508 



EEPOKX— 1891. 



The temperature is taken about 8 feet from the bank, where the depth 

 of water varies from 4 feet to 12 feet or more. 



The Braan is a rapid mountain stream ; about half a mile above the 

 point at which observations are taken, it emerges from a deep narrow rocky 

 ravine well wooded and about one and a half mile long. Before its 

 junction with the Tay a mill- lead is cut from it to the Tay, and the 

 temperature is observed near the outlet of this artificial channel. 



Both sets of observations are shown in the same diagram. Curve XXVI. 

 The Braan is always a few degrees colder than the Tay and follows the 

 variations of air-temperature much more closely than does the larger 

 river. The Tay was almost always warmer than the air, and this was 

 particularly the case in the summer of 1889, when for more than a month 

 both rivers had a temperature over 60°, a temperature which the air did 

 not reach on any occasion as a weekly mean. It must, however, be 

 noticed that the early hour of observation would account for a low air- 

 temperature. The river-temperature runs much closer to the air- 

 temperature during the period of heating up than while cooling down. 



Curve XXVI.— Tay and Braan, Dunkeld. 8.15 A.M. 

 Air JKwer I'ay jtta/<.r iSraxui. 



