404 An Account of the Measurement 
To show with distinctness and sufficient perspicuity, the 
manner in which the cross wires are sustained in the tube, also 
the means by which they are adjusted, figures are given in 
Plate XII. These also show the micrometer-screw, and the 
mode of clamping it to the proper arch. In the horizontal re- 
presentation of the end of the telescope, the wires are seen in 
the centre, and also the two screws, with the helical spring for 
adjusting and retaining them. EF is a strong brass arch, with 
an edge bar IK, placed parallel to the divided arch. At EF are 
seen two milled-headed screws, passing through a metallic 
embracement of the bar and arch, which are firmly connected 
with the apparatus belonging to the micrometer by their means. 
At S is a piece of hard polished steel, against which the point 
of the micrometer-screw rests ; and, as the arch EF is the seg- 
ment of a circle whose radius is its distance from the axis of 
the sector, the point of that screw always buts against the steel 
head in the same place. In this plate is also seen a vertical 
section of the same parts. Here, EF is the back arch, and MS 
the micrometer-screw.. This figure also shows the means by 
which the pieces carrying the wires, and inserted into the end 
of the telescope, are retained in their proper places. CA and 
DB are two long pillars, which pass through an annular piece 
of brass parallel to the end of the tube. 
A screw with a windlas-like head is seen at G, from the 
turning of which, the wires are moved in one of their proper 
directions. A screw for giving them a motion at right angles 
to that obtained by the fore-mentioned one at G, is seen in the 
horizontal view of the end of the telescope at H. 
It would be swelling this account to an inconvenient size, if 
I were to attempt any farther explanation of the plates ; I shall, 
