( 4° ) 
The Tube T P, with this apparatus, being fil- 
led with Mercury, and plunged into the Bafon V, 
which hangs by two or more Wires upon the lower 
End of the Tube M N, muft be fo poized as to floit 
in the Liquor contained in the Veffel AB CD, and 
then' it will rife when the Atmofphere becomes lighter, 
and e contra. 
Let the fpecifick Gravity of Quickfilver be to that 
of Water, or to the Liquor the Barometer floats in, as 
s to i : and if it be propofed that the Variations of this 
Compound Barometer lhall be to the contemporary 
Variations of the common Barometer in the given 
•Ratio of * to i, this Effect will be obtain’d by ma- 
king the Diameter of the Rod S T to the Diameter 
of the Cavitity of the Tube H I,, as V'— to i 
ns ■’ 
which may be thus demonftrated. 
Let us luppofe that the Variation of the Height of 
the Quickfilver in the common Barometer, called v, 
‘is fuch, that a Cubic Inch of Quickfilver lhall rife 
into the Vacuum XT; in order to which a Cubic 
Inch of Quickfilver muff rife from the Veffel V, that 
is, the Surface P muft fubfide fo far, that a Cubic Inch 
of Water (if that be the Fluid made ufe of) lhall en- 
ter the Veffel V, by which Means the Barometer 
'with the Parts annexed will be heavier by a Cubic 
Inch of the Fluid. 
Now this additional Weight of a Cubic Inch of 
'Fluid will make the Whole Barometer fubfide (ac- 
cording to the Laws of Hydroftaticks) ’till a Cubick 
Inch of the Rod H S, immediately extant above the 
Surface at W, lhall come under it ; but the Length 
