VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 6 17 



which the line of the hooks is made parallel to the line of collimation in this di- 

 rection, with the greatest precision. The level may be suspended on these pins 

 in the same manner as on the horizontal axis. 



The cross wires, in the common focus of the object and eye-glasses, are fixed 

 at right angles to each other; but instead of being placed horizontally and ver- 

 tically, as in the common way, they make each an angle of 45° with the plane 

 of the horizon. This mode of fixing wires is of the greatest advantage in 

 making nice observations, as it remedies the inconvenience and error arising 

 from their thickness. To bring the line of collimation in the telescope at right 

 angles to the horizontal or transverse axis, there are 2 nuts for the purpose, 

 one on each side of the box, which serve to move the intersection of these 

 wires towards the right or left. 



In the eye end of the telescope is a micrometer, which serves to measure small 

 angles of elevation or depression. It consists of a moveable horizontal wire, 

 placed as close as possible to the cross wires already mentioned. By turning 

 the micrometer-screw, this wire is moved across the field of the telescope, and 

 the space which it moves through is shown in revolutions of the micrometer- 

 screw, by means of an index, moveable in a slit, and the divisions on the stem. 

 The parts of a revolution are shown in lOOths by an index, on the micrometer 

 head. 



In tracing out a base by intermediate stations, the instrument must be fre- 

 quently shifted to the right or left, till the telescope shows that the middle of 

 its axis and the extremities of the base are in the same vertical plane. To ex- 

 pedite this operation, there are slight cuts 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, while the stand remains fixed. Over another slit in the top, and directly 

 under the centre of the axis of the telescope, 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 boning telescope is in every respect the same as that which was made 

 use of by General Roy; it will therefore only be necessary to explain its appli- 

 cation, 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 on it by means of a 20-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 teles- 



VOL. XVII. 4 K 



