C 1 5 1 ] 
the Air is quite cleared out which curious as well 
as fatiguing Operation is continued for the Space of 
four Hours : And when no more Bubbles would rife 
in the Tube, it remain’d whole, with its Mercury of 
a moft lively fparkling Brightnefs, with this Difference 
only, that the Mercury , fo purged from its Air, did 
not fill the Tube fo high as when firft put in by about 
two Inches ; which is a plain Demonftration, that in 
that Tube, which was 49 Inches long, there was in- 
terfpers’d in the Mercury at firft filling it, fo much Air 
as would fill two Inches of the faid Tube, which was 
a 24th Part of the faid Space. 
The whole Operation I myfelf attended the 2.0th 
of January 1734-y. 
And further I can affirm, that every Part of the 
Mercury boiled for a long time, and the Tube was 
gradatim fo red-hot, that with a warm Knife I could 
make Impreffions in any part of it. 
And this I the rather mention, by reafon I have 
heard feveral Perfons, and thofe not incurious, affirm 
it was impoflible. 
And that this is the moft fure and certain (if not 
the only) Method for getting out all the Air, may be 
judged by the boiling of Water, which in its Ebulli- •. 
tioa does emit a great Quantity of Air for a long Space 
of Time. 
The Perfection of thefe Barometers, ‘which 
exceed all others I have ever obferved in 
the following ‘Particulars. 
x. They arefenfible of the moft minute Changes of 
the Air whatfoever. 
2. They 
