322

LECTURE XXVII.

must be measured by the product of the quantity multiplied by the heightfrom which it descends: for example, a hogshead of water capable of descend-ing from a height of 10 feet, possesses the same power as 10 hogsheads des-cending from a height of one foot; and a cistern tilled to the height of 10feet above its orifice possesses 100 times as much power as the same cisternfilled to the height of one foot only.

When, therefore, the fall is sufficiently great, an overshot wheel is far pre-ferable to an undershot wheel, and where the tall is too small for an overshotwheel, it is most advisable to employ a breast wheel, which partakes ot itsproperties ; its tloatboards consisting of two portions meeting at an angle, soas to approach to the nature of buckets, and the water being also in some measureconfined within them by the assistance of a sweep or arched channel which fol-lows the curve of the wheel, without coming too nearly into contact with it,so as to produce unnecessary friction. When the circumstances do notadmit even of a breast wheel, we must be contented with an undershotwheel : it is recommended, for such a wheel, that the tloatboards be so placedas to be perpendicular to the surface of the water at the time that they riseout of it: that only one halt ot each should ever be below the surface, andthat from three to five should be immersed at once, according- to the mac-nitude of the wheel. Sometimes, however, it has been thought eligible toemploy a much smaller number: thus the water wheel which propels Mr.Symington’s steam boat has only six tloatboards in its whole circumference.(Plate XXII. Fig. 291, 292.)

Since the water escaping front an undershot wheel still retains a part ofits Velocity, it is obvious that this may be employed for turning a secondwheel, if it be desirable to preserve as much as possible of the force. In thiscase,by causing the first wheel to move with two thirds of the velocity of thestream, the whole effect of both will be one third greater than that of a singlewheel placed in the same stream ; but it must be considered that the expenseot the machinery will also be materially increased.

Consideiable enors have frequently been made by mathematicians andpractical mechanics in the estimation of the force of the wind or the wateron oblique surfaces; they have generally arisen from inattention to the distinc-