The wheels scatter their burden of powdered sand into the axle-boxes, and it grinds its way inside the rod-brasses, and part of it gets wafted upon the guides; and in all these positions it is matter decidedly in the wrong place. And this body of sand under the wheels increases the resistance in the same way, as a wagon is harder to pull among gravel than it is on a clean, hard road: the indiscreet engineer complains about the train being stiff to haul; and the chances are, that he goes twice up the hill before the whole train is got over. Uncle Toby’s plan is, when pulling on a heavy grade, to open the valve enough to let the drivers leave a slight white impression on the rails. If they slip, he gives a few particles more sand, but decreases the supply again so soon as the drivers will hold with the diminished quantity. Uncle Toby seldom needs to double a hill.
SLIPPERY ENGINES.
These remarks apply to ordinary engines with ordinary rail-conditions. Occasionally we find an engine inveterately given to slipping, and no conditions seem able to keep it down. Such an engine is as ready to whirl its wheels as an ugly mule is to kick up its heels, and upon as little provocation. With a dirty, half-wet rail, an engine of this kind loses half its power. The causes that make an engine bad for slipping are various. Very hard steel tires, or excess of cylinder power, are the most frequent causes of slipping; but badly worn tires sometimes produce a similar effect; or the blame may rest in a short-wheel base, deficient in weight, or in too flexible driving-springs. To get a slippery engine over the road when the rails are moist and dirty, requires the exercise of unmeasured patience by the engineer. Job was a cantankerous old Arab beside the engineer who passes cheerfully through this ordeal. The tendency of an engine to slip may be checked to some extent by working with the lever well ahead towards full stroke, and throttling the steam. This gives a more uniform piston-pressure than is possible while working expansively. Of two evils, it is best to choose the least. The smallest in this case is losing the benefits of expansion, and getting over the road.
FEEDING THE BOILER.
Some engineers claim that the most economical results can be obtained from an engine by running with the water as low as possible, consistent with safety. They hold, that, so long as the water is sufficiently high to cover the heating-surfaces, there is enough to make steam from; and the ample steam-room remaining above the water, assures a more perfect supply of dry steam for the cylinders than can be had from the more contracted space left above a high-water line. Old engineers, running locomotives furnished with entirely reliable feeding-apparatus, may be able to carry a low-water level advantageously, especially with light trains and level roads; but with ordinary men, average pumps or injectors, and the common run of roads, a high-water level is safest. With a high-water level the temperature of the boiler can be kept nearly uniform; for the increased volume of water holds an accumulated store of heat, which is not readily affected by the feed. And the surplus store is convenient to draw upon in making the best of a time-order, or in getting over a heavy grade. Then, if the pumps or injectors fail, a full boiler of water often enables a man to examine the delinquent feeding-apparatus, and set it going; whereas, with low water, the only resource would be to dump the fire.
CHOICE OF PUMP AND INJECTOR.
The engine on this train has one pump and one injector. The pump is preferred for ordinary feeding-purposes, and is kept graduated to supply the needs of the boiler while the engine is working, without the foot-cock being moved. On a heavy pull, the pump in this condition would not keep up the water-level; so the injector is called upon to make up the deficiency. When the engine gets upon the heavy part of the grade, it makes steam very freely; and, when the indications of getting hot appear, the injector is started. During the remainder of the ascent, the water is supplied as liberally as it can be carried; and the top of the grade finds the engine with a full boiler. This enables the engineer to preserve a tolerably even boiler temperature; for in running down the long descent which follows, where the engine runs two miles without working steam, the pump can be shut off, and sudden cooling of the boiler avoided. The preservation of flues and fire-box sheets depends very much upon the manner of feeding the water. Some men are intensely careless in this matter. In climbing a grade, they let the water run down till there is scarcely enough left to cover the crown-sheet when they reach the summit. Then they dash on the feed, and plunge cold water into the hot boiler, which is then peculiarly liable to be easily cooled down, owing to the limited quantity of hot water it contains. The fact of having the steam shut off, greatly aggravates the evil; for there is then no intensity of heat passing through the flues to counteract the chilling effect of the feed-water. If it is necessary to pump while running with the steam shut off, the blower should be kept going; which will, in some measure, prevent the change of temperature from being dangerously sudden. There will probably be some loss from steam blowing off, but that is the smaller of two evils.
Engineers are not likely to feed the boiler too lavishly when working hard, for the injection of cold water instantly shows its effect by reducing the steam-pressure. But this is not the case when running with the throttle closed. The circulation in the boiler is then so sluggish, that the temperature of the water may be reduced many degrees, while the steam continues to show its highest pressure.
Writers on physical science tell us that the temperature of water and steam in a boiler is always the same, and varies according to pressure; that, at the atmosphere’s pressure, water boils at 212 degrees, and produces steam of the same temperature. At 10 pounds above the atmospheric pressure, the water will not evaporate into steam until it has reached a temperature of 240 degrees, and so on: as the pressure increases, the temperature of water and steam rises. But under all circumstances, while the water and steam remain in the same vessel, their temperature is the same. This is an acknowledged law of physical science; yet every locomotive engineer of reflection, who has run on a hilly road, knows that circumstances daily happen where the law does not hold good.