CHAPTER IV. 



HYDROSTATIC AND HYDRAULIC MACHINES. 



139. Definitions. There are several simple machines 

 whose action depends on the properties of air and water ; a 

 brief description of some of these machines will now be 

 given, sufficient to exhibit the principles involved in their 

 construction and use. 



Hitherto the energy exerted by means of a head of water 

 has been wholly employed in overcoming frictional resist- 

 ances, and in generating the velocity with which the water 

 is delivered at some given point. In the cases which we 

 have now to consider, only a fraction of the head is required 

 for these purposes; the remainder, therefore, becomes a 

 source of energy at the point of delivery by means of which 

 useful work may be done. 



Hydraulic energy may exist in three forms, according as 

 it is due to motion, elevation, or pressure. In the first two 

 cases the energy is inherent in the water itself, being a con- 

 sequence of its motion or position, as in the case of any 

 other heavy body. In the third it is due to the action of 

 gravity or some other force, sometimes on the water itself, 

 but oftener on other bodies ; the water then only transmits 

 the energy, and is not directly the source of it.* 



140. The Hydrostatic Bellows. This machine pre- 

 sents an illustration of the principle of the transmission of 

 fluid pressure (Art. 8). It consists of a cylinder CDEF 

 (Fig. 70), with its sides made of leather or other flexible 

 material, and a pipe ABF leading into it If water is 



* Cotterill's App. Mecte., p. 483, 



