others also \\\\o, having received an instrument in apparently good 

 condition, allow the screws to rust in. * 



Prof. Walter Harris of Princeton, Prof. J. M. Porter of 

 Lafayette, and Pi of. H. K. Vedder of Mich. Agri. Coll., amoii 

 others who are alert to the practical necessities of the case, have 

 given us the assurance that no student is allowed to "pass" until 

 he has stood a satisfactory examination in adjustments. Indifference 

 to the necessity or reason for instrumental adjustments has caused 

 many embarassing situations in the field. When a polygon will not 

 close, either the graduation is defective or the proper precautions 

 in the routine of work have been slighted. 



In starting the adjustments set up the tripod firmly on a sub- 

 stantial footing. The wing nuts at the top of the tripod legs 

 should be screwed home tightly so that when held in the air the leg 

 will not drop of its own weight. Much depends upon the fit of 

 the bolts, both in the wooden forks and in the brass lug of the 

 tripod head. The tripod itself should not be too light, merely to 

 save weight. 



The survey, no doubt, is affected by unstable conditions 

 in the tripod. With all of our portable azimuth instruments 

 we offer an optional choice between the No. 75 Tripod with 60-in. 

 legs, weighing 9/4 Ibs., and the No. 76, weighing \\Yz Ibs. The 

 No. 74 Tripod, designed for the Reconnoissance Transit, the Moun- 

 tain Tachymeter and the No. 110 Compensation Level, has 57-in. 

 legsand weighs only 7%\\is. To secure greater stability, many engin- 

 eers prefer to order, for small instruments, the No. 75 Tripod with 

 a Reducing Ring, shown in Fig 108 which expedient, in fact, makes 

 it possible to use one tripod for two different sized instruments. 



I. To Adjust the Tangent Axes of the Plate Bubbles 

 Perpendicular to the Vertical Axis and Parallel with the 

 Graduated Plates. 



(a) The bubble vials are usually supplied in pairs and placed 

 at right angles on the plate. Unclamp the vertical axis and turn 

 the alidade so that each bubble will lie parallel with one set of 

 leveling screws. If the 3-screw base is used, turn one bubble 

 parallel to any two screws; the other will then lie in a co-ordinate 

 plane controlled by the third screw. It is advisable to adjust only 

 one vial at a time, but not much dexterity is required to adjust them 

 simultaneously. Bring the bubble to the center of its scale with 

 the leveling screws. Revolve the instrument 180, preferably by 

 vernier, on the vertical axis and note the error of displacement. 

 Correct half with the leveling screws and the other half with the 

 adjusting stud of the vial mount. 



The leveling screw threads are usually too coarse to get perfect 

 results at first, but center the bubble carefully and swing back to 



* Manual of Adjustments II. ( . /->.-, 1W6, p. 2. 



62 



