224 OSTRICH-FARMING IN SOUTH AFRICA. 



More dams are lost through the overflow being too 

 small than by any other cause. A man looks at the 

 rmi of water in the kloof, and makes his outlet 

 accordingly, quite forgetting that the water there has 

 got a straight flow, on an incline, and with an accumu- 

 lated velocity ; consequently, a very much larger 

 amount of water will pass in the same space than will 

 pass out of the overflow where the water is at rest, and 

 generally flows out on a level. The same mistake is 

 made where men talk of a spring, saying it would fill 

 an inch ])ipe ; whereas, a pipe lying level is one thing, 

 and a pipe inclining down, sa}^, to an angle of 45^, is 

 quite another thing. 



Another mistake that is commonly made is sup- 

 posing the strength of the embankment depends on the 

 distance the water is thrown back. This makes no 

 difference whatever : the pressure entirely depends upon 

 the depth. Thus, suppose an underground tank 10 ft. 

 deep : it will not make the slightest difference to the 

 strength required whether the tank is built 4 ft. 

 square or 20 ft. square. Or, in other words, the 

 w^eight of a column of water 1 in. square and 32 ft. 

 high is 15 lbs., so that the pressure on the bottom of 

 our tube will be 15 lbs. ; but if we take a tube a foot 

 square and 32 ft. high the pressure on any given 

 square inch will only be 15 lbs., whilst if we take a 



