60 Analyses of Books. ath [Juny, 
cold water to the outside of the condenser, was soon found in 
convenient from the great size and expense attendant. on the use 
of this apparatus; and Mr. Watt introduced an internal jet of 
cold water, which, striking against the steam, instantaneously 
reduced it to its original bulk, and thus formed a vacuum. To 
draw off the condensing water, as well as to get rid of the air 
that was extricated during condensation, he found it necessary 
to employ a small pump, worked by the engine, the size of 
which was proportioned to the amount of air and water gene- 
rated in the condenser. In one of the early engines upon this 
construction, erected at Bedworth, three air-pumps were used ; 
two below, worked by chains connected with the beam, anda 
third, placed above, which received the hot water raised by the 
others. In the engines now constructed, only one alr-pump is 
employed, and this fully answers the intended purpose. 
* Another improvement introduced by Mr. Watt, consisted in 
surrounding the upper part of the cylinder with a cap, through 
a hole in the centre of which the piston rod worked air-tight. 
The force of steam was then substituted for that of the atmo- 
sphere, and at a pressure of more than fifteen pounds on the 
square inch ; so that when a vacuum was formed beneath the 
piston, steam of considerable impellent force was entering the 
anys end of the cylinder, by means of a pipe connected with a 
oiler, 
“ By thus substituting the force of highly elastic vapour, for 
the ordinary pressure of the atmosphere, the upper and under 
side of the piston were preserved at the same temperature, and 
the supply of steam being regulated by the width of the aper- 
ture, any givenamount of force might readily be produced. In 
the atmospheric engine this could not be effected, as the whole 
ressure of the atmosphere was made to act on the piston, the 
instant the vacuum was formed by the condensation of the va- 
pour beneath ; so that in the event ofa pump-rod breaking, by 
which the elevation of the water might be impeded, and the la- 
bour of the engine taken off, the rapid descent of the piston 
would evidently cause the destruction of the entire apparatus. 
“Soon after the completion of his first model, Mr. Watt erected 
an engine for his friend Dr. Roebuck of Kinneil, near Borrow- 
stownness, with whom he was afterwards associated in the 
manufacture of his improved engine: the latter gentleman, 
however, in 1774, disposed of his share of the business to Mr, 
Boulton, of Soho.” 
Want of room prevents our making any additional extracts 
from Mr, P.’s work, or attempting an enumeration of the various 
engines he describes, which could only be satisfactorily accom- . 
plished by reference to the numerous plates employed for their 
illustration.—But it may be adviseable before we finally dismiss 
the subject, to briefly notice another work of a more general 
nature, but with much higher pretensions, announced as far 
