48 
On Water Supplies suited to 
= 2J square feet The float went a distance of 120 feet in one 
minute: therefore, the average speed was 120 X "8 = 96 feet 
per minute, and the volume of water passing down the brook is 
96 feet X 2^ square feet = 224 cubic feet per minute, or 1389 
gallons ; hence the supply of water would be ample for the pro- 
posed installation. 
But very commonly the stream is so irregular in its cross 
section and rate of fall, that a satisfactory test cannot be made 
in the manner just explained; it is best then to place a board 
across the stream so as to dam it completely, and to cut a 
rectangular notch in the top edge of the board sufficiently wide 
and deep to allow all the water to flow through as a cascade.* 
The notch should be about ten times as broad as it is deep, 
and the edges should be chamfered, on the down-stream side 
of the board, so that the thickness of the edges of the notch 
should not be more than ^-inch. Two or three feet up stream, 
from the notch, drive a peg into the bed of the stream near its 
margin, and set the peg by means of a level exactly level with 
the bottom of the notch ; then, when the water is flowing 
through, measure the depth of water over the top of the peg,^ 
and the discharge will be found by the help of the following 
table, which gives the quantity of water passing over a notch 
1 inch long for various depths : — 
Depth of water. 
Gallons per minate. 
per inch la length. 
i inch 
•944 
i „ 
1-734 
1 „ 
2-670 
li „ 
3-818 
u „ 
4-905 
li „ 
6-167 
2 „ 
7-52-2 
2i „ 
10-55 
3 „ 
13-87 
3i „ 
17-48 
4 „ 
21-36 
The gallons discharged per minute, multiplied by the length 
of the notch in inches, will give the volume of water passing 
down the stream per minute. 
Suppose that the observed depth was 2J inches, and the 
length of the notch 28 inches. The discharge for 1 inch long 
and 2^ inches deep by the table is 10-55 gallons per minute ; 
then the discharge would be = 10*55 gallons X 28 inches = 
295 gallons per minute, or sufficient for the contemplated 
supply. 
* An illustration of this test is given in toI. siv. of the ' Journal,' page 53. 
