36 TRANSACTIONS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
own banks as it wandered, and spreading out that rich carpet of 
alluvial detritus which forms the basis of all fertility. 
VELOCITY. 
The Tay carries more water to the sea than any other river in the 
United Kingdom, and in velocity it yields only to the Spey. The 
velocity of the river varies to some extent according as it is in a 
normal or flooded state. There are very few data regarding the 
velocity of the Tay, but the following are copied from Stevenson’s 
“Canal and River Engineering” (3rd ed., 1886):— 
PER HOUR. 
Velocity of Tay at Perth, - - - 3*09 miles. 
,, Site of Old Bridge, 3-17 „ 
„ Willowgate, - - i '55 ^ 
,, Mugdrum, - - 2 to 2|- ,, 
VOLUME. 
The variation in the volume of the water is very considerable, as 
must be evident to any observer who has seen the river at periods both 
of winter floods and summer droughts. The average discharge for 
the year at Perth is 207,000 cubic feet per minute, but this may in¬ 
clude a variation of from 40,000 to 500,000 cubic feet per minute, so 
that the river may contain at one time 12J times more water than it 
does at another. 
RAINFALL. 
The Tay in its course from the mountains to the sea passes 
through a. series of regions exhibiting every variation of annual rain¬ 
fall which is possible within the limits of Scotland. Among the 
mountains of Argyllshire, in the neighbourhood of which it rises, the 
rainfall exceeds 80 inches. At Loch Tay the rainfall is reduced to 
between 60 and 40 inches. At the junction with the Garry 351 - 
inches are recorded, 277 at Perth, and finally 23 inches at Dundee. 
It will thus be observed that the cultivation of the land in the 
different regions varies inversely as the rainfall. The same cause 
which brings the heavy rainfall on the hills—namely, their steep 
slopes and great elevation—allows of the weathered rock-crust being 
washed away by the copious showers, so that soil forms only in ihe 
valley bottoms. The smaller rainfall in the Carse lands, where the 
soil is productive, is powerless to wash away the soil firmly spread 
out on the level land. 
UTILITY. 
The composition of the water in a river often plays an important 
part in its destiny, and the Tay affords a very clear illustration 
of this fact. It is well known that the banks of the Tay have 
