2l8 
MR. ROBERT GURNEY ON THE TIDES OF 
distinction must always be borne in mind. The tide, as 
generated in the Pacific Ocean, consists simply in the forma- 
tion of two waves, of small height, which follow the moon’s 
course round the earth without necessarily any horizontal 
movement of the water. But when the waves propagated 
from the primary waves enter our shallow seas their progress 
is impeded, their height is increased, and a current is induced. 
In the sea, wave and current are generally so closely associated 
that, for practical purposes, they may be treated as together 
forming “ the tide,” but in rivers wave and current may 
come to have such different periods that a distinction is very 
necessary. In the sea ebb and flow may be simultaneous 
with fall and rise, but in rivers this is never the case. 
In the sea, as has been said, the current, as a rule, ebbs 
when the water level falls and flows when it rises, but in 
a river the current has this peculiarity, that the flood current 
continues to run after the level has begun to fall, and the 
ebb current runs after it has begun to rise. The reason of 
this is that there is always a certain mean level of water in 
the river such that the water rises as much above it at high 
water as it falls below it at low water, and when the water 
rises above the mean the current will flow upstream, and 
whenever it falls below the mean the current will set down- 
stream. It follows that at low water the current will be 
downstream and will continue to be so until the level has 
risen to, or above, the mean, when the current will be reversed. 
But, as the flood current has to create its own head while 
the ebb has the natural stream behind it, the ebb current 
will always tend to run longer than the flood, and the 
difference will be the more pronounced according to the amount 
of water discharged by the river and the distance from its 
mouth. “ The downward stream, in fact, lasts longer than 
the upward one. The moments at which the currents change 
will differ in each river according to the depth, the rise and 
fall of the tide at the mouth, and the amount of water delivered 
by the river. An obvious consequence of this is that in rivers 
the tide rises quicker than it falls, so that a shorter time 
