240 AN ACCOUNT OF THE © 
height as was equal to this quantity,+ the depth of the rods. From 
this arrangement the line traced through the air, and the inclination 
ef which was observed, was really above the surface of the rods, three 
inches, but parallel to it, and care was therefore palcans, before remov- 
ing the first set of rods of any hypothenuse, to slink the theodolite on a 
stand with an elevating screw, so that the height of the axis of the teles- 
cope, when directed to the transverse piece of the cross placed on the 
picket, should be exactly equal to this quantity. Three stands out of nine 
(the number required for a pair of rods) being thus. adjusted, that is the 
two outer and the middle one—both as to the alignment and hypothenusal 
direction, the others were quickly brought to correspond by means of a 
strong twine stretched along the nine. The stands being moved to right 
or left, and the forks raised or lowered tll they were all so adjusted, that 
the twine lay in the middle of the forks and barely touching them. The 
hinder pair of rods were now brought forward, to be laid on the stands 
previously adjusted. It has been already noticed in the description of the 
rods, that the two pairs were perfectly independent of each other, and 
generally one inch asunder. 'Mh's afforded a sufficient precaution against 
the fixed or fore pair being moved, in bringing forward the hinder, but to 
guard against the possibility of such a thing, which would have vitiated 
the whole operation, I determined to trust to no one but myself, in a matter 
of this kind, and I therefore never allowed the hinder rods, after being 
adjusted, and read off, to be touched without being myself present, at the 
junction of the two pairs, to be satisfied, that in removing thera, no shock 
or derangement had happened to.the fore pair. In like manner, in laying 
this hinder pair‘in advance ofthe other, I was equally particular in seeing, 
