39 
upon the compressed air contained therein. From the experi- 
ments of several philosophers, it is rendered pretty certain, 
that the capacity of air for heat, follows some reciprocal ratio 
to its density; hence when the air by expansion becomes rari- 
fied, a thermometer within the receiver falls several degrees, 
and by the sudden cold thus produced, the vapour of the air is 
condensed and becomes visible : on the contrary, when air is 
compressed, its temperature is proportionably raised, and if 
water or moisture be present, it immediately converts as much 
of it into invisible vapour, as will again reduce its temperature 
to an equilibrium with the water or other surrounding bodies. 
This well-known fact gives a very clear and just idea of the 
process continually going on in the water regulator ; every 
succeeding cylinder of air thrown into the chest, enters with 
a temperature elevated in proportion to the degree of pres- 
sure it sustains ; but so much as comes in contact with the 
water in the chest, is immediately reduced to the standard 
temperature, its excess of heat uniting with water and pro- 
ducing vapour ; and by continuation of this process the super- 
incumbent strata of air become successively reduced. This, 
however, requires some time to effect, and hence it appears 
that the larger the waterhouse, or the nearer the surface of 
the water the eduction pipe is fixed, the more perfectly will the 
air be reduced in its temperature, or saturated with moisture. 
We should be glad to know of any series of experi- 
ments being made to determine the relative capacities for 
heat, of confined air under different pressures ; as then the 
additional quantity of vapour taken up thereby might be 
ascertained. 
Lambert and Saussure's tables, of the proportions of vapour 
under different pressures, being calculated for the open air, 
are not, we think, directly applicable in this case. 
That a quantity is taken up, and a considerable one 
too, is however sufficiently manifest, from Mr. Roebuck's 
observations, respecting the air vault at Devon Iron Works; 
