287
The total friction of the engine, with 4-feet wheels, webefore found to be 384 lbs. Then, as 5 : 411 384 t 308 lbs.the friction of the engine with 5-feet wheels. Supposing thesame load taken, the resistance will now be 308 4- 448 —756 lbs., moved over in one revolution of the wheel, or in15.7 feet, — 11869, the resistance; and, if the density ofsteam in the boiler be the same, the power will, also, remainas before.
f 25446 power,
\V hence \ r .
I 11869 resistance,
equal to 46.8 per cent.
The relative resistances, at 6 and 8 miles an hour, willtherefore be as 10400 : 11869, or as 34 ; 39.
liy reducing the load equivalent to thatamount of resistance, or from 40 to 35 tons,the engine Mould then travel at the rate of8 miles an hour M'ith the same expenditure ofsteam as at 6 miles an hour; but, as the effectwill invariably be greatest when the loadbears as great a proportion to the friction ofthe engine as possible, it will always be ad-visable to make the load approach the nearestto that which the engine can overcome by theadhesion of its wheels upon the rails. If,therefore, instead of diminishing the load, weproduce an additional quantity of steam byincreasing the surface of the fire, or, in fact,the size of the tube, we can, either by enlarg-ing the aperture through which the steamissues to the cylinder, or by increasing the area