43 



In practically applying this table, it is preferable to take the mean of several 

 readings of the rod in each position of the gradienter screw.* 



Thus, with the target near the foot of the rod, and then moved to correspond to 

 two revolutions of the gradienter screw, three readings in each position were a* 

 follows : 



I. II. 



Altitude 18 20" 



ft. 



0.625 

 0.627 

 0.625 



Means, . . . 0.626 



Difference, . . 2.752 



3.378 

 0.626 



Factor for direct distance for 18 = 94.80 

 44 " 1S30'= 94.51 



Differences, 



0.29 



For Horizontal Distance = 90.17 



44 44 44 =89.63 



0.54 



Therefore, the factor for 18 20' will be for the direct distance 94.80, z /$ of 0.29 

 94.61, and for the horizontal distance, 90.17 ^ of 0.54 = 89.81. 



Then we have, 2.752 X 94.61 = 

 2.752X89.81 = 



260.37 = the direct distance. 

 247.15 = the horizontal distance. 



This direct distance being the distance from the position of the foot of the rod 

 or the lower target to the center of the horizontal axis of the telescope,t and the 

 horizontal distance, the one usually desired, that distance reduced to a level line. 



The mean value of two revolutions of the Gradienter Screw in arc, is 34' 23". 

 Hence the value in arc of one of its smallest divisions on the head is 20" nearly. 

 Vertical angles therefore may be laid off with facility when they are confined to 

 the range of the screw. 



*To insure at all times accurate results, the telescope axis should revolve free, but without any looseness in 

 the bearings. The engineer should examine these bearings from time to time, and, if necessary, fresh and pure 

 watch oil must be applied. 



To make a measurement with a micrometer screw, its graduated head should be set back slightly, then brine 

 ** up to the readings in the same direction in which the measurement must be effected. 



t Should the engineer desire the direct distance between the foot of the rod, and the faint over -which tht 

 is suspended^ it may be found by the following formula. 



X = 



or putting it in a shape adapted for logarithmic computation, 



Where x =the distance from the point under the plumb-bob to the foot of the vertical rod. 

 <f=the direct distance obtained as above. 



/ = the distance from the center of the horizontal axis is to the point under the pltxnb-bob. 

 E =the angle of elevation of the foot of the rod, as above. 



