[ 4M ] 
included Air has no Communication with the Ex- 
ternal, its being clofe confin'd gives this Advantage, 
that it prevents the Mercury, in its vibratory Mo- 
tion, from being quick and tremulous. 
The Bottoms of the brafs Cylinder that the glafs 
Tubes are fix'd in, muft in the Infide be made Tun- 
nel-wife, that the Mercury may not lodge behind. 
The Hole at the Top, and the Pin, is for taking out 
or putting in Mercury, if Occafion 5 as alfo to clean 
the T ubes with a W ire. The perpendicular Tubes muft 
at leaft be twice the Diameter of the long BafeTube, 
for this Reafon among others, that the dilating and 
condenfing of the Mercury, from Heat or Cold, may 
not be fenfible in the perpendicular Tubes 5 and alfo 
that the BafeTube muft be as long as the Index or 
Label will admit, and the Tube thereof to be as 
fmall as can be, but fo as to admit a Paffage for 
the Mercury. This Paffage flhould be through a 
fmall glafs Tube inclofed in Wood, &c. The Cy- 
linders muft not be folder'd with foft Solder nor 
Silver: The Mercury will affeft it. 
Note, If the Mercury fhould be feparated by an 
Air-bubble in the Tube, incline the In- 
ftrument till the Mercury difappears in 
the Tube below the Bafe, and it will 
take it out. The true Level is when the 
little convex Surfaces of the Mercury juft 
appear above the Level Rings 5 then it is 
rightly trimm'd 5 and when you obferve, 
you look only at one of them, viz. that 
at the Centre, the Shade Vane co-inciding at 
the fame time on the Horizon Vane. 
March 11. 1738. 
III. A 
