and the Storage of Water. 
57 
for domestic use, for irrigation, or for driving machinery, it is 
necessary to ascertain the yield of the stream or spring, and to 
measure the quantity of water available. For small springs the 
simplest plan is to let the water run into a vessel of known capa- 
city, and to note the time required to fill it. For streams an ap- 
proximate estimate may be formed by selecting a straight length 
of the channel, free from obstruction, which will give a fair re- 
presentation of the current generally; and, having measured the 
distance, to ascertain the time taken by a float to pass from one 
point to the other. The float should be so constructed as just to 
move on the surface of the water, Avith the greater part of its 
body below, so as not to be acted upon by the wind. A tuft of 
grass with a portion of the earth adhering to the roots will answer 
the purpose. This will give the surface-velocity in the centre 
of the stream, which, being greater than that at the sides and 
bottom, owing to the friction of the rubbing surface, must be 
reduced by 16 per cent., or by multiplying the number of feet 
per minute by '84, the product of which, multiplied by the 
sectional area of the stream, will give the discharge in cubic 
feet per minute ; multiplying this again by 6*23, will give 
the equivalent number of gallons. Where greater accuracy 
is required, a weir or dam should be made across the stream 
with planks and clay, and the depth of water measured either 
passing over the top of the weir or through a notch. In 
small streams a weir may be formed with a plank from half 
an inch to an inch in thickness, having a V-notch cut in its 
upper edge, the sides of the notch meeting at a right angle. 
The edges should be chamfered off, so as to leave as little 
thickness as possible in contact with the water, and the plank 
should be set perfectly horizontal with a spirit-level. At a 
distance above, sufficient to avoid the curvature of surface 
which the water assumes as it approaches the weir, a peg with 
a step cut in it requires to be driven into the stream, at a point 
accessible from the side, the step being exactly level with the 
bottom of the notch. This point may be ascertained either by a 
spirit-level or by means of the water as it gradually rises to the 
level of the notch. In small streams the distance of the peg from 
the weir may be from 3 to 4 feet. (See illustration. Fig. ] 1, p. 58.) 
After the water has settled down from the disturbance caused by 
placing the dam, the height of the water on the step of the peg 
must be measured in inches. The fifth power of the square-root 
of the height or head (/t) in inches, multiplied by 0'32, will 
give the cubic feet (D) passing through the notch every minute 
(D = 0'32 /('). For example, supposing the height of the 
water on the step of the peg is 4 inches, the quantity would be 
10^ cubic feet, or 63| gallons. Care must be taken that the 
