37 © Mr. mann’s Treatife 
3dly, Through unequal fedlions, in equal times, and 
with equal velocities, the quantities of water which pafs 
tfre in a direct ratio of their refpedtive fedtions. 
4thly, Through unequal fedtions, and with unequal 
velocities, the quantities of water which flow in equal 
times, are in a combined ratio of the fedtions and mean 
velocities (N° 8.) together. 
In a word, the fedtions of the bed, the mean veloci- 
ties, the times of flowing, and the quantity which flows, 
are univerfally in a combined ratio together; and this 
combination is what is called the momentum of a river ; 
and this momentum of the fame flowing water is univer- 
fally equal. 
19. From hence may eafily be deduced the principles 
for calculating the quantity of diminution of the water 
in a lake, pond, or veflel, by any determinate flowings 
whatfoever : for as the furface of the lake, See. is to the 
fedlion of the current which carries off the waters ; fo is 
the mean velocity of the current in this fedlion to the 
decreafe of the waters in the lake, 8ec. and vice verfa. 
III. On 
