148 Transactions. 
given, first, the approximate indicated horse-power ; the quantity of water 
to be evaporated to supply the engines at a given speed and pressure, with a 
given known consumption. Secondly—To calculate in the same way the results 
that would probably be obtained if the same engines were converted into 
compound engines. 
This seems to be a subject of importance to every one concerned or 
interested in the use or science of steam, but it must not be understood that I 
pretend to satisfactorily solve the question of the superiority of the compound 
principle, but to bring it before the notice of this Society for discussion. — 
At the same time I shall endeavour to show the results of a few simple 
calculations from the three different examples given. 
Commencing then with the before-mentioned examples of engines, whose _ 
cylinders are 44 inches diameter, with a piston speed of 297 feet per minute, 
and an initial pressure of 15 lbs. on the square inch, cutting off at 3 of the 
stroke, and a mean vacuum of 26 inches, the consumption of iis being 
18 tons per 24 hours, or 15 ewt. per hour. Working this out in the usual 
way, these engines might be expected to indicate 7:11 horse-power, and the 
quantity of water that would have to be evaporated to supply the engines at 
the above-named pressure and speed is 241 cubic feet per hour. This is equal 
to one pound of coal evaporating 10 lbs. of water in the hour, or a consump- 
tion of 2°36 Ibs. per indicated horse-power per hour. 
Secondly.— We will now compare the above results with what would 
probably be obtained if these engines were compounded with two high-pressure 
cylinders (similar to those in the “Star of the South ”) of 22 inches diameter, 
with a boiler pressure of 80 lbs. per inch, and cutting off at half-stroke in all 
the cylinders. Still retaining the same piston speed, we shall have for the 
upper cylinders an initial pressure of 75 Ibs. per inch, the mean pressure 
63 Ibs. The initial pressure in lower cylinders being 9:375 lbs., the back 
pressure in upper cylinders will be 54 lbs. per square inch, and the ratio of 
expansion in upper cylinders being 2 to 1, the terminal pressure equalling 
37:5 lbs., from which data, using the same formula as in the preceding 
examples, the two high pressure cylinders would indicate 372-4 horse-power. 
The ratio of the areas of the upper and lower cylinders being 4 to 1, the 
initial pressure in lower cylinders will be say 9:3 lbs., cutting off at half- 
stroke ; mean pressure = 7°6 lbs., but deducting 3 Ibs. for loss of steam travell- 
ing from one cylinder to the other, and condensation, we shall have a total 
effective pressure of 4:6 + 13= 17-6 lbs. in lower cylinders, which will give 
481 horse-power for the lower cylinders, making a total of 854 horse-power 
for the combined engines, and this with the steam expanding eight times to one. 
The quantity of water required to be evaporated to supply the engines at 
the speed and pressure above stated will be 165 cubic feet per hour ; allowing 
