SMALL SAWMILLS, THEIR EQUIPMENT, ETC. 61 
the shape of two hemispheres joined together at the center by a 
straight thin rib. By this means the jet is split and returned with- 
out serious shock. The speed of the rim should be one-half of the 
jet velocity to give the highest efficiency. The water works by im- 
pact and pressure and not by weight, as in vertical wheels, and 
greater speed is directly transmitted, which makes a turbine wheel 
better adapted for sawmill purposes than a vertical one. 
The advantage of water-power mills is less operating expense. 
The disadvantages are: Water-power mills are not portable; the 
source of power not uniform or continuous ; not as capable of control 
as steam; and in the majority of cases results in low speed and low 
-capacity. 
ENGINE FRICTION BRAKE. 
The engine friction brake is an instrument for measuring the 
power which an engine can give off for external use. It is usually 
applied to a flywheel and will give results if the belting is properly 
adjusted and the shafting in line. The object of a flywheel is to 
compensate for the irregular turning movements, and to prevent the 
consequent variation of speed from exceeding certain predetermined 
limits. The energy stored in a flywheel varies as the square of the 
velocity and directly as the weight. In the case of solid cast iron of 
good quality the velocity of the rim should not exceed 80 feet per 
second and in built-up wheels 50 feet. 
STANDARD HORSEPOWER. 
Standard horsepower is 33,000 foot-pounds. A foot-pound is 
1 pound lifted 1 foot per minute or any equivalent amount of force, 
such as one-half pound lifted 2 feet or 12 pounds lifted 1 inch per 
minute. The horsepower of an engine may be determined by the 
2 x PLAN 
following formula: H. P.— , When P=mean effective 
pressure, L= length of the stroke in feet, A=average net area of 
piston, and N= number of revolutions per minute. To calculate 
horsepower, multiply the area of the piston in square inches by the 
speed of the piston in feet per minute and divide the product by 
33,000. The result is the horsepower value of 1 pound mean effective 
pressure, which, if multiplied by the whole mean effective pressure, 
will give the indicated horsepower. The net effective horsepower is 
the indicated horsepower less the friction of the engine. 
GEARS. 
The word " diameter " when applied to gears is always understood 
to mean the pitch diameters. Diameter pitch of the gear is the num- 
ber of teeth to each inch of its pitch diameter. If a gear has 40 teeth 
