THE RIVER THAMES AT LONDON BRIDGE AND THE SEA. 
171 
Loosen now the clamp which confines the telescope over the foot-screw, and 
adjust the bubble exactly to the centre ; bring the telescope and limb half 
round on its conical centre, and observe the bubble ; half the number of divi- 
sions, in error on the scale, must be corrected by the foot-screw, and the other 
half by the vertical motion to the Y, securing it, when it is sufficiently adjusted, 
by the little side screw. 
Fourth, To place the vertical and horizontal wires at right angles, and to 
connect them with the little levels on the telescope : 
Place one of the station-staves about five feet from the ground in a horizontal 
position (by means of a small hand level and two nails) ; from the centre thereof, 
suspend a white plumb-line ; adjust the vertical wire to the horizontal one, by 
loosening the two screws, which admit of its moving diagonally, and making 
them coincide with the surface of the staff and the plumb-line ; then by loosen- 
ing the screws of the small level at the eye end, move it until the bubble rests 
in the centre, reverse the position of the wires, making the vertical horizontal, 
and adjust in the same manner the other little level. 
Fifth, To make the hanging level parallel to the plane of the instrument : 
Adjust by the foot-screw, until the bubble of the lower level is in its position ; 
then observe the variation of the riding level, and alter it one half the error by 
the vertical screw on the cock, and the other half by the pushing screws on 
the riding level itself. 
Or, By the foot-screw, bring the bubble of the hanging level in the centre ; 
then reverse it on the cocks ; one half the difference is to be altered by means 
of the adjustment on the level itself, and one half by the foot-screw : now place 
the lower level perfectly horizontal, and by the vertical screw on the cock, 
bring the bubble of the hanging level to correspond. 
The usual adjustment for collimation is here purposely omitted. The eye 
tube of the telescope altering continually in its position, renders it most diffi- 
cult to make this adjustment correctly ; for although it may be found in per- 
fect "adjustment for a distant object, when directed to a near one there will be 
a considerable error, which would affect the results in levelling, if the station- 
staves were not equidistant from the instrument. 
To avoid this inconvenience therefore, as well as to avert the difficulty of 
placing the extra lenses for short distances, so as not to alter the line of colli- 
z 2 
