7{ FLORIDA GEOLOGICAL SURVEY-THIRD ANNUAL REPORT.
slight a limit is thereby placed on the effectiveness of the well. Unless the flow of water at the-bottom of the well is tree the in-take of water is necessarily limited.
Assuming free movement of the vater at the bottom of the well, the rapidity of in-take and hence the efficiency of the well is influenced 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 increased diameter. The area of the section of the pipe is proportionate 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. Moreover, 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 intake. 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 33 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 of 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 64.321
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*u.~ S. Geol. Surv. Water Supply Paper, I45, p. 36, 1905. R. E. IHortoni.