554 On the Force of Steam or Vapour 
Having procured a tin tube of 4 inches in di- 
ameter and 2 feet long, with a circular plate of 
the same soldered to one end having a round 
hole in the centre, like the tube of a reflecting 
telescope, I got another smaller tube of the 
same length soldered into the larger, so as to be 
in the axis or centfe of it: the small tube was 
open at both ends, and on this construction water 
could be poured into the large vessel to fill it, 
whilst the central tube was exposed to its tem- 
perature. Into this central tube I could insert 
the upper half of a syphon barometer, and fix it 
by a cork, the top of the narrow tube also being 
corked: thus the effect of any temperature un- 
der 212° could be ascertained, the depression of 
the mercurial column being known by the ascent 
in the exterior leg of the syphon. 
The force of vapour from water, between 86 
and 212° may also be determined by means of 
an air-pump ; and the results exactly agree with 
those determined as above, Take a Florence 
flask half filled with hot water, into which in- 
sert the bulb of a thermometer; then cover the 
whole with a receiver on one of the pump plates, 
and place a barometer gage on the other; the 
air being slowly exhausted, mark both the thers 
mometer and barometer at the moment ebullition 
commences, and the height of the barometer 
gage will denote the force of vapour from 
i ne 
