Water Mills

water mill

Water mill is the name by which all mills are designated that receive their motion from the impulse of the water.

As each of these mills will come under their respective heads, we shall, in the present article, confine ourselves to a minute description of the different kinds of water wheels, by whose axis the force with which they have been impressed may be transmitted to move any species of machinery, however simple or complex.

But, notwithstanding the extensive signification of the term water-mill when applied to the different branches of manufacture carried on therein, we have another, and still, more simple division, arising from the peculiar construction of the water-wheel, termed the undershot-mill, the overshot mill, and the breast-mill. There is also another called the mill with horizontal wheels ; but as this is very disadvantageous in point of practical utility, we shall forbear to describe it.

The undershot-wheel is used only in streams, and is acted upon by the water striking the float-boards at the lower circumference of the wheel. In the overshot-wheel the water is poured over the top of the wheel, and is received in buckets formed all round the wheel for that purpose.

And in the breast-wheel the water falls down upon the wheel at right angles to the float-boards, or buckets placed round the circumference of the wheel to receive it.

Smeaton

John Smeaton has made numerous experiments upon the different kinds of water-wheels, the results of which were laid before the Royal Society. The time that has elapsed since the period when they were first given to the world, has been sufficient to prove their fallacy if any had existed; and the high estimation in which they still continue to be held by mathematicians and mechanics, is certain evidence of their value and importance.

Mr. Smeaton prefaces a minute description of the machines and models used by him for his experiments, with an observation, that what he has to communicate on the subject was originally deduced from experiments, which he looks upon as the best means of obtaining the outlines in mechanical inquiry.

“But in such cases,” says he, ” it is very necessary to distinguish the circumstances in which a model differs from a machine in large; otherwise a model is more apt to lead us from the truth than towards it : and, indeed, though the utmost circumspection be used in this way, the best structure of machines cannot be fully ascertained but by making trials with them, when made of their proper size. It was for this reason, though the models and experiments referred to were made in the years 1752 and 1753, that I have deferred offering them to the Society until I had an opportunity of putting the deductions made therefrom in real practice, in a variety of cases, and for various purposes, so as to be able to assure the Society that / have found them to answer.”

Mr. Smeaton then remarks, that the word power, as used in practical mechanics, signifies the exertion of strength, gravitation, impulse, or pressure, so as to produce motion: and by means of strength, gravitation, impulse, or pressure, compounded with motion, to be capable of producing an effect; and that no effect is properly mechanical, but what requires such a kind of power to produce it.

Smeaton’s Observations

Having described the models and machines used for making his experiments, he observes that with regard to power, it is most properly measured by the raising of a weight, the relative height to which it can be raised in a given time being the actual extent; or, in other words, if the weight raised be multiplied by the height to which it can be raised in a given time, the product is the measure of the power raising it ; and, consequently, all those powers are equal, whose products, made by such multiplication, are the same: for if a power can raise twice the weight to the same height, or the same weight to twice the height, in the same time that another power can, the first power is double the second ; but if the power can only raise half the weight to double the height, or double the weight to half the height, in the same time that another can, those two powers are equal.

This, however, must be understood to be only in cases of slow and equable motion, where there is no acceleration or retardation.

In comparing the effects produced by water-wheels with the powers producing them, or, in other words, to know what part of the original power is necessarily lost in the application, we must previously know how much of the power is spent in overcoming the friction of the machinery and the resistance of the air; also, what is the real velocity of the water at the instant that it strikes the wheel, and the real quantity of water expended in a given time.

From the velocity of the water at the instant that it strikes the wheel, the height of head productive of such velocity can be deduced, from acknowledged and experimented principles of hydrostatics : so that by multiplying the quantity or weight of water really expended in a given time, by the height of a head so obtained, which must be considered as the height from which that weight of water had descended in such given time, we shall have a product equal to the original power of the water, and clear of all uncertainty that would arise from the friction of the water, in passing small apertures, and from all doubts arising from the different measure of spouting waters, assigned by different writers.

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