(320.) 
Imperfec- 
tions of at- 
mospheric 
engine. 
(321.) 
Watt's 
principle 
of separate 
condensa- 
tion, 
868 
When Watt discovered by means of his model that 
the condensation and loss of steam in the cylinder at 
every stroke of the piston exceeded that which is use- 
fully employed in producing the vacuum, he proceed- 
ed very methodically to ascertain the chief numerical 
data or constants upon which the working of steam~- 
engines depend ; as, the bulk of steam of given elas- 
ticity compared with that of the water producing it, the 
elasticities of steam at different temperatures, espe- 
cially above the boiling point, the evaporating power 
of a pound of coals, and finally, the expenditure of 
steam and of injection-water for a single stroke, The 
last, the amount of cold water required effectually to 
condense the cylinderful of steam, appeared so very 
large that it at once betrayed the source of the waste- 
ful expenditure of fuel. All the heat which was ab- 
stracted by the injection-water had to be supplied 
afresh at a vast expense of steam, and consequently 
of coal, before a fresh stroke could be made. The 
mere amount of steam necessary to fill the cylinder 
and eleyate the piston was a trifle in comparison. He 
naturally ascribed it to the absorption of heat by the 
material of the cylinder, which no doubt was an im- 
portant element; and he tried cylinders of different 
materials, and ascertained (in most instances for the 
first time) the specific heats of the substances used. 
But still the amount of injection-water remained un- 
accounted for, and the regulation of it was the great 
difficulty of the old engine, If a tolerable vacuum 
was desired, the cylinder must be flooded with cold 
water, and all the heat abstracted must be restored 
before the next stroke was produced. If, on the other 
hand, the injection was imperfect, so that the con- 
densed steam and water together retained a high 
temperature, an atmosphere of vapour so elastic 
spoiled the vacuum, and in many cases the working 
efficiency of the engine was reduced to about half of 
the atmospheric pressure. 
The theory of latent heat, showing that steam con- 
tains as much heat as if the water yielding it had 
been raised through 1000° of temperature, gave a 
clear explanation of the enormous amount of injec- 
tion-water necessary. How to escape from the diffi- 
culty was another question, which was solved solely 
by Mr Watt, who, early in 1765, first conceived the 
idea of retaining the cylinder always at the boiling 
heat, and effecting condensation in a separate vessel 
kept constantly as cold as possible, in which injection 
should also take place, and into which the steam of 
the cylinder would spontaneously rush, on a ecommu- 
nieation being opened at the proper instant. The suc- 
cess of this device was complete, and it was speedily 
followed by a series of admirable and mutually de- 
MATHEMATICAL AND PHYSICAL SCIENCE. 
[Diss. VI. 
pendent improvements which Watt introduced. To 
keep the cylinder as warm as possible, it was first 
provided with a cover, through which the piston-rod 
passed by means of a stuffing-box. Then steam it- 
self was employed to press down the piston, instead 
of atmospheric air which cooled the cylinder injuri- 
ously, 
Having thus steam on each side of the piston, the 
step was easy to make the engine a double-acting Double-act- 
one; so that the volume of steam which by its elas- 
ticity impelled the piston during its descent, being 
thrown into the condenser, produced the vacuum 
necessary to make the ascending stroke equally ad- 
yantageous.' But in pumping-engines, which had 
hitherto been the main employment of steam, or jire 
engines (as they were then called), it was sufficient 
that the power was produced in one direction, that 
is, to lift the pump-rods ;? but the double action of a 
pushing and pulling foree was evidently applicable 
to every sort of machinery by the use of a crank and 
fly-wheel. A difficulty, however, was experienced in 
applying this push-and-pull movement to the oscil- 
lating beam which had so long formed a part of the 
atmospheric engine, and which it was so convenient to 
use. 
jointed links which effectually united the piston-rod 
with the end of the beam, the former moving in a 
vertical straight line, the latter in a cireular are, so 
that pressure was transmitted in the manner of either 
push or pull without the slightest practical incon- 
venience, No doubt the locus of the motion of the 
piston-rod head is not rigorously straight. It is, when 
fully developed, a curve of the fourth order, resem- 
bling a very elongated figure 8; but the portion of 
the curve in use is practically straight, This happy 
solution, the most elegant perhaps in the science of 
pure machinery, or kinematics, was not the result of 
a formal mathematical research, but presented itself 
to the mind of Watt, as he tells us, almost intui- 
tively. It became from henceforth an important ele- 
ment of mechanical combinations. 
Other methods of producing direct rotation oc- 
curred to the ingenious mind of Watt, and many of Expansive 
them were shown to be practicable by being actually 
executed in model size. But the next great practi- 
cal improvement introduced by him was the princi- 
ciple of expansive working, whereby a saying of steam 
is produced (particularly in single-acting engines), 
second in importance only to the result of the sepa- 
rate condensation, This consists in allowing the 
steam to flowin upon the piston during only the first 
part of the stroke; the rest being completed partly 
1 This happy idea seems to have been suggested by Dr Small of Birmingham in a letter to Watt, of date 5th November 1769. 
“ T see no reason 
why you should not condense at both ends of your cylinder, and drive all before you, back stroke and 
fore stroke.” —Mechanical Inventions of James Watt, i. p. 81. The uncommon sagacity of Dr Small, and the important services 
which he rendered in many ways to Watt, come out clearly for the first time in this work. » 
2 So associated were these engines with the operation of raising water, that we find they were at one time used to supply 
common water-wheels with a constant stream, and thus to produce continued motion for mill-work, &c. 
Mr Watt obviated it with his customary saga- Parallel 
city, and succeeded in inventing a connection of rigid motion. 
