Water-wheels, next to steam-engines, are the most common motive agents. For centuries water-wheels remained without much improvement or change down to the period of turbine wheels, when it was discovered that instead of being a very simple matter, the science of hydraulics and water-wheels involved some very intricate conditions, giving rise to many problems of scientific interest, that in the end have produced the class known as turbine wheels.
A modern turbine water-wheel, one of the best construction, operating under favourable conditions, gives a percentage of the power of the water which, after deducting the friction of the wheel, almost reaches the theoretical coefficient or equals the gravity of the water; it may therefore be assumed that there will in the future be but little improvement made in such water-wheels except in the way of simplifying and cheapening their construction. There is, in fact, no other class of machines which seem to have reached the same state of improvement as water-wheels, nor any other class of machinery that is constructed with as much uniformity of design and arrangement, in different countries, and by different makers.
Water-wheels, or water-power, as a mechanical subject, is apparently quite disconnected with shop manipulation, but will serve as an example for conveying general ideas of force and motion, and, on these grounds, will warrant a more extended notice than the seeming connection with the general subject calls for.
In the remarks upon steam-engines it was explained that power is derived from heat, and that the water and the engine were both to be regarded as agents through which power was applied, and further, that power is always a product of heat. There is, perhaps, no problem in the whole range of mechanics more interesting than to trace the application of this principle in machinery; one that is not only interesting but instructive, and may suggest to the mind of an apprentice a course of investigation that will apply to many other matters connected [36] with power and mechanics.
Power derived from water by means of wheels is due to the gravity of the water in descending from a higher to a lower level; but the question arises, What has heat to do with this? If heat is the source of power, and power a product of heat, there must be a connection somewhere between heat and the descent of the water. Water, in descending from one level to another, can give out no more power than was consumed in raising it to the higher level, and this power employed to raise the water is found to be heat. Water is evaporated by heat of the sun, expanded until it is lighter than the atmosphere, rises through the air, and by condensation falls in the form of rain over the earth's surface; then drains into the ocean through streams and rivers, to again resume its round by another course of evaporation, giving out in its descent power that we turn to useful account by means of water-wheels. This principle of evaporation is continually going on; the fall of rain is likewise quite constant, so that streams are maintained within a sufficient regularity to be available for operating machinery.
The analogy between steam-power and water-power is therefore quite complete. Water is in both cases the medium through which power is obtained; evaporation is also the leading principle in both, the main difference being that in the case of steam-power the force employed is directly from the expansion of water by heat, and in water-power the force is an indirect result of expansion of water by heat.
Every one remembers the classification of water-wheels met with in the older school-books on natural philosophy, where we are informed that there are three kinds of wheels, as there were "three kinds of levers"—namely, overshot, undershot, and breast wheels—with a brief notice of Barker's mill, which ran apparently without any sufficient cause for doing so. Without finding fault with the plan of describing water-power commonly adopted in elementary books, farther than to say that some explanation of the principles by which power is derived from the water would have been more useful, I will venture upon a different classification of water-wheels, more in accord with modern practice, but without reference to the special mechanism of the different wheels, except when unavoidable. Water-wheels can be divided into four general types.
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First. Gravity wheels, acting directly from the weight of the water which is loaded upon a wheel revolving in a vertical plane, the weight resting upon the descending side until the water has reached the lowest point, where it is discharged.
Second. Impact wheels, driven by the force of spouting water that expends its percussive force or momentum against the vanes tangental to the course of rotation, and at a right angle to the face of the vanes or floats.
Third. Reaction wheels, that are "enclosed," as it is termed, and filled with water, which is allowed to escape under pressure through tangental orifices, the propelling force being derived from the unbalanced pressure within the wheel, or from the reaction due to the weight and force of the water thrown off from the periphery.
Fourth. Pressure wheels, acting in every respect upon the principle of a rotary steam-engine, except in the differences that arise from operating with an elastic and a non-elastic fluid; the pressure of the water resting continually against the vanes and "abutment," without means of escape except by the rotation of the wheel.
To this classification may be added combination wheels, acting partly by the gravity and partly by the percussion force of the water, by impact combined with reaction, or by impact and maintained pressure.
Gravity, or "overshot" wheels, as they are called, for some reasons will seem to be the most effective, and capable of utilising the whole effect due to the gravity of the water; but in practice this is not the case, and it is only under peculiar conditions that wheels of this class are preferable to turbine wheels, and in no case will they give out a greater per cent. of power than turbine wheels of the best class. The reasons for this will be apparent by examining the conditions of their operation.
A gravity wheel must have a diameter equal to the fall of water, or, to use the technical name, the height of the head. The speed at the periphery of the wheel cannot well exceed sixteen feet per second without losing a part of the effect by the wheel anticipating or overrunning the water. This, from the large diameter of the wheels, produces a very slow axial speed, and a train of multiplying gearing becomes necessary in order to reach the speed required in most operations where power is applied. This train of gearing, besides being liable to wear [38]and accident, and costing usually a large amount as an investment, consumes a considerable part of the power by frictional resistance, especially when such gearing consists of tooth wheels. Gravity wheels, from their large size and their necessarily exposed situation, are subject to be frozen up in cold climates; and as the parts are liable to be first wet and then dry, or warm and cold by exposure to the air and the water alternately, the tendency to corrosion if constructed of iron, or to decay if of wood, is much greater than in submerged wheels. Gravity wheels, to realise the highest measure of effect from the water, require a diameter so gr............