t, or glass) and admitting the external Air, you will find your self
unable to see the utmost ends of the object; but the terminating rayes AE
and AD (which were before refracted to G and F by the rarified Air) will
proceed almost directly to I and H; which alteration of the rayes (seeing
there is no other alteration made in the Organ by which the Experiment is
tryed, save only the admission, or exclusion of the condens'd Air) must
necessarily be caused by the variation of the _medium_ contain'd in the
Glass B; the greatest difficulty in the making of which Experiment, is from
the uneven surfaces of the bubble, which will represent an uneven image of
the object.

Now, that there is such a difference of the upper and under parts of the
Air is clear enough evinc'd from the late improvement of the _Torricellian_
Experiment, which has been tryed at the tops and feet of Mountains; and may
be further illustrated, and inquired into, by a means, which some whiles
since I thought of, and us'd, for the finding by what degrees the Air
passes from such a degree of Density to such a degree of Rarity. And
another, for the finding what pressure was requisite to make it pass from
such a degree of Rarefaction to a determinate Density: Which Experiments,
because they may be useful to illustrate the present Inquiry, I shall
briefly describe.

[16] I took then a small Glass-pipe AB, about the bigness of a Swans quill,
and about four foot long, which was very equally drawn, so that, as far as
I could perceive, no one part was bigger then another: This Tube (being
open at both ends) I fitted into another small Tube DE, that had a small
bore just big enough to contain the small Pipe, and this was seal'd up at
one, and open at the other, end; about which open end I fastned a small
wooden box C with cement, so that filling the bigger Tube, and part of the
box, with Quicksilver, I could thrust the smaller Tube into it, till it
were all covered with the Quicksilver: Having thus done, I fastned my
bigger Tube against the side of a wall, that it might stand the steadier,
and plunging the small Tube cleer under the _Mercury_ in the box, I stopt
the upper end of it very fast with cement, then lifting up the small Tube,
I drew it up by a small pully, and a string that I had fastned to the top
of the Room, and found the height of the _Mercurial Cylinder_ to be about
twenty nine inches.

Then letting down the Tube again, I opened the top, and then thru