LEYELIXG IXSTEUMENTS. 215 



locity per second, winch would bo about three gallons per 

 second, or three hogsheads per minute. 



An open ditch, therefore, with smooth sides, conveying 

 a stream of this size, would carry off, in one hour, from 

 an acre of land, all the water which might fall by half an 

 inch of rain, during the wet season ; for half an inch of 

 rain would be one hundred and eighty hogsheads per 

 acre, which would pass off in one hour ; or it would sup- 

 ply in one hour, by the process of irrigation, as much 

 water as a heavy shower of half an inch. Where the 

 descent is greater, the increased quantity may be i-eadily 

 calculated by the rule given. The capacity of smooth- 

 sided underground channels may be determined in the 

 same way ; but if built of rough stones, great allowance 

 must be made, as they will retard the flow of water. 



In common practice, too, even with straight, open 

 ditches, the velocity will be much diminished by the 

 rough sides. 



LEVELING I^'STRUMENTS. 



The simplest mode of leveling^ or ascertaining the slope 

 for ditches, is to cut a few yards of the ditch, so that wa- 

 ter may stand in it, and then to set two sticks perpendicu- 

 larly, both rising to an equal height above the surface. 



I 



Simple Method cf Taldnrj Levels. 



The sticks should be measured at equal distances from 

 the top downward, and marked, and then pressed into 

 the earth, till the water reaches the mark. The level may 

 then be determined with much accuracy by " sighting " 

 over the tops of these sticks. Fig. 238 exhibits this ar- 

 rangement. The shorter the sticks, and the longer the 

 piece of water, the less will be the liability to ovror. 



