522 LECTURE XXVII. 



must be measured by the product of the quantity multiplied by the height 

 from which it descends: for example, a hogsiiead 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 filled to the height of 10 

 feet above its orifice possesses 100 times as much power as the same cistern 

 filled 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 fall is too small for an overshot 

 wheel, it is most advisable to employ a breast wheel, which partakes of its 

 properties ; its floatboards consisting of two portions meeting at an angle, so 

 as to approach to the nature of buckets, and the water being also in some measure 

 confined 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 not 

 admit even of a breast wheel, we must be contented with an undershot 

 wheel : it is recommended, for such a wheel, that the floatboards be so placed 

 as to be perpendicular to the surface of the water at the time that they rise 

 out of it: that only one half of each should ever be below the surface, and 

 that from three to five should be immersed at once, according to the mag- 

 nitude of the wheel. Sometimes, however, it has been thought eligible to 

 employ a much smaller number : thus the water wheel which propels Mr. 

 Symington's steam boat has only six floatboards in its whole circumference. 

 (Plate XXII. Fig. 291, 292.) 



V 



Since the water escaping from an undershot wheel still retains a part of 

 its velocity, it is obvious that this may be employed for turning a second 

 wheel, if it be desirable to preserve as much as possible of the force. In this 

 case, by causing the first wheel to move with two thirds of the velocity of the 

 stream, the whole effect of both will l:>e one third greater than that of a single 

 wheel placed in the same stream ; but it must be considered that the expense 

 of the machinery will also be materially increased. 



Considerable errors have frequently been made by mathematicians and 

 practical mechanics in the estimation of the force of the wind or the water 

 on oblioue surfaces : they h^^ve generally arisen from inattention to the distinc- 



