( * 7 * ■) 
Ends will agree, and the Degree and Parts marked 
muft be added to, or fubftra&ed from the Altitude 
(hewn by the Index, according to the Directions : If 
the Ends do not exactly agree, take the Mean be- 
tween them. If you ufe the under Scale, fubftraQ: 
the lefs Number from the greater, and add, or fub- 
ftra<Et the Excefs, the Number refulting will fhew 
the mean Elevation of the Index during the latter 
Part of the Obfervation, and will differ from the 
true Altitude of the ObjeCt about half fo much as 
the Vibrations of its Image above and below the afore- 
mentioned Thread on the Vane fail of compenfating 
one another during that time. If either End of the 
Spirit leave the Scale, the Index muft be remov’d 
three or four Degrees, and the Obfervationrepeated. 
Inftead of the Curve Tubes A and B, two ftrait 
ones might be ufed, fet together fo as to make a very 
obtufe Angle in the Middle; but then it will be con- 
venient to have the Quantity of Spirit more exaftly 
fitted to the Scale, becaufe the allowing for the Dif- 
ference will be fomething more troublefome. 
If the Obferver have an Affiftant to attend to the 
Level, while he himfelf obferves the Objeft, the 
whole Apparatus of the Brafs Tube, and Stop-cock, 
maybe omitted, fubftituting in its room only a Plug 
with a fmall Hole in it, which may be wrapped 
round with a very thin Slice of Cork, and fo thruft 
down into the middle of theGlafs Tube. The cut- 
ting the Glafs Tube in half in the Middle may likewife 
be avoided, if, inftead of the Stop-cock at G, there be 
one fixed in one or both of the Pipes I and K, to open 
and flop thePaffageof the Air, having a larger Hole in 
their Keys, there being alfo a Plug with a fmall Hole, 
thruft down into the Middle of the Tube, as before. 
Z % The 
r 
