MEASURING CANALS. 245 



found profitable to extend its length around the curve. 

 It will be so in cases where the soil of the yalley can be 

 brought under irrigation/ and where it is compact and 

 capable of retaining the water. Where these circum 

 stances are not present, it will be best to carry the water 

 across the depression by a wooden channel, supported up 

 on timbers, or by an inverted siphon resting upon the 

 surface and covered with a bank of earth, or buried 

 wholly beneath the surface. If an inverted siphon is 

 used, it must be remembered that the confined water 

 exerts considerable pressure, which must be provided for 

 by securely strengthening the tube. 



The capacity of the canal is an element which enters, 

 in an important degree, into the calculation as to its con 

 struction. To estimate the capacity of a stream of water 

 it is necessary to find the area of the cross section of the 

 stream in feet, and to multiply this by the velocity in 

 feet per second or minute. This should be reduced one- 





Fig. 129. PLAN OF MEASURING A CROSS-SECTION. 



fifth, to allow for the lesser velocity at the bottom and sides, 

 before explained. The result is the cubic feet of water 

 passing down the canal or river in the time indicated. A 

 cubic foot of water weighs 62 1 1, pounds, and measures 

 about 7 1 1 3 gallons. To find the cross section of a stream, 

 the figure formed by the surface of the bed and that of 

 the stream is taken and averaged, or reduced to determin 

 ed geometrical outline. Thus a stream one foot deep in 

 the center, four feet wide on the surface, two feet at the 

 bottom, with banks sloping at an angle of 45 degrees, 

 will have a cross section of three feet. This result is ob- 



