The rules by which the necessary dimensions to perform any required duty are fixed, depend upon the very simplest mechanical laws.
NOTE.—The formulæ expressing the most proper relations to exist between the several steam-producing and steam-consuming parts are more reliable than the assertions of any machinist in America, and though taken from books, are the result of the experience of the most able and practical men for twenty years. Operatives are too apt to despise book knowledge, forgetting that the very knowledge so despised is the result of more practice than a lifetime can afford them. Railroad managers are too apt to receive as indisputable, the opinions of men who are _practical_, simply because they understand nothing of _principle_.
Since the work of D. K. Clark (England) has appeared, any dimension from the beginning to the end of a locomotive may be fixed, to the eighth of an inch, with absolute correctness, and there is no excuse for departing from the proper proportions. It does not follow that because a locomotive does actually start off and draw the train, that it is properly made. A race-horse can draw a plough, and a yoke of oxen a “trotting buggy,” but this is by no means the correct adaptation of power.
310. The elements which govern the requirements of power are
The maximum grades. The weight of the train. The required speed.
And the elements which govern our ability to produce the power needed,
The grate area. The heating surface. The cylinder diameter. The steam pressure. The stroke. The diameter of wheels. The weight upon driving wheels.
MECHANICAL AND PHYSICAL PRINCIPLES GOVERNING THE CONSTRUCTION OF THE LOCOMOTIVE ENGINE.
RESISTANCE TO THE MOTION OF RAILROAD TRAINS.
311. The exact resistance to the motion of a railroad train cannot be determined, as some of the elements are so variable; for example, the state of the weather. An approximate estimate, near enough for practice, is easily obtained. To arrive at correct data the observations must be made upon trains working under the same conditions that they are subject to in practice.
The whole resistance is made up of several partial resistances, some of which are constant at all speeds, and some of which increase with the velocity.