LECTURES AND ESSAYS READ AT INSTITUTES. 



279 



TABLE. 



The following table I first computed from tables given by B. Latham, in 

 his work on sewers, in 1877. His table shows discharge in cubic feet per 

 minute, when full. So far as I know, it is the only table which gives the 

 acres drained by a given-sized tile on a given grade. The rainfall to be 

 removed is considered equal to one-half inch in depth each day of twenty-four 

 hours. The table has been used quite extensively by drainage engineers in 

 this State, and has been found to give good results. I have revised it, cor- 

 rected some errors and indicated, by omission of figures, what should be the 

 maximum and minimum grade for each size of tile. 



[Note.— Tile should not be laid to grades when numbers are replaced by a dash.] 



CONSTRUCTION OF A DITCH. 



In almost every instance this work will have to be begun at the outlet of the 

 drain, in order that water may not interfere with the work. If the soil will 

 stand for a long time — that is, until the excavation for the whole ditch is fin- 

 ished — it is advisable to commence to lay tile at the upper end of the ditch and 

 work towards the outlet. This method permits only clear water to enter the 

 drain. I have, however, never seen a case in which it was practicable, but I 

 have no doubt there are localities in which the soil will stand until the whole 

 ditch can be excavated. Good working under-drains, even in very flat land, 

 could be constructed without leveling under such circumstances; for, after the- 

 ditch is excavated for its whole length, irregularities in the bottom could be 

 detected by pouring in or allowing water to run through it. 



In nearly every case, however, in order to prevent caving, tile must be laid 

 as fast as the ditch is excavated, and in such a case even the test of running 

 water is worthless. Suppose, for instance, you are to build a drain 40 rods 

 long, in which the maximum fall is only two feet. The profile of almost all 

 these drains will show a rapid rise close to the outlet, and near the upper end 

 little or no rise; frequently the drain must cut through a bank or dam. Now, 

 in working by a water level, there are no means of ascertaining the amount of 

 rise per rod, and a ditcher is almost certain to rise too fast at first, and, in order 



