THE FLOW OF WATER IN DRAIN TILE. 45 



the tile during the experiments indicated that there is additional 

 resistance to the flow caused by the action of the air on the water sur- 

 face in tile flowing partly full, to which may be due the decided decrease 

 in the velocity at the partial depths. Therefore it was decided to vary 

 the formula for concrete tile, 



17=51 #0.56 S 0.5 ( 59 ) 



A 

 by substituting ^p for D, thus, 



/ A \0-56 



and to determine the proper values of /for the various depths of flow. 

 In the formula 60, the values of V, s, A, and P from the experiments 

 (Ffrom Plate IX, the others from Table 4) were used, and the values 

 of/ were found to be as follows: 



Tile flowing Ml 1. 00 



Tile flowing 0.9 depth '. 1. 15 



Tile flowing 0.8 depth 1. 20 



Tile flowing 0.7 depth 1. 25 



Tile flowing 0.6 depth 1. 35 



Tile flowing 0.5 depth 1. 50 



The above discussion is given merely as a method of making allow- 

 ance for air resistance to flow. Further experiments are considered 

 necessary to establish the necessity of making such allowance. 



A comparison of the various f ormulse which have here been derived 

 may be made from Table 7. The formulae have been arranged sys- 

 tematically, so that variations may be noted at a glance. They have 

 been so classified that all involving the hydraulic radius are in one 

 column, while those involving the diameter are in another column. 

 The number opposite each formula refers to the corresponding equa- 

 tion in the text. 



