series of tests should be gone over as a means of checking. It will 

 often be found necessary to make slight corrections which exemplify 

 the need of much attention during the initial adjustment. Before 

 making any one of the adjustments, note that the instrument is 

 properly seated at leveling screws and that the horizontal base plate 

 is level. Check for levels before each of adjustments 2, 3, and 4 is 

 attempted. 



23. The adjustments of the theodolite are such as to cause (1) the 

 instrument to revolve in a horizontal plane about a vertical axis, 



(2) the line of coUimation to generate a vertical plane through the 

 instrument axis when the telescope is revolved on its horizontal axis, 



(3) the horizontal axis of the telescope to be perpendicular to the verti- 



