C 4*9 3 
The firft Inftrument that I made conformable to this 
Principle, was with a Water-Level, but finding that 
Water was fubjed tofome Inconveniencies, I altered 
the Apparatus, and changed the Fluid from Water to 
Mercury : This Alteration and Improvement will more 
intelligibly appear by the Inftrument, and alfo the 
Draughthereuntofubjoined,TAB.I. Fig. i. where 
CD, reprefents the Segments of two different Circles 
that are concentrick 5 F,the common Centre, in which 
moves the Pin or Axis fitted to the Index or Label E F* 
on which Label is alfo fix'd the horizontal Tube Gg y 
which has a Communication with the two glafs ver- 
tical Tubes Eh,dh , in which moves the Mercury. On 
each Top of the vertical Tubes are fix'd a large hollow 
brafs Cylinder hh, having in their Tops a Pin, by 
clofing of 'which, the included Air is prevented from 
any Communication with the External 5 by which 
means this Advantage is obtained, that it prevents, in 
a great meafure, that too quick and vibratory Motion 
that is natural to the Fluidity join'd to the Gravity of 
Mercury when moved, and at the fame time, by 
having a fufficient Space and Quantity of Air in the 
Cylinders 'at Top, does not in the leaft impede the true 
Level ; but notwithstanding this Precaution, the Mer- 
cury ftill would be fubjed to a tremulous Motion, 
were it not that the Diameters of the vertical Tubes, 
to that of the horizontal, are as 2 to 1, and confe- 
quently the Area 4 to 1 5 by which means this Incon- 
veniency is alfo removed, without any way affeding 
the horizontal Level. 
The firft trimming or preparing the Tubes with 
Mercury is fufficient, and when the two little convex 
Surfaces of the Mercury appear juft vifiblc above the 
level 
