"Usually it will only be necessary to use two deflected lines to 

 pass a tree or other obstruction, by deflecting 1 % to the right fol- 

 lowed immediately by an equal deflection to the left, measuring 

 equal distances, on the two lines. If the deflected line departs 1 % 

 from the true direction, it is much easier to compute the offset to 

 the line than if the deflection be 30' (0.874# ); and not only easier 

 but more accurate, for the least count of the gradienter is .0001 and 

 by estimation 0.1 of this making the reading accurate to .00001 

 or 0'.034, whereas the least count of the vernier is usually 1'. It 

 may be said that the accuracy of the gradienter method, by this 

 comparison, is about thirty times as great as deflection by vernier. 



"With the azimuth gradienter it is possible to measure horizon- 

 tal distances if the rod is held horizontal instead of vertical, and 

 no correction has to be made if the sight should be inclined. Fig. 

 70 (supposing GL horizontal) TG = 100 GL and GF TF tan a. 

 The simplicity of these formulae is apparent compared with those 

 given above." 



Specifications 

 Bausch & Lomb 4V2-inch Tachymeter 



tl Mountain" 



Telescope Erecting, 8 in. long; aperture, 1^ in.; power, v18. 



Inverting, 8^4 in. long; aperture, 1 in.; power, , 16 or ,\20. 



Both types transit at both ends. Cryptic focus. 

 Telescope Bubble 3^ in. long, 2mm divisions, sensibility, 30". 

 Plate Bubbles 1>< in. long, 2mm divisions, sensibility, 60". 

 Needle 3 in. long, tubular, sapphire mount; weight, 0.50 grms. 

 Vertical Limb 2j^-in. radius, open or closed cover guard, as 



desired. 

 Horizontal Limb 2f-in. radius, 5^-in. outside diameter of 



plates. 

 Standards Aluminum bronze, bell metal journals; 5J^ in. high, 



2^-in. spread; 3^-in. diameter of compass ring. 

 Leveling Screws German silver, l^j-in. head, dust caps and 



guards. 



Shifting Center X-in. range. 

 Height 11 '4 in. with circle. 

 Finish Bakelite yellow and Kahki pyrolin. 

 Weight 9^ Ibs.; in case with accessories, 14 MIS. 



Split leg tripod, No. 74, 57-in. legs, 1} Ibs. lixt. leg tripod, 

 ., 7% Ibs. 



126 



