1we muſt anſwer, That of thoſe which are
eaſily intelligible without ocular demon
ſtration, we can at preſent ſuggeſt upon
our own tryals no better then theſe. Firſt,
at the open end of the Tube the Glaſs
muſt not onely be made as even at the ed
ges as you can, but it is very conveni
ent (eſpecially if the Tube be large) that
the bottom be every way bent inwards,
that ſo the Orifice, not much exceeding a
quarter of an Inch in Diameter, may be
the more eaſily and exactly ſtopp'd by the
Experimenter's finger; between which
and the Quick-ſilver, that there may be
no Air intercepted (as very often it hap
pens that there is) it is requiſite that the
Tube be fill'd as full as poſſibly it can be,
that the finger which is to ſtop it, preſſing
upon the accumulated and protuberant
Mercury, may rather throw down ſome,
then not finde enough exactly to keep out
the Air. It is alſo an uſeful and compen
dious way not to fill the Tube at firſt
quite ful of Mercury, but to leave near the
top about a qnarter of an Inch empty; for
if you then ſtop the open end with your
finger, and invert the Tube that quarter
of an Inch of Air will aſcend in a great
bubble to the top, and in its paſſage thi-
eaſily intelligible without ocular demon
ſtration, we can at preſent ſuggeſt upon
our own tryals no better then theſe. Firſt,
at the open end of the Tube the Glaſs
muſt not onely be made as even at the ed
ges as you can, but it is very conveni
ent (eſpecially if the Tube be large) that
the bottom be every way bent inwards,
that ſo the Orifice, not much exceeding a
quarter of an Inch in Diameter, may be
the more eaſily and exactly ſtopp'd by the
Experimenter's finger; between which
and the Quick-ſilver, that there may be
no Air intercepted (as very often it hap
pens that there is) it is requiſite that the
Tube be fill'd as full as poſſibly it can be,
that the finger which is to ſtop it, preſſing
upon the accumulated and protuberant
Mercury, may rather throw down ſome,
then not finde enough exactly to keep out
the Air. It is alſo an uſeful and compen
dious way not to fill the Tube at firſt
quite ful of Mercury, but to leave near the
top about a qnarter of an Inch empty; for
if you then ſtop the open end with your
finger, and invert the Tube that quarter
of an Inch of Air will aſcend in a great
bubble to the top, and in its paſſage thi-
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