The next illustration represents the portable steam-engine as built by one of the oldest and most experienced manufacturers of such engines in the United States. In the boilers of these engines the heating-surface is given less extent than in the stationary engine-boiler, but much greater than in the locomotive, and varies from 10 to 20 square feet per horse-power. The boilers are made very strong, to enable them to withstand the strains due to the attached engine, which are estimated as equivalent to from one-tenth to one-fifth that due to the steam-pressure. The boiler is sometimes given even double the strength usual with stationary boilers of similar capacity. The engine is mounted, in this example, directly over the boiler, and all parts are in sight and readily accessible to the engineer. [Illustration: FIG. 116.--The Portable Steam-Engine, 1878.] One of these engines, of 20 horse-power, has a steam-cylinder 10 inches in diameter and 18 inches stroke of piston, making 125 revolutions per minute, and has 9 square feet of grate-surface and 288 feet of heating-surface. It weighs about 4-1/2 tons. Steam is carried at 125 pounds. In the class of engines just described, the draught is obtained by the blast of the exhaust-steam which is led into the chimney. Such engines are now sold at from $120 to $150 per horse-power, according to size and quality, the smaller engines costing most. The usual consumption of fuel is from 4 to 6 pounds per hour and per horse-power, burning from 15 to 20 pounds on each square foot of grate, and each pound evaporating about 8 pounds of water. A usual weight is, for the larger sizes, 500 pounds per horse-power. [Illustration: FIG. 117.--The Thrashers' Road-Engine, 1878.] These engines are sometimes arranged to propel themselves, as in the Mills "Thrashers'" road-engine or locomotive, of which the accompanying engraving is a good representation. This engine is proportioned for hauling a tank containing 10 barrels, or more, of water and a grain-separator over all ordinary roads, and to drive a thrashing-machine or saw-mill, developing 20 or 25 horse-power. This example of the road-engine has a boiler built to work at 250 pounds of steam; the engine is designed for a maximum power of 30 horses. This engine has a balanced valve and automatic cut-off, and is fitted with a reversing-gear for use on the road. The driving-wheels are of wrought-iron, 56 inches diameter and 8 inches wide, with cast-iron driving-arms. Both wheels are drivers on curves as well as on straight lines. The engine is guided and fired by one man, and the total weight is so small that it will pass safely over any good country bridge. A brake is attached, to insure safety when going down-hill. Although designed to move at a speed of about three miles per hour, the velocity of the piston may be increased so that four miles per hour may be accomplished when necessary. [Illustration: FIG. 118.--Fisher's Steam-Carriage.]