22 



MECHANICS. 



of the wheel, so that the'water may be 

 retained between the float-boards and 

 the mill-course, and that it may act by 

 its weight until it has arrived nearly at 

 the lowest point of the wheel. 



(50.) An undershot-wheel is driven 

 exclusively by the momentum or moving 

 force of water, and is quite independent 

 of its weight. Like the breast- wheel it 

 is furnished with float-boards, against 

 which the water is delivered by a sloping 

 canal at the under part of the wheel, as 

 represented in fig. 23. 



fig. 23. 



The effect of such a wheel depends on 

 the quantity of water in the mill-course, 

 and the velocity with which it strikes 

 the float-boards. The velocity will de- 

 pend on the height of the fall, which 

 therefore should be as much increased 

 as the peculiar circumstances of the 

 situation will admit. 



Much difference of opinion has sub- 

 sisted among mechanical writers re- 

 specting the best number of float-boards 

 in undershot-wheels. Bossut shows, 

 that when the velocity of the wheel is 

 given, there is a certain number of float- 

 boards, which produces the greatest ef- 

 fect. 



fig. 24. 



The rule of Bossut, however, is not 

 simple enough to be of use to the prac- 

 tical mechanic. In general, it may be 

 observed that the wheel may be fur- 

 nished with as great a number of float- 

 boards as the strength of the rim to 

 which they are attached will conveni- 

 ently admit, care being taken at the same 

 time not to overload the wheel by their 

 weight. The injury arising from having 

 too limited a number of float-boards is 

 much greater than any which could 

 possibly arise from the opposite error. 



(51.) In estimating the power of a 

 stream on an undershot-wheel, it is 

 frequently necessary to measure the 

 velocity of the stream, and the quan- 

 tity of water which flows through its 

 bed. Various methods have been sug- 

 gested of measuring the velocity of 

 a stream. One of the most simple is 

 to stretch two strings across it, each 

 perpendicular to its course, and at as 

 great a distance one from the other 

 as may be found convenient. Let a 

 light floating body be flung into the 

 stream, above the higher string, and let 

 the moment of its passing under each 

 string be observed by a clock which 

 beats half or quarter seconds. The 

 time of the passage of the floating body 

 from the one string to the other, will 

 thus be obtained, and by measuring the 

 distance between the two strings, the 

 rate at which it was earned along by the 

 stream will be found, which will be the 

 velocity of the stream. 



A more accurate method of deter- 

 mining the velocity of a stream is, by a 

 small wheel, furnished with float-boards 

 as represented in fig. 24. This wheel is 



about one foot in diameter, and moves 

 upon a fine screw a b, passing through 

 its axle B b. When the float-boards are 

 impelled by the stream, the axle B b will 

 be turned upon the screw a b, and will 

 approach toward D, each revolution 

 moving it through one thread of the 



screw. An index o h is fixed to "the 

 movable axle at h, and when the wheel 

 commences its motion, the point of the 

 index is at on the fixed scale m a. As 

 the wheel moves towards D, each revo- 

 lution moves the index oh over one 

 division of the graduated scale, so that 



