212 IEBIGATION. 



and rather than submit to pressure, its particles may be 

 forced through the infinitely small pores of cast iron, if 

 the iron is strong enough to resist the enormous pressure 

 required. So many instances of this property of water 

 are supposed to be popularly known, that a statement 

 to the contrary not only misleads and confuses those who 

 'read it, but tends to cast doubt and suspicion upon what- 

 ever else the writer may say. 



To expect, therefore, that by the use of diverging 

 entrances to a canal, or by the use of a funnel-shaped 

 sluice, a larger quantity of water may be forced into a 

 channel, will be found fallacious, and will lead to disap- 

 pointment. A funnel-shaped box will pass no more water 

 through it, than can be passed through another with 

 straight sides, and of the same diameter as the narrow 

 throat of the funnel, unless the inclination is changed 

 and the velocity increased. This is an established prin- 

 ciple of hydraulics. Other principles of hydraulics, which 

 relate to the construction and use of dams, are, that the 

 pressure of water is equal in all directions ; that it is 

 exerted only in proportion to the hight and area of the 

 base of the column of water resting upon a given space ; 

 that water will always seek and maintain an exact level, 

 and that the disturbance of the level sets it into im- 

 mediate motion. 



The pressure of a body of water upon a perpendicular 

 wall, a dam, or any other obstacle to its flow, is exerted 

 to force it forwards in the direction of the stream. A 

 dam placed directly across the stream is, therefore, weak 

 and faulty. It will be rendered very much stronger by 

 being placed across the stream in a curved, or angular, 

 form, with its apex towards the resistance, and giving it 

 somewhat the shape, and consequent strength, of an arch. 

 The material of the dam should be selected for its im- 

 permeability to water, and for its more perfect capacity 

 for binding together, and resisting disintegration. There 



