Portable Agricultural Steam Engines at Newcastle. 693 
jackets bears some relation to the amount of work done in the 
cylinder, and to the rate of expansion. In large condensing 
engines, in which the circulation of steam in the jackets is 
thoroughly efficient, and where the rates of expansion are high, 
it has been found that the heat corresponding to the steam con- 
densed represents A'ery nearly the amount of heat converted into 
work. Though we have no experimental proof of the extent 
to which this law applies to small non-condensing engines, yet 
the assumption that it does so is a rational one, and we have 
therefore added the calculated weight of water so condensed in 
order to get an approximately correct estimate of the evaporative 
duty of the boilers. There can, at any rate, be no doubt, that 
with the very perfectly arranged jackets in use, and with the 
considerable expansion employed, that duty will be determined 
with a nearer approach to accuracy by making the proposed 
allowance for jacket-water than would be attained were no 
account whatever taken thereof. 
We may illustrate the nature of the calculations by reference 
to Mr. E. Foden's "Simple" traction engine. No. 3114. 
During the trial run, which lasted 263^ minutes, 138j lbs. of 
coal were consumed, and a total of 1413 lbs. of water at 63° were 
supplied to the engine. The condensed exhaust-steam raised 
the temperature of this water from 63° to an average of 83 "6^, 
or 20 • 6°. We may assume that the exhaust-steam in the heater 
was about 1 lb. per square inch above the atmospheric pressure, 
and under such circumstances the total heat above the freezing- 
point per 1 lb. weight of steam condensed would be 1147 '2 units. 
The steam was condensed at 83 • 6°, or 51 • 6° above the freezing- 
point ; hence the number of units imparted to the water supplied 
per 1 lb. weight of steam condensed would be 1147 "2 — 51 • 6 = 
1095*6° units. The temperature of the 1413 lbs. of fresh water 
supplied rose 20 • 6°, which represented the absorption of 
29,107 "8 units, and therefore, dividing by 1095*6 units, we 
^et 26*57 lbs. of steam condensed in heating the feed-water, 
ind consequently the total quantity pumped into the boiler was 
1413 lbs. -f 26 -57 lbs. = 1439* 57 lbs. 
But in order to compare the work done by different boilers, 
t is necessary to reduce the water evaporated to a standard 
emperature and pressure, because the quantity of heat which 
nust be imparted to generate steam depends on these two points. 
The standard usually adopted is the quantity of heat needed 
0 evaporate water at the boiling-point, or 212° temperature, in 
in open boiler at atmospheric pressure. It requires 966*6 units 
)f heat to evaporate one pound of water under these conditions, 
f water be supplied at a lower temperature than 212°, the heat 
lecessary to raise it to that temperature must be added. 
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