22 
THE CORNISH PUMPING ENGINE. 
4th. The substitution of the expansive force of steam to press on the piston, 
instead of the pressure of the atmosphere; whereby the engine ceased to be an 
atmospheric, and became a steam 42 engine. 
5th. The use of oleaginous matter to render the piston steam-tight, instead 
of water, as formerly employed. 
6th. The invention and adaptation of the principle of expansion, by cutting 
off the steam before the piston had finished its stroke. (See Art. 40.) 
7th. The introduction of the double-acting engine. (See Art. 48.) 
8th. The arrangement known by the name of the parallel motion. This was 
first designed for the double-acting engine, to replace the toothed rack and 
sector required for giving an upward as well as a downward propulsion. It 
was afterwards applied to single-acting engines, being a much more suitable 
method of connecting the piston rod to the beam than the original arc and 
chain. 
33. All the above-named inventions were protected to Watt by patent; but inde 
pendently of those thus secured, he made other alterations of very considerable 
value. 
He changed the dome boiler, formerly used for the atmospheric engine, to the 
waggon-shaped one, which has since been applied generally to low-pressure engines. 
By this change more heating surface was gained for a given cubic content, and the 
boiler at the same time rendered easier of manufacture and of a more convenient 
form for setting. 
He also removed the boiler from immediately beneath the cylinder, and placed 
the latter on a separate foundation. This was a great improvement; for in the 
atmospheric engines, the cylinders of which stood upon the top of the dome 
boilers, 43 the vibration caused by the motion and concussions of the engine soon 
rendered the boilers leaky, and deranged the position of the cylinders; while in 
Watt’s engine the cylinder was much more permanently and steadily fixed, and 
42 Sir F. Blake, as early as 1750, suggested the use of the term steam engine, in lieu of the then common 
appellation of fire engine. See Phil. Trans, vol. xlvii. p. 197. 
43 The boiler of Mr. Smeaton’s portable engine (see Farey, Plate IX.) was not immediately under the 
cylinder, but his stationary engines appear to have been, in this respect, constructed on the old plan. 
There is a report that an atmospheric engine near Camborne, made by Bridge between 1775 and 1778, 
had the cylinder separated from the boiler; but this may have been in imitation of Watt’s practice, which 
must have been then known. This engine has been mentioned by Mr. Davies Gilbert, and others, as the 
best in the county at that time. 
Vide also Robison’s Mech. Phil. vol. ii. p. 79, where the removal of the boiler to the side of the cylinder 
of the atmospheric engine is noticed, but there is no intimation of this having been done before Watt’s 
time.