89 
Steam Engine. 
thence the quantity of water required to form as much 
steam as would supply each stroke of the engine, he pro- 
ceeded to examine how much cold water was used for 
injection, and what heat it gained ; which, to his very 
great surprise, he found to be many times the number of 
degrees which could have been communicated to it by a 
quantity of boiling water equal to that of which the steam 
was composed. Suspecting, however, that there might 
foe some fallacy in these deductions, he made a direct ex- 
periment to ascertain the degree of heat communicated by 
steam to water ; when it clearly appeared, that one part 
of water, in the form of steam, at 212 degrees, had com- 
municated about 140 degrees of heat to six parts of water^ 
The fact, thus confirmed, was so contrary to all his pre- 
vious conceptions, that he at first saw no means of ex- 
plaining it. Dr. Black indeed had, some time before, 
made his discovery of latent heat ; but Mr. Watt’s mind 
being otherwise engaged, he had not attended sufficiently 
to it, to make himself much acquainted with the doctrine : 
but upon communicating his observations to the doctor, 
he received from him a full explanation of his theory ; 
and this induced him to make farther experiments, by 
which he ascertained the latent heat of steam to be above 
900 degrees. 
The causes of the defects of Newcomen’s engines were 
now evident. It appeared that the steam could not be 
condensed so as to form an approximation to a vacuum, 
unless the cylinder, and the water it contained, were cool- 
ed down to less than 100 degrees ; and that, at greater de- 
grees of heat, the water in the cylinder must produce 
steam, which would in part resist the pressure of the at- 
mosphere. On the other hand, when greater degrees of 
exhaustion were attempted, the quantities of injection 
water required to be increased in a very great ratio ; and 
\'oi. IL M 
