In some cases the wheel is turned by the weight of animals placed at its 

 circumference, who move forward as fast as the wheel descends, so as to main- 



Fig. 12. 



Fig. 13. 



tain their position continually at the extremity of the horizontal diameter. The 

 treadmill, fig. 13, and certain cranes, such as fig. 14, are examples of this. 



Fig. 14. 



In water-wheels, the power is the weight of water contained in buckets at 

 the circumference, as in fig. 15, which is called an overshot wheel ; and some- 

 times the impulse of water against float-boards at the circumference, as in the 



Fig. is. 



Fig. 17. 



undershot wheel, fig. 16. Both these principles act in the breast- wheel, 

 fig. 17. 



In the paddle-wheel of a steamboat, the power is the resistance which the 

 water offers to the motion of the paddle-boards. 



In windmills, the power is the force of the wind acting on various parts of 

 the arms, and may be considered as different powers simultaneously acting on 

 different wheels having the same axle. 



In most cases in which the wheel and axle is used, the action of the powei 

 is liable to occasional suspension or intermission, in which case some contri- 

 vance is necessary to prevent the recoil of the weight. A ratchet-wheel, R, 

 fig. 10, is provided for this purpose, which is a contrivance which permits the 

 wheel to turn in one direction ; but a catch which falls between the teeth of a 

 fixed wheel prevents its motion in the other direction. The effect of the power 

 or weight is sometimes transmitted to the wheel or axle by means of a straight 



