and by a small Syphon I draw out the Mercury so long, till I find the surface of it A B in the head to touch exactly the line X Y; at which time I immediately take away the Syphon, and if by chance it be run somewhat below the line X Y, 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 A B. 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 M N O P, whose outmost limb is divided into two hundred equal parts; this by certain small Pillars is fixt on the Frame R T, 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 K L, 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 G H; 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 K L.
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 discerned, which the Nose it self is not able to find. This I have describ'd in the following Tract in the Description of the Beard of a wild Oat. Others there, are, may be discovered both by the Nose, and by other wayes also. Thus the smoak of burning Wood is smelt, seen, and sufficiently felt by the eyes: The fumes of burning Brimstone are smelt and discovered also by the destroying the Colours of Bodies, as by the whitening of a red Rose: And who knows, but that the Industry of man, following this method, may find out wayes of improving this sense to as great a degree of perfection at it is in any Animal, and perhaps yet higher.
'Tis not improbable also, but that our taste may be very much improv'd either by preparing our tast for the Body, as, after eating bitter things, Wine, or other Vinous liquors, are more sensibly tasted; or else bypre-
