in the
small Pipe E, it may ascend into the Pipe F, and not into the Pipe DC:
Having thus erected it, and hung it by the hole Q, or fixt it
perpendicularly by any other means, I open the end F, and by a small
_Syphon_ I draw out the _Mercury_ so long, till I find the surface of
it AB in the head to touch exactly the line XY; at which time I
immediately take away the _Syphon_, and if by chance it be run somewhat
below the line XY, by pouring in gently a little _Mercury_ at F, I
raise it again to its desired height, by this contrivance I make all
the sensible rising and falling of the _Mercury_ to be visible in the
surface of the _Mercury_ in the Pipe F, and scarce any in the head AB.
But because there really is some small change of the upper surface
also, I find by several Observations how much it rises in the Ball, and
falls in the Pipe F, to make the distance between the two surfaces an
inch greater then it was before; and the measure that it falls in the
Pipe is the length of the inch by which I am to mark the parts of the
Tube F, or the Board on which it lyes, into inches and Decimals: Having
thus justned and divided it, I have a large Wheel MNOP, whose outmost
limb is divided into two hundred equal parts; this by certain small
Pillars is fixt on the Frame RT, in the manner exprest in the Figure.
In the middle of this, on the back side, in a convenient frame, is
placed a small Cylinder, whose circumference is equal to twice the
length of one of those divisions, which I find answer to an inch of
ascent, or descent, of _Mercury_: This Cylinder I, is movable on a very
small Needle, on the end of which is fixt a very light Index KL, all
which are so pois'd on the Axis, or Needle, that no part is heavier
then another: Then about this Cylinder is wound a small Clew of Silk,
with two small steel Bullets at each end of it GH; one of these, which
is somewhat the heavier, ought to be so big, as freely to move to and
fro in the Pipe F; by means of which contrivance, every the least
variation of the height of the _Mercury_ will be made exceeding visible
by the motion to and fro of the small Index KL.

But this is but one way of discovering the _effluvia_ of the Earth mixt
with the Air; there may be, perhaps many others, witness the _Hygroscope_,
an Instrument whereby the watery steams volatile in the Air are