258 MOTIVE POWER EMPLOYED

therefore produced by the diminution of all therubbing - parts of the engine, compared with theprogressive motion. In travelling 48 mileswith the 4-feet wheels, the same number of re-volutions of the engine-wheels, and conse-quently the same extent of rubbing actionoccurs, as before took place in travelling 36miles with 3-feet wheels: and hence we findthe consumption of coals nearly the same inperforming both journies.

From the diminution of friction, by the in-crease of one foot in the diameter of the wheels,we therefore find the whole friction of theengine.

The ratio of the wheels were as 3 : 4, there-fore, by the application of 4-feet wheels, one-fourth of the friction in passing over the samespace will be annihilated ; whence 128 x 4=512/6s. the total amount of friction with the3-feet wheels, and as 4 : 3 :: 512 : 384 lbs. thefriction of the engine with 4:-feet wheels.

We see, therefore, the reason why the samequantity of fuel produced different effects with

3 and with 4-feet wheels; in the former, thefriction was 5121bs.; increasing the wheels to

4 feet, diminished that friction 128lbs.; andthis diminution of friction amounts to morethan the resistance of 10 tons upon a Rail-way,which enables the fuel to produce a correspond-ing increase of effect.