and the Storage of Water. 
23 
than that given in the table, the banks require protection by 
osiers, fascines, stone, or other means. 
Feet per 
Minute. 
jVIiles per 
Hour. 
Material. 
15 
017 
Soft clay. 
30 
0-34 
Fine sand. 
40 
0-45 
Coarse sand, and gravel as largo as peas, and clay. 
120 
1-36 
Gravel, 1 inch in diameter. 
200 
2-27 
Pebbles, 1| inch iu diameter. 
240 
2-72 
Heavy sliinglo. 
300 
3-40 
Soft rock. 
400 
4-54 
Eock and shingle. 
Under ordinary conditions the surface inclination of the water 
follows that of the bottom of the cut ; but it is not necessary 
for the motion of the water that any inclination should be given 
to the bed of the channel. In low flat countries, such as the 
Fens, it is not unusual to lay out the bed of the larger cuts or 
drains at a dead level. When floods come, the whole body of 
water is set in motion, and there can be no dispute that water 
running over water suffers less friction than when running over 
soil ; the drain serves also as a reservoir for water when the 
sluices are closed in summer. In the smaller drains a fall of 
from 4 to 6 inches in the mile is deemed sufficient. 
The ordinary velocity of streams passing through cultivated 
lands, where it naturally follows that the fall is not very great, 
is from three-quarters of a mile to 3 miles an hour. Mr. Beard- 
more records a velocity of 9 miles an hour in the River Lea, one 
of the tributaries of the Thames ;* but this was under very 
exceptional circumstances during the great flood of 1852, and 
such a rate is rarely attained except in mountain torrents. 
Where there is any considerable body and depth of water, a 
stream will continue in motion with a mean inclination on the 
surface as low as 2 inches per mile. The inclination of the 
larger rivers varies from 4 to 12 inches in the mile. The 
Tha mes varies from about 18 inches per mile from Lechlade to 
Teddington, to 1^ inch from Teddington to London Bridge, 
and rather more than half an inch from London to Yantlet 
Creek. t Du Buat considered that one-eighth of an inch per 
mile is the smallest possible rate of inclination that can be 
given to a canal to produce sensible motion.!]: 
In providing a system of arterial drainage, attention must be 
directed to :he fact that in making provision for the admission 
* " Fresh Water Floods," ' Trans. Instit. Civil Engineers,' vol. xxvii. 
t Renn.e's ' Report on Hydraulics.' 
X ' Canal and River Engineering.' By Stevenson. Black, 1872. 
