60 Steam Engine^ 
within can be observed from the depression of the mercn- 
rial column. In this way I have had water as high as 
155° surrounding the vacuum : but as the higher tempe- 
ratures might endanger a glass apparatus ; instead of it I 
used the following 
Having procured a tin tube of four inches in diameter 
and two i’eet long, with a circular plate of the same sol- 
dered to one end having a round tube 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 centre 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 temperature. 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 under 212^^ 
could be ascertained, the depression of tfie mercurial co- 
lumn being known by the ascent in the exterior leg of 
the syphon. 
The force of vapour from water between 80° 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 insert the bulb of a thermometer ; then cover the 
whole v/ith a receiver on one of the pump-plates, and 
place a barometer gage on the other : the air being slowly 
exhausted, mark both the thermometer and barometer 
at the moment ebullition commences, and the height of 
the barometer gage will denote the force of vapour from 
water of the observed tem]3crature. This method may 
also lie used for other liquids. It may be proper to ob- 
serve, the various tliermometers used in these experiments 
were duly adjusted a good standard one. 
