The overshot wheel is by far the most powerful; both because it receives the water at the very commencement of de scent, and that the buckets with which this kind of wheel is ordinarily furnished retain the power until they gradually dis charge their contents, as these buckets successively become inferior parts of the circumference. It should be stated in this place, that muck may be effected by al lowing the water merely to,flow upon the upper part of the wheel into the superior buckets, whereby an immense auxiliary power is erected as they successively be come filled. Add to this the discovery made by Mr, Smeaton, that " the more slqwly any body descends by the force of gravity, while acting upon any piece of machinery, the more of that force will be spent upon it, and consequently the effect will be the greater." That effect is by no means increased in proportion to the ve locity of the wheel's motion ; on the con trary, Mr. Smeaton found, that when his wheel, which was two feet in diameter, re volved 20 times in a minute, its effect was greatest : when it made only 18} turns the effect was irregular ; and when so laden as not to make 18 turns, the wheel was overpowered by the load. He found that 30 turns in the minute occasioned a loss of about and that when turned 30 times in a minute, the diminution of ef fect was nearly one fourth of its powers. This proportion may be easily" estimated on any wheel of greater extent, by com puting the proportion of accumulated power lost by greater velocity, than may be sufficient to load the wheels by means of the buckets being filled ; observing that the progress of a machine may be so much retarded as to cause the effect to be irrelevant of the purpose, although the machine may be kept in motion. Some machines do their work well, simply in consequence of a certain celerity, as is generally the case in a grinding apparatus: thus also every person conversant in the practice of agriculture is sensible, that when a plough is drawn at a certain pace, it will cut the soil regularly and freely, while, on the other hand, the same cattle proceeding at a very slow pace shall be more fatigued with doing less work, and that work by no means so neatly executed. All things considered, it will perhaps be found, that the great wheels of all ma chines ought to move at such a rate, as to cause their circumferences to pass over three feet in each second of time. We could instance several very large wheels, erected within the last five or six years, which scarcely make more than one re volution in the minute, but which ope rate so forcibly on the counter-wheels, as to give an astonishing degree of firmness as well as of regularity to their motions.
The maximum load for an overshot wheel is that which reduces the circum ference of the wheel to its proper veloci ty, which is known by dividing the effect it ought to produce in a given time by the space intended to be described by the circumference of the wheel in the same time. The quotient will be the resist ance overcome at the circumference of the wheel ; it is equal to the load requir ed, including the friction and the resist ance of the machinery. So much, how ever, depends on the proper precautions for reducing the friction of the several moving parts, that too much stress cannot be laid on that highly important consider ation. We therefore solicit those readers,
who may wish to render themselves con versant in this branch of science, and es pecially if practical knowledge is in view, to refer to the article FRICTION, where they will find many very necessary points treated of, with as much atten tion to their interests as our limits could allow.
We may, in theory, suppose a wheel to be capable of overcoming any resistance whatever; yet we always find, in prac tice, that the wheel stops, or at least is incapable of progressive motion, when the opposition or load is equal to the sum of the water contained in all the buckets. In this we speak of overshot wheels, which designation includes all that carry the water with them in their descent, and do not depend so much on its velocity as its weight : henee many kinds of breast wheels, which are constructed according to the above plan, are by many persons classed with overshots; the latter, how ever, strictly speaking, applies solely to such as receive their impulse somewhere above their centres ; generally indeed at their summits. The breast-wheel, when well constructed, may carry an effect equal to half, or even to three-fifths, of the power; while the overshot wheel or dinarily works with a result equal to four fifths of the momentum : but Mr. Smeaton thinks the (owing to a want of exact levels, and of a due fitting and squaring of the parts, together with an in attention to the removal of friction,) do not perform work beyond half the power. Many attempt too great velocity, which, as already shown, produces considerable diminution of power. Mons. Parent, whose principles were considered by Desaguliers and Maclaurin to be perfect ly correct, considered that the wheel should move with about Iths the velocity of the water, the ratio combining the es sential points of receiving the full force of the stream, and enabling the engineer to regulate the interior or dependent parts in such a manner as might answer their intended purposes, and give per fect effect to the whole. As to the velo city of the stream, that cannot always be made to equal our wishes, on account of the scanty supplies in the many instances, where greater falls, or more impetuous force would prove highly valuable. It is, however, generally in our power to di minish the velocity by means of sluices, overflows, &c. so as to carry off any re dundancy, and to limit the power within the bounds of safety and utility. But we trust it has already been partially shown, that, by confining a stream within more narrow bounds than its natural banks may afford, the velocity may be considerably increased ; and we presume it must have been already understood, that by giving additional height to the fall, or head, whence the water flows upon the wheel, velocity, or at least power, may be great ly augmented.
While on this part of our subject, it may be proper to state, that it is in al most every instance strongly advisable to form a large, reservoir, and to uphold a sufficient quantity of water, by means of a dam, &c. to afford a supply in case of long-continued drought. Such an excess can rarely prove inconvenient ; the only cases in which it might perhaps not be eligible are, where the supply may be considered as infallible, or the expense prove too great a drawback on the pro fits of the concern.