SECT. II.] 
PROPERTIES OF STEAM. 
99 
repaired a working model of a steam engine for the university of Glasgow, in 
1763. The cylinder of the model was 05 feet stroke, and 2 inches, or one- 
sixth of a foot diameter. He “was surprised to find that its boiler would not 
supply it with steam, though apparently quite large enough.” By blowing the 
fire it was made to make a few strokes, but required an enormous quantity of 
injection water, though it was very lightly loaded by the column of water in the 
pump. It soon occurred, that this was caused by the little cylinder exposing 
a greater surface to condense the steam, than the cylinders of larger engines did 
in proportion to their respective contents. 1 
There is no doubt this difficulty was the cause of Mr. Watt turning his thoughts 
to improve the steam engine. Our rule being applied to this case, l = %, cl = 
and, 
of steam which would fill the cylinder would be consumed to condense at 160°. 
By lessening the load lifted, and consequently not condensing the steam to so low 
a temperature, Mr. Watt made the engine work. 
165. Now in our formula it will be observed, that 26° is half the degrees the 
temperature of the steam falls by condensation, and that if we lessen this, the 
quantity of heat lost will lessen in the same proportion, but the loss by uncon 
densed steam will be greater. The effect of the engine will be greatest when 
the sum of these losses is a minimum, and its load should be arranoed 
accordingly. 
The loss by cooling the cylinder is 
25 (212 - t) (l + d) ' 
, 
when t is the temperature of condensation. 
The loss by imperfect condensation is 
1127 f 
30 ’ 
but by our formula (art. 86.) 
Hence, with respect to t, 
1127 (t 4-100) 6 
30 x 1776 + 
25 (212 - t) (l + d) 
Id 
— a minimum. 
1 Robison’s Meehan. Phil. vol. ii. p. 114. Note by Mr. Watt.