thereby free the Quicksilver and Glass from all the bubbles or parcels of lurking Air; then inverting it as before, I fill it top full with clear and well strain’d Quicksilver, and having made ready a small ball of pretty hard Cement, by heat made very soft, I press it into the hole E, and thereby stop it very fast; and to secure this Cement from flying out afterward, I bind over it a piece of Leather, that is spread over in the inside with Cement, and wound about it while the Cement is hot: Having thus fastned it, I gently erect again the Glass after this manner: I first let the Frame down edge-wayes, till the edge RV touch the Floor, or ly horizontal; and then in that edging posture raise the end RS; this I do, that if there chance to be any Air hidden 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