CHAPTER VIII.

OF THE ADHESION.

§ 1. Measure of that Force.

The series of experiments we have described above, on the velocity and load of the engines, solves also another question in regard to the motion of locomotive engines, of which we have not yet spoken. That is the adhesion of the wheel to the rails.

We have remarked in describing the engine, that the power of the steam being applied to the wheel, the engine is in the same situation as a carriage which is made to advance by pushing at the spokes. Thus, as in that action, the only fulcrum of the moving power exists in the adhesion of the wheel to the rail, if that adhesion is not sufficient, the force of the steam will indeed make the wheels turn, but the wheels, slipping on the rails instead of adhering to them, will revolve, and the engine will remain in the same place.

The more considerable the train the engine

draws, the more power it must employ, and the more resistance it must consequently find in the point on which it rests, for executing the motion. It was therefore to be feared, that with considerable trains, the engines would be unable to advance; not that the force would be wanting in the moving power itself, but in the fulcrum of the motion.

The experiments related above, establish the measure of that adhesion in the fine season of the year. Among all these experiments, not one is to be found where the motion has been stopped or even slackened for want of adhesion, and nevertheless we see loads that amount to more than 200 t.

If we take, for instance, the first experiment made with the FURY, on July 24; during a part of the journey, that engine drew 244 t. The engine advancing with that load, the adhesion must necessarily have been sufficient. Now the weight of the FURY is 8.20 t., and that weight is divided in such a manner, that 5.5 t. are supported on the two hind wheels, which are the only working wheels, the others not serving to push the engine forward, but only to carry it. We have thus a weight of 5.5 t., drawing 244 t., or a load 44 times as considerable as itself. The result of this is, that an engine having its four wheels coupled, and which consequently adheres by its whole weight, is able to draw a load 441 times its own mass.

We have said that the FURY engine adhered only by two of its wheels. On the Liverpool Railway

that disposition is generally adopted for all trip engines, because the adhesion of two wheels is sufficient for the loads they have to draw. As for the helping engines, they work by the adhesion of their four wheels, as has been said elsewhere. The ATLAS is the only one of the former class that differs from the others in that respect. This engine has six wheels, four of which are of equal size, and worked by the piston. The two others, which are smaller, and have no flange, can be raised out of contact with the rails, by the action of the steam on a moveable piston. That ingenious arrangement, which may have more than one useful application, in permitting the weight of an engine to be distributed upon six wheels, without making the engine more embarrassing than if it had only four, is due to Mr. J. Melling, of Liverpool, who, in this instance, made use of it in order to give the engine a much larger firebox, and, consequently, the power of generating a greater quantity of steam.

We have now expressed the adhesion, by giving the measure of its effects; but the power itself may be expressed in a direct manner. The load of 244 t. produced a resistance, or required a traction of 1,952 lbs.; the adhesion was thus equal at least to 1,952 lbs., else the wheel would have turned without advancing. Now the adhering weight was 5.5 t., or expressed in pounds 12,320 lbs. ; we see then that the force of adhesion was equal to about of the adhering weight. Considering that

every

8 lbs. force corresponds with the traction of a ton on a level, this expression is exactly similar to the first.

In winter when the rails are greasy and dirty, in consequence of damp weather, the adhesion diminishes considerably. However, except in very extraordinary circumstances, the engines are always able to draw a load of 15 waggons, or 75 t., tender included, that is to say, 14 times their adhering weight. In other words, the resistance of 75 t. being 600 lbs., the force of adhesion is always at least of the adhering weight.

Adhesion being indispensable to the creation of a progressive motion, two conditions are necessary in order that an engine may draw a given load. 1st. That the dimensions and proportions of the engine and its boiler enable it to produce on the piston, by means of the steam, the necessary pressure, which constitutes what is properly termed the power of the engine; and, 2nd, that the weight of the engine be such as to give a sufficient adhesion to the wheel on the rail. These two conditions of power and weight must be in concordance with each other; for, if there is a great power of steam and little adhesion, the latter will limit the effect of the engine, and there will be steam lost; if, on the other hand, there is too much weight for the steam, that weight will be an useless burthen, the limit of load being in that case marked by the steam.