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 14 J- 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 



