FLOW Ot WATER THROUGH PIPES. 1 33 



plying it for determining the size of main drains the 

 question arises how much water per acre should be 

 taken off the land in a given time Many engineers 

 who discuss this branch of the drainage subject pro- 

 pose some formula which is used for water supply or for 

 city sewerage and by it compute the size a drain should 

 be to carry off i inch or J inch of water in twenty- 

 four hours These results differ so widely from the 

 most approved practice in drainage that they are in dis- 

 repute among practical men and are not sustained by 

 the test of experience. The discrepancy is explained 

 by saying that drainage is not thorough as practised. 

 Be that as it may, observation and experiment demon- 

 strate quite conclusively that where thorough work is 

 practised, tile drains do not remove ^ inch of water 

 from the soil in twenty-four hours. While this standard 

 is about correct for open ditches, it does not apply to 

 ordinary tile drainage, except under special conditions. 



The results of experiments and observations in re- 

 cent years seem to sustain the practice of many careful 

 engineers who state that the removal of J inch depth 

 of water in twenty-fours hours by tile drains meets the 

 requirements of the average soil, and may be regarded 

 as a basis upon which to make computations for the size 

 of mains, subject to such changes as the special tract to 

 be drained may demand. The following are some of 

 the considerations which have a bearing upon this 

 question. 



A drained soil is a reservoir. The ideal drained 

 field is one in which water sinks downward through the 

 soil from the point where it falls, the surplus passing 

 into some drain near by and thence into the main. If 



