When an engine is required for the transport of very heavy loads, such as those of merchandise, the adhesion of one pair of working wheels is sufficient ; and, in such cases, one of the two pairs of wheels L or M is made of the same di ameter as the driving wheels, and a bar is attached to points on the outside of the wheels, at equal distances from their cen tre, connecting them in such a manner that any force applied to make one pair of wheels, revolve must necessarily nn part the same motion to the other pair. By such means the force of the steam is made to drive both pairs of wheels, and consequently a proportionally increased adhesion is obtained.
The speed which an engine is capable of imparting depends on the rate at which the pistons are capable of being moved in the cylinders. By every motion of each piston backward and forward one revolu tion of the driving-wheel is produced; and by each revolution of the driving-: wheels supposing them not to slip upon the rails, the load is driven through a distance equal to their circumference. As the two cylinders work together, it follows that a quantity of steam sufficient to fill four cylinders must be supplied by the boiler to the engine, to move the train through a distance equal to the circum ference of the driving-wheels ; and in ac complishing this, each piston must move twice from end to end of the cylinder, each cylinder most be twice filled with steam from the boiler, and that steam must be twice discharged from the blast. pipe into the chimney. If the driving wheels be 5 feet in diameter, their cir cumference will be 15 feet 7 inches. To drive a train with a velocity of 30 miles an hour, it is necessary that the engine be propelled through 45 feet per second ; and to accomplish this with 4 feet wheels they must make nearly 3 revolutions per second ; and as each revolution requires two motions of the piston in the cylinder, it follows that each piston must move three times forward and three times back ward in the cylinder in a second ; that steam must be admitted six times per second to each cylinder, and discharged 12 times per second through the blast pipe: the motion of the slides and other reciprocating parts of the machinery must correspond.
This rapid reciprocating motion being injurious to the machinery, attempts have been made to diminish it by the adoption of larger working-wheels, and the driv ing-wheels on several of the great lines have been accordingly increased to 5} and 6 feet in diameter. Such engines have not been yet sufficiently long in use to afford a practical estimate of the effects of this change. Experiments of a much bolder
kind have been tried in England on the Great Western Railroad, where driving wheels of 10 feet in diameter have been worked. From a course of experiments, however, made by Dr. Lardner with those engines, it did not appear that they had. ;lurid; antage over those constructed with smaller and lighter wheels. Experience appears to have since confirmed this, as they are now for the most part abandon ed. The pressure of steam in the boiler is usually limited by N, safety-valves— one represented at N, under the control of the engineer ; and the other at 0, which cannot be approached byhim. The safety-valve at N is held down by a lever r, which is attached to a spiral spring, and which may, by an adjusting screw, be made to press on the valve with any re quired force. The second valve 0 is press ed by several small elliptical springs, placed one above another over the valve, and held down by a screw, which turns in a frame fixed into the valve scat. By this screw the pressure on the valve can be adjusted.
In order to give notice of the approach of the train, a steam-whistle Z is placed immediately above the fire-box at the back of the engine. This is an apparatus composed of two small hemispheres of brass, separated one from the other by a small space. Steam is made to pass through a hollow space formed in the lower hemisphere, and escapes from a very narrow circular opening round the edge of that hemisphere. The edge of the upper hemisphere presented down wards encounters this steam, and an effect is produced similar to the action of air in organ pipes. A shrill whistle is produced, which can be heard at a great distance, and differing from all ordinary sounds, never fails to give notice of the approach of a train.
It is not usual to express the power of locomotives, in the same manner as that of other engines, by the term horse power. Indeed, until the actual amount of resist ance encountered by these machines shall be more certainly ascertained, it is impossible that their efficiency can be es timated. The quantity of water evapo rated supplies a major limit to the power exerted ; but even this necessary element is not ascertained. Mr. Stevenson states that mm engine such as that above de scribed is capable of evaporating only 77 cubic feet of water per hour ; but Dr. Lardner found that the mean evaporation obtained by a very accurately conducted experiment over 200 miles of railway, with an engine called the "Heels," simi lar to the above, was 90 cubic feet per hour very nearly.