CANAL CONSTRUCTION. 79 



may be lifted. In constru(5tion the gate is most simple, 

 any carpenter or farmer being able to build one. A 

 tight-fitting slide over the end of the box or pipe outlet 

 is all that is necessary to shut off the water. The gate 

 may be raised or lowered by a stick of 2 x 4 bolted to 

 the front of the gate and leading up through slides or 

 guide holes in the end of the walk. Simple means may 

 be provided for fastening the gate either up or down. 

 The pressure of the water against the gate will keep it 

 in position and preserve a tight joint if the sliding sur- 

 faces have been properly dressed or surfaced. Grooves 

 should be provided in the sliding supports so as to 

 make sure that the gate will return to its seat when it 

 is desired to lower it. Modifications of detail are many 

 and will suggest themselves to any one as the condi- 

 tions of the work or the setting may require. One of 

 these is a cast-iron lift gate working in an iron frame 

 with grooves, as seen in Fig. 17. 



Evaporation and Seepage.— Evaporation is 

 greatest during warm or windy weather, greater in 

 shallow than in deep water, and greater in running than 

 in still water. The evaporation of a canal during June, 

 July, and August will rarely exceed three to four inches 

 a day. During the remaining months the average will 

 be about one inch, making for the year from three to 

 five feet of loss by evaporation. To the loss in this 

 must be added the loss by seepage or filtration either 

 into the earth or through the banks. The amount of 

 seepage through the banks will depend not only upon 

 the charadler of the soil of which they are made, but also 

 upon the solidity with which they are thrown up. So 

 with seepage into the earth. If the soil is of soft loam, 



