co Dr. Darwin’s /rigor ific Experiments 
that vapour becomes condenfed, and is precipitated in (bowers, 
as is vifible in the receiver of an air-pump above mentioned. 
There are, however, two other curious circumftances be- 
longiug to the devaporation of water, which have not been 
perhaps much attended to. 
Firft, that the deduction of a fmall quantity of heat from a 
cloud or province of vapour, compared with the quantity of 
heat which was necefiary to raife that vapour from water, will 
devaporate the whole. This circumftance is evident in the opera- 
tion of common (team-engines, in which a fmall jet of water, 
whofe heat is often above 48 degrees, perpetually devaporates the 
fteam raifed by a comparatively very great quantity of heat 
under the boiler. This difficult problem is explicable from 
the principles before eftabliffied : if a fmall part of a province 
of vapour be fuddenly condenfed, a vacuity takes place, and 
the contiguous walls of vapour expand themfelves into this 
vacuity ; and thus a large area of vapour, perhaps of many 
miles in circumference, becomes more or lefs expanded ; by 
this expanfiqn cold is produced (that is, its capacity of receiv- 
ing heat is increafed), and the whole is devaporated. 
This very circumftance exadUy takes place in the famous 
fleam-engine of Meffi Watt and Boulton ; which, from 
the happy combination of chemical and mechanic power, may 
juftly be efteemed the ftrft machine of human invention. In 
this excellent machine, after the cylinder is filled with fteam, 
a communication is opened between this refervoir of fteam and 
a fmall cell, which is kept cold by furrounding water, and 
free from air by an air-fyringe adapted to it. What then hap- 
pens ? The corner of the fteam in the cylinder next to this 
vacuum (with which it now communicates) rufties into it, 
and the whole fteam in the cylinder is thus fuddenly expanded, 
and 
