Gen . PvOy’s Account of 
1 4 % 
Art. IX, Spirit Lev eh. 
The inftrument has two very good fpirit levels, that are 
fitted with the feverai means of adjuftment, as is uiual in fuch 
cafes, the detail of which it is unneceffary here to enter into. 
The firft or axis level, becaufe it is only applied on the axis of 
the telefcope, is that whereby it is fet horizontal, as in the 
ordinary tranfit inftrument ; and it is likewife ufed for placing 
the conical axis truly vertical, fo that the inftrument may turn 
round without fenfible alteration of the level, previoufty to 
obfervations of the pole ftar, or of other heavenly bodies. 
The fecond, or elevation level, is that whereby the telefcope 
is brought to be truly horizontal, when angles of elevation or 
depreffion are to be taken. At fuch times it is fufpended on a 
rod attached to the outfide of the telefcope, to whofe axis of 
viiion the rod, by adjuftment, can be made parallel, as will 
readily be conceived, by obferving the reprefentation of thefe 
parts in the right hand feCtion of Plate IV. 
When the angles of elevation or depreffion to be deter- 
mined are very fmall, they are meafured by the motion of an. 
horizontal wire in the focus of the eye-glafs of the telefcope ; 
but when great, their quantity is meafured by the arc of mo- 
tion of the femicircle, as fhewn by its proper horizontal 
microfcope. 
The elevation level is likewife made ufe of for levelling the 
inftrument when horizontal angles only are to be taken, for 
which purpofe it is fufpended on two pins, which are feen 
projecting from the horizontal bar in the plan, and one of 
them in each of the feClions in Plate IV. This was the ordi- 
nary pofition of the elevation level when the angles of the 
2 triangles 
