SMALL SAWMILLS, THEIR EQUIPMENT, ETC. 59 
water or water at a lower temperature than that of the steam, when 
introduced into saturated steam, will condense some of it, thus lower- 
ing both the temperature and pressure of the rest until the tempera- 
ture again equals that due its pressure. The speed of steam for 
designers of steam piping is about a mile and one-half per minute 
and sometimes much more. 
There is a saving of approximately 1 per cent in fuel burned for 
every 11° that feed water is warmed before being run into the boiler. 
If sufficient exhaust steam is available and cold water at 70° is 
raised to 210° the saving in fuel will approximate 12 per cent. 
FUEL. 
One cord of air-dried hickory or hard maple weighs about 4,500 
pounds and is equal as fuel to 2,000 pounds coal. 
One cord of air-dried white oak weighs about 3,850 pounds and is 
equal to 1,715 pounds coal. 
One cord of air-dried red oak or black oak weighs 3,250 pounds 
and is equal to 1,150 pounds coal. 
One cord air-dried poplar, chestnut, or elm weighs 2,350 pounds 
and is equal to 1,050 pounds coal. 
One cord of air-dried yellow, white, or lodgepole pine weighs 
2,000 pounds and is equal to 625 pounds coal. 
From the foregoing data it is safe to assume that 2 J pounds dry 
wood is equal to 1 pound average-quality soft coal, and that the fuel 
value of the same weight of different woods is very nearly the same. 
That is, 1 pound of hickory is worth no more for fuel than a pound 
of pine, assuming both to be dry. It is important that the wood be 
dry, as each 10 per cent of water or moisture will detract 12 per cent 
from its value as fuel. This fact should be noted by small mill 
operators who use green slabs for fuel and sell the dry ones. In one 
case observed by the writer the dry slabs were sold in the mill yard 
at 1 cent each and the green ones used to fire with. During the after- 
noon the mill stopped four times, averaging 15 minutes each time, in 
order to get sufficient steam to run the saw, and then the sawyer had 
to " back up " the carriage several times to enable the saw to gather 
sufficient speed to cut a foot or two farther into the log. 
Evaporative power of 1 pound of various fuels at atmospheric 
pressure (1^.7 pounds). — 1 pound good coal will evaporate 10 pounds 
water; 1 pound crude petroleum will evaporate 16 pounds water; 1 
pound natural gas (25 cubic feet) will evaporate 20 pounds water. 
WATER WHEELS. 
The horse power of falling water is expressed in the formula 
T r X-^X62 5 
~' , which means that the volume of discharge in cubic feet 
