For work of this nature, where the elevation of 

 the station in the foot plate at each level is to be 

 carefully determined as a datum point for that gallery, 

 the height of instrument (H. I.) and the height of 

 point (H. P.) should be determined as suggested in 

 Fig. 99. 



For this purpose, as well as for any other where 

 the height of the instrument enters into the calcula- 

 tion, we have designed a height-of-instrument plummet 

 (H. I. P.) as shown in the accompanying illustration, 

 which is to be carefully adjusted to each instrument 

 by allowing for the distance between the horizontal 

 axis and the hook in the plummet chain. 



It consists of a circular nickel-plated brass case, 

 1% inches in diameter, containing a steel band some- 

 what over 8 ft. in length, graduated to hundredths of 

 a foot on one side and millimeters on the other. It 

 can be conveniently carried in the pocket and will 

 serve other useful purposes. Attach to hook of 

 plummet chain, pull down until tip of plummet 

 touches the station, then read the H. I. direct 2.32 

 as in the figure. If this amount is added to the 

 known elevation of the station, the elevation of the in- 

 strument above the datum is at once available. 



Pence & Ketchum (p. 140) recommend that for 

 the rapid determination of H. I. in stadia work, the 

 tripod leg should be graduated by experiment, so that 

 when the plumb bob is swung out against it, the 

 amount will be indicated at the point; but the means 

 here described is more direct, convenient and certain. 



The Tunnel Trivet is a rigid ribbed support 

 with a neck high enough to be easily grasped for 

 carrying, or for suspending a plumb line beneath the 

 instrument when overhead plumbing is not desirable 

 Fig 100 or not convenient. It is a very substantial brass 



Structure as shown in connection with Fig. 101. 

 The Lateral Adjuster, as shown also in connection with Fig. 

 101, is used for the final accurate placement of the theodolite when 

 ranging the sight line into an alignment indicated by two wire 

 plumb lines suspended in a vertical shaft. The feed screw of this 

 apparatus has a pitch of 100 threads per ft., and the knurled head 

 is provided with ten points in order that intervals of Viooo ft. can 



176 



