To find the value in seconds for each division of the 

 bubble scale, run the bubble near to the objective end of the tube, 

 reading each end of the bubble against the graduation. Take a 

 reading on a rod, say 400 ft. from center of instrument, move the 

 bubble toward the eye-end and note the number of divisions 

 traversed by each end of the bubble. Take a second rod reading 

 and get rod interval. 



Let E = divisions traversed by eye-end of bubble. 

 " O = " " object-end of bubble. 



" R rod interval in feet. 



Then V" R= F ^ n (Log. Sin. 1" = 4.6855749) 



400 Jz^Lyn 1" 



Slight inequalities in the collars will be difficult of apprehension 

 by end for-end reversals, particularly as the upper arc is not taken 

 into account; but this influence on a collimation test, conducted at 

 some distance, is quite a different matter. The length of base for the 

 bubble test is limited to the distance between the collars. Assuming 

 the angular value of the error to be the same, it is likely to be tangible 

 in the collimation test when an ordinary bubble would not indicate it. 



In a case of this kind it is always preferable to make the bubble 

 test by the peg method as explained on page 19. The peg method 

 does not take the collars into consideration and their inequalities 

 are of no concern. 

 Disparity in con- 

 tour or diameter 

 of the collars can 

 not be known ex- 

 cept by testing 

 either with a 

 striding level, as 

 shown in Fig. 6, or 

 \\ith a reversion Fig. 6 



bubble. * 



The design of our Precise Wye Level is such that a supple- 

 mentary bubble can be suspended between the wyes, and, if this 

 bubble is of the reversion type, the above requirement is not only 

 provided for but the instrument is incidently supplied with a vial 

 of lower sensitiveness which can be used in conjunction with a lower 

 power eyepiece for the less accurate and less expensive work. 



*Stf Theory and Practice of Surveying, Johnson-Smith. Page 695. 

 Surveying Manual, Pence and Ketchum, Page 75. 



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