422 The Account of a 
operation, there are slits cut through the top of the mahogany 
board, for receiving the screws which fasten the supports of 
the telescope; by which means the telescope, with its supports, 
can be moved a little to the right or left, whilst the stand re- 
gains fixed. Over another slit in the top, and directly under 
the centre of the axis of the telescope at R, is a small hole for 
a wire or thread to pass through, suspending a plummet for 
marking a point on the ground, when the telescope is brought 
into the desired vertical plane. 
The method of levelling the axis, adjusting the line of col- 
limation, &c. are similar to those for the upper telescope of 
the great theodolite, as described in the Philosophical Trans- 
actions. 
2. The Boning Telescope. 
This telescope is in every respect the same as that which 
was made use of by General Roy, therefore it will only be 
necessary, to explain the application of it, for fixing the pickets 
in the direction of the base, with the tops of those belonging to 
the same hypotenuse in the same right line. 
A rope being stretched along the ground, in the direction of 
the base, distances of 100 feet were marked upon it by means 
of a twenty-feet deal rod. After a sufficient number of these 
distances were set off, the telescope was laid on a narrow piece 
of board, truly planed, and fixed to the top of the picket at the 
beginning of the hypotenuse; and another picket was driven 
into the ground at a convenient height at the other end. To 
the top of this last, a thin deal spar was fixed, and the telescope 
directed to it, whilst the intermediate pickets were driven to 
