140 THE LIFE OF THE SALMON 
ture seems to play in the entrance of tributaries by 
salmon which have ascended a main river. This 
subject has been chiefly studied in the Tay, and no 
river in Scotland seems to offer better facilities, 
since the tributaries of the Tay are numerous, and 
are in several instances rivers of considerable size, 
which present varying physical conditions. Read- 
ings were taken in the main river at Perth—which 
is at the mouth, but at the top of tide reach—and 
at Grandtully about forty miles up stream. Above 
Grandtully the river Lyon enters, and in this 
readings were taken. Below Grandtully the river 
Tummel enters, carrying with it the water of the 
Perthshire Garry, and in both of these were readings 
also taken. Curves were plotted for each series of 
readings. Of the two from the main river it is 
noticeable that the curve from the mouth of the 
river is lower than that for the upper river. This 
is no doubt caused by the entrance of the large 
tributaries below Grandtully, the Tummel and Garry, 
and the Isla, but especially perhaps the Almond, the 
waters of which are found to be colder than the 
waters of the other Tay tributaries. The Lyon 
alone shows a curve of temperatures very similar to 
the Tay. All other tributaries for which readings 
are available are found to be colder. Loch Tay, 
from which the main river springs, is 144 miles long, 
and has a mean depth of 199 ft. and a maximum 
depth of 508 ft. The water which flows from this 
large loch is naturally much more uniform in tem- 
perature than is the water of rivers draining high 
surrounding land. A chart of curves showing 
