22 On some Geological Agents. [Sess. 
pension. In mountain torrents we can sometimes hear the 
erinding action of the stones as they are rolled along, and 
we can also hear the dull thud as they are knocked against 
each other by the force of the current. 
The power of water to move heavy bodies depends upon 
its velocity, and the velocity on the volume of water and the 
angle of slope. Hopkins of Cambridge has stated it in this 
way: “The force exerted on a surface given in magnitude 
and position is found to increase with the square of the 
velocity; and if the force of the current be estimated by 
the weight of the largest block of a given form which it 
is capable of transporting, it is found that the force varies 
as the sixth power of the velocity of the current. Thus a 
current being able to move a cube of a given weight, another 
current of double the velocity would move a cube of 64 times 
the weight of the former; and if the velocity were trebled, 
that of the first case, the cube moved might be 729 times as 
great, and so on.’ Prestwich says, supplementary to the 
above: “In this manner a spherical block of 5 tons might 
be moved by a current of 10 miles an hour; a current of 
15 miles per hour would move a block of 56 tons, and a 
current of 20 miles per hour a block of 350 tons and 
upwards. 
In order to make the geological work of rivers—running 
water—more clear and definite, I will take as an instance 
the river Tay, the largest of our Scottish rivers, with the 
view of showing you by figures the mechanical work done 
by water. But in considering this work, we must always 
bear in mind that gravel and stones lose about one-third 
of their weight when immersed in water. The Tay drains 
an area of some 2400 square miles, and the rainfall in 
the western gathering-ground is heavy. It discharges about 
144,000,000,000 cubic feet of water annually, and the esti- 
mated discharge of sediment during the same period is 
49,660,000 cubic feet. Now the upper waters or feeders 
of the Tay have several settling-ponds to keep back the 
sediment brought down by these feeders: Loch Tay holds 
back all that is brought down by the Dochart and the Lochy ; 
Lochs Ericht, Rannoch, and Tummel all that comes from the 
north and west; and yet, with all these four settling-ponds as 
