HYDRAULICS. 7 1 



under-shot, or over-shot accordingly as the water passes horizontally over 

 the floats, or acts beneath them. These wheels are used in relation to the 

 fall of water. If there is plenty of water and a slight fall, the under-shot 

 wheel is used. If there is a good fall less water will suffice, as the weight 

 and momentum of the falling liquid upon the paddles will turn the wheel. 

 Here is the Persian water-wheel, used for irrigation (fig. 75). The Archime- 

 dian Screw, called after its inventor, was one of the earliest modes of raising 

 water. It consists of a cylinder somewhat inclined, and a tube bent like a 

 screw within it. By turning the handle of the screw the water is drawn up 

 and flows out from the top. 



Fig- 75- Irrigation wheel in Egypt. 



The Water Ram is a machine used for raising water to a great height 

 "by means of the momentum of falling water. 



The Hydraulic Lift is familiar to us all, as it acts in our hotels, and 

 we need only mention these appliances here ; full descriptions will be found 

 in Cyclopaedias. 



We have by no means exhausted the subject of Water in this chapter. 

 Far from it. But when we come to Chemistry and Physical Geography we 

 shall have more to tell, and our remarks as to the application of science to 

 Domestic Economy, in accordance with our plan, will also lead us up to 

 some of the uses of water. So for the present we will take our leave of 

 water in a liquid form, and meet it again under the application of Heat, 

 -which subject will take us to Ice and Steam, two very different conditions 

 of water. 



