204 



MEASURING APPARATUS OF THE 



' There are seven sets of microscopes, each consisting of two common 

 microscopes, and one telescopic microscope. The use of these is as 

 follows. The two common microscopes are compensated by means 

 of parallel bars of brass and iron in the same manner with the mea- 

 suring apparatus, and the condition of compensation requires that, 

 when each microscope is in focal distance in respect to the dot of 

 the bar to which it is attached, the horizontal distance between the 

 lines of collimation of the two microscopes shall always be the same 

 quantity, which ought to be six inches of Parliamentary measure at the 

 temperature of 62° 



There are six of these compound bars which are aligned in succession 

 by means of an instrument called a boning instrument, placed at a certain 

 number of paces in the rear. 



The first operation is to drive a set of pickets into the ground to 

 support the tressels, two of which are required for each bar ; this must be 

 accomplished by measurement with a common chain or rope, the line of 

 the pickets being traced with respect to the middle of the bars, and not 

 that of the dots or microscopes. 



The pickets being driven, one of the bars having a set of microscopes 

 at each end, is brought into the alignment, by placing the telescopic 

 microscope over the limit of the line to be measured. 



Over the eye tube of this telescopic microscope a small brass 

 upright called a director, is then raised, and the boning instrument, 

 by means of this and the other limit, being now properly placed in 

 alignments, the other end of the first bar is also placed right by means 

 of its director. 



