152 L. Nickerson on the Periodic action of Water. 
Afterward, I watched this place for hours at a time, unlor 
tunately without timing, but yet with so distinct and definite’ 
I take it to be thus: a 
Ist. That a certain quantity of water arrives at the pool, and ‘ 
obvious. 
3d. That the decrement of velocity, and corresponding ince 
ment of section, is greater at a point nearly under the point of 
greatest depression of the curve of amplitude. 
4th. That outside of this stream the pool is made up of walét 
In a state of slow motion, at rest, or even in some cases of te 
k 
its changes, For this we take the formulx of permanent 
which, though not exact, is sufficiently characteristic, a8 
from Weisbach, 
when : 
‘ 1 = distance between a and 4%: 
sin @ = slope of original stream) a = depth of dam or known er 
P = whetted perimeter, ; E 
_ al = transverse section, Z = coefficient of resistance, 
v = velocity, g@ == $22, ora gravitation. 
The form of this curve is represented in the works of alm ie 
all hydraulic authors, and its equation shows it to be meee 
_tetic to the original surface. It is easily seen above eet 
sin.« of the original surface becomes equal to 2— 39 and the 
: ae: : : 
fore-equal to sin a’ of a transverse section of the pool ; a—a,0 
¢ 
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