2353 AN ACCOUNT OF THE 
10. Tue rods being so loig and thin were necessarily extremely 
pliable, so that supposing the forked slides of the stands to be laid quite 
correctly in the hypothenusal plane and the rods consequently adjusted in 
one sense, still it was by no means likely they would be correct with 
respect to the vertical plane; without which it is. evident the distance be- 
tween the extremities of the rods must be continually changing. ‘To guard 
against this error a brass wire about }, of an iach diameier, was stretched 
along the middle of the rod, sufficiently light to leave no doubt of its 
straightness of direction, At convenient distances small flat bridges were 
attached to the rod of the same height as the wire, and in their middle a 
marrow eroove of about — of an inch. The-rod was easily brought into. 
such a position by means of small wedges pressing against the prongs of 
ihe forked slides, that the wire lay freely in this groove without touching 
either side of it. The rod was then known to be straight. This: wire 
had also a second use, and no inconsiderable one. ‘The forked. slides were 
to be brought in to the hypothenusal plane by a boring telescope, placed 
on the hinder rod, the adjustment being made by means of a small cross of 
wood, the transverse piece of which was fixed at exactly the same height 
as the cross wires of the telescope, when placed on the rod. But it was 
found that this manner of adjusting the forks was not entirely satisfactory, 
as there was always a trifling deviation in most of them. ‘The reason of this 
will appear evidentif it be considered that the slides being raised or-depressed_ 
by jirks, were necessarily very difficult to be gct guite correct. This difficulty 
had been foreseen from. the first, and indeed the: chief object of the a 
was tocorrect this defect. Although it be certain, mathematically speaking, 
that no wire or cord stretched between two supports can ever be perfectly 
