the Trigonometrical Operation • t 
to give fufficient room for the adjuftment of the inftrument, 
which by them is brought to be level. 
Art. XIX. Adjufment of the axis Level. 
The axis of the upper or tranfit telefcope being brought over 
any one of the feet, and the circle being clamped, hang the 
axis level on the pivots or anfce of the telefcope, and bring the 
bubble to the two indexes ; then reverfe the level, that is, turn 
it end for end, and note the difference. Bifecft this difference, 
one half by the level’s proper adjufting fcrew, and the other 
half by that foot-fcrew only which is in a line with the axis. 
This operation being repeated until the difference wholly va- 
nifhes, the level will be truly adjufted, that is to fay, the bub- 
ble will reft between the fame points in both portions. 
Art. XX. Adjujlment of the elevation LeveL 
This level being fufpended on the rod attached to the out- 
fide of the tranfit telefcope, fcrew the ere£t eye-tube on, to 
make that end preponderate. Adjuft the bubble to the indexes 
by the fteel finger-fcrew at the tail of the femicircle’s clamp. 
Reverfe the level, and note the difference. Then bife<ft that 
difference, and corre£t one half by the finger fcrew, and the 
other half by the proper adjufting fcrew under the level, and 
fo on repeatedly until the difference wholly vaniffies. The 
level may then be hung on the two pins that project from the 
horizontal bar which carries the telefcope, where, being pa- 
rallel to the axis level, it will fhew when that is removed (as 
is commonly the cafe when terreftrial objects only are obferved) 
whether the plane of the inftrument fuffers any alteration. 
If this fliould have happened, the level on the horizontal bar 
is at all times fufficient to correft it. 
7 
5 
Art. 
