the Trigonometrical Operation. ■ i $ t 
conceived by obferving attentively the top of the vertical axis 
in the fefiion towards the right hand of Plate IV. On the edge 
of the femircircle may likevvifebe feen a moveable clamD, eafilv 
made to flip, with the hand only, around its circumference, 
and it carries with it a very fine flee! fcrew. When the fp mi- 
circle is towards the left hand of the telefcope, which is its 
ordinary polition, the point of the fteel fcrew refcs, or may be 
made to reft, perpendicularly on the furface of the plane 
that is on the left of the vertical axis. But when the tele- 
fcope is inverted in its Y’s, or turned upfide down, as is the 
cafe in adjufting the line of collimation, the femicircle being 
then on the right of the telefcope, and the clamp neceflarily 
brought down, the point of the fteel fcrew accordingly refts 
perpendicularly on the furface of the plane to the right of the 
vertical axis. Thus it will be readily conceived, that in ad- 
jufting the telefcope by the level for elevations, which is then 
conftantly fufpended on its proper rod, parallel to the axis of 
vifion, the action of the fteel fcrew on the bell-metal plane 
ferves not only for the adjuftment of the telefcope in a truly 
horizontal pofition, for angles of elevation or depreffion, by 
the motion of a wire in the focus of its eye-glafs, in the man- 
ner hereafter to be defcribed, but alfo to keep it in that pofi- 
tion, by the fuperior weight of the eye end, rendered fo on 
purpofe. By the fame means the telefcope remains fteadily on 
any objedfc that it may be directed to for interfefiion, whether 
above or below the plane of the horizon * 
One thing more with regard to the femicircle muft be men- 
tioned, namely, that it gives angles of elevation \z // too great, 
and thofe of depreflion i 2 y/ too little. It is very eafy to con- 
ceive, that this arofe from the impoffibility of dividing it on 
the axis of the telefcope to which it is fixed, and through the 
centers 
