93 
of the Equatorial Instrument. 
in which the level G, H, I, about 12 inches long, swings upon 
two conical centres a , i, truly turned, of polished steel. G I, 
is an hollow brass tube, about i inch in diameter, containing 
within it a steel triangular axis, one extremity of which is 
terminated by the conical centre a, and, by means of a spiral 
spring winding round the axis, is made to contract its length 
by pulling the trigger g, towards I, by which means the 
point or centre a, retires within the tube G I ; so that the 
two centres may easily be released from the steel pivots 
wherein they play, and the level may be reversed, if neces- 
sary, to adjust it. It is evident that the axis a, i, should be 
parallel to the line b, b, which may be taken for a tangent 
to the curvature of the tube at k, and this is procured by 
the capstan headed screw c; it is also necessary that this 
same axis should be parallel to the line AB, or rather to the 
line of collimation of the telescope, in a north and south 
direction, and this is obtained by the capstan screw d. 
Finally, it is expedient that this axis be parallel to the line 
of collimation, in a direction east and west ; and this is ef- 
fected by the screw h. e, e, are two indexes to mark each 
end of the bubble, and, being fixed to the two concentric 
sliders/,/, which embrace y of the circumference of the tube 
GI, are moveable any where at pleasure, h is one of two 
screws, the other being opposite and out of sight, for ad- 
justing the axis a, i, parallel to the line of collimation, east- 
ward or westward, as has been said, and at right angles to 
the declination axis. I proceed next to the divisions. 
(21.) The superior advantages of an entire circle/ over a 
* Vide Observations Astronomicce Annis 1781, 1782, 1783, institute in Observa- 
torio Regio Hauniensi, auctore Thom a Bugge, Hauniae, 1784. Quarto. Cap. 5. 
