Reaction wheels are employed to a limited extent only, and will soon, no doubt, be extinct as a class of water-wheels. In speaking of reaction wheels, I will select what is called Barker's mill for an example, because of the familiarity with which it is known, although its construction is greatly at variance with modern reaction wheels.
There is a problem as to the principle of action in a Barker wheel, which although it may be very clear in a scientific sense, remains a puzzle to the minds of many who are well versed in mechanics, some contending that the power is directly from pressure, others that it is from the dynamic effect due to reaction. It is one of the problems so difficult to determine by ordinary standards, that it serves as a matter of endless debate between those who hold different views; and considering the advantage usually derived from such controversies, perhaps the best manner of disposing of the problem here is to state the two sides as clearly as possible, and leave the reader to determine for himself which he thinks right.
Presuming the vertical shaft and the horizontal arms of a Barker wheel to be filled with water under a head of sixteen feet, there would be a pressure of about seven pounds upon each superficial inch of surface within the cross arm, exerting an equal force in every direction. By opening an orifice at the sides of these arms equal to one inch of area, the pressure would at that point be relieved by the escape of the water, and the internal pressure be unbalanced to that extent. In other words, opposite this orifice, and on the other side of the arm, there would be a force of seven pounds, which being unbalanced, acts as a propelling power to drive the wheel.
This is one theory of the principle upon which the Barker wheel operates, which has been laid down in Vogdes' "Mensuration," and perhaps elsewhere. The other theory alluded to is that, direct action and reaction being equal, ponderable matter discharged tangentally from the periphery of a wheel must create a reactive force equal to the direct force with which the weight is thrown off. To state it more plainly, the spouting water that issues from the arm of a Barker wheel must react in the opposite course in proportion to its weight.
The two propositions may be consistent with each other or even identical, but there still remains an apparent difference.
The latter seems a plausible theory, and perhaps a correct one; but there are two facts in connection with the operation of reaction water-wheels which seem to controvert the latter and favour the first theory, namely, that reaction wheels in actual practice seldom utilise more than forty per cent. of useful effect from the water, and that their speed may exceed the initial velocity of the water. With this the subject is left as one for argument or investigation on the part of the reader.
Pressure wheels, like gravity wheels, should, from theoretical inference, be expected to give a high per cent. of power. The water resting with the whole of its weight against the vanes or abutments, and without chance of escape except by turning the wheel, seems to meet the conditions of realising the whole effect due to the gravity of the water, and such wheels would no doubt be economical if they had not to contend with certain mechanical difficulties that render them impracticable in most cases.
A pressure wheel, like a steam-engine, must include running contact between water-tight surfaces, and like a rotary steam-engine, this contact is between surfaces which move at different rates of speed in the same joint, so that the wear is unequal, and increases as the speed or the distance from the axis. When it is considered that the most careful workmanship has never produced rotary engines that would surmount these difficulties in working steam, it can hardly be expected they can be overcome in using water, which is not only liable to be filled with grit and sediment, but lacks the peculiar lubricating properties of steam. A rotary steam-engine is in effect the same as a pressure water-wheel, and the apprentice in studying one will fully understand the principles of the other.
(1.) What analogy may be found between steam and water power?—(2.) What is the derivation of the name turbine?—(3.) To what class of water-wheels is this name applicable?—(4.) How may water-wheels be classified?—(5.) Upon what principle does a reaction water-wheel operate?—(6.) Can ponderable weight and pressure be independently considered in the case?—(7.) Why cannot radial running joints be maintained in machines?—(8.) Describe the mechanism in common use for sustaining the weight of turbine wheels, and the thrust of propeller shafts.