150 FLORIDA GEOLOGICAL SURVEY SIXTH ANNUAL REPORT. 



water at the bottom of the well is free the in-take of water is 

 necessarily limited. 



Assuming free movement of the water at the bottom of the well, 

 the rapidity of in-take and hence the efficiency of the well is in- 

 fluenced by (a) size of well; (b) construction of well; (c) depth 

 of water above the mouth of the pipe; (d) distance from the top 

 of the pipe to the underground water level.* 



(a) The capacity of a drain pipe increases rapidly with in- 

 creased diameter. The area of the section of the pipe is propor- 

 tionate to the square of the diameter. Thus the area of the cross 

 section of a 12-inch well is nine times that of a 4-inch well. More- 

 over, for a given velocity the friction of movement is less in a large 

 than in a small pipe. 



(b) The construction of a well also affects its rapidity of in- 

 take. When the pipe is cut off squarely at the top according to the 

 usual custom, the full capacity of the well is not realized. The 

 rapidity of in-take may be appreciably increased by the use of a 

 flared or bell-shaped mouth at the top of the pipe. 



(c) If the underground water level lies some distance from 

 the surface and if there is free discharge at the bottom of the well, 

 siphonage or draft-tube action increases the rate of flow. When the 

 distance from the top of the pipe to the underground water level is 

 37, feet or over, the maximum possible draft-tube head of 32.8 feet 

 may be available. 



(d) The influence of the depth of water above the mouth 

 cf the pipe is as follows : Assuming that the water flows into the 

 pipe as through an orifice, the in-take at the mouth of the pipe will 

 be proportionate to the square root of the depth of the water above 

 the mouth of the pipe. 



The velocity of flow in the drainage well may be measured by 

 means of Pitot's tube. This is a bent tube one arm of which is 

 graduated, used to determine the velocity of running water. To 

 make the measurement insert the tube vertically in the top of the 

 pipe, the short end projecting upward and having its mouth a few 

 inches below the top of the drain pipe. The velocity of flow in the 

 pipe is expressed within close limits by the following formula in 

 which h is the height in inches to which the water rises in the long 

 arm above the surface of the lake.* 



v= V^^W = '■'' V' 



*U. S. Geol. Surv. Water Supply Paper, 145, p. 36, 1905. R. E. Horton. 



