14 Dr Davy's Meteorolo(jical Observations. 



tlie frost on the 21st of March, owing to copious mild rain, 

 and the compaivatively little change of the temperature of the 

 Eotha itself, with the intention of reverting to the subject. 

 This I shall now very briefly do. It has been stated already, 

 that when the lower part of Rydal Lake was frozen over, 

 and the greater pox'tion of Grasmere, that the water which 

 flowed from under the ice forming the Rotha was about 40°, 

 whilst that which entered the Lake of Grasmere was 36°. 

 The day after the heavy fall of rain, I examined these lakes, 

 and the water entering and running out of them. The 

 flowing out water was 41° where coldest ; the flowing in 

 water about 48° ; and the surface water of each lake about 

 47°, though only the day before covered with thick ice ; 

 difi^erences of temperature in accordance with what might 

 be expected, keeping in mind the peculiar pi-operty of water 

 as to density in connection with temperature, owing to which, 

 when brought to 40°, it will sink and tend to run out, and if 

 of higher temperature, as was the rain-water in this instance, 

 to float and spread over the surface. In agreement with 

 this, streams not flowing from lakes should have their 

 temperature more suddenly raised by warm rains than rivers 

 from lakes, and be subject to greater vicissitudes of tempera- 

 ture, both which I have always found to be the case. May 

 it not be owing to these circumstances, that fish, especially 

 the trout, quits the lake and enters the river when flooded, 

 and returns to the lake as the river subsides, especially in 

 spring, when in good condition and highly fed, and at a dis- 

 tance from the spawning season ? I believe it will generally 

 be found, that the warmest rivers are the first which afi"ord 

 sport to the angler, and that in his choice he may often with 

 profit be guided by the thermometer. 



4. The appearance of dew is generally considered as con- 

 nected with, and dependent on, radiation, owing to which the 

 moisture in the air, existing in the state of transparent va- 

 pour, is precipitated on the grass, and other objects on which 

 the dew appears, these being colder than the air. Many 

 times I have observed what may be considered as an excep- 

 tion to this rule — the appearance of dew on the grass, when 

 a thermometer laid on it stood higher than when suspended 



