466 



PHYSICAL GEOGRAPHY. 



to the interior ; whereas in water above 40 Fahrenheit, as soon as the upper stratum is 

 cooled, whether by radiation or evaporation, it sinks in the mass of fluid, and its place is 

 supplied by water from below, and till the temperature of the whole mass is "reduced 

 to nearly 40 Fahrenheit, the surface cannot be the coolest part. It follows, therefore, 

 that wherever water exists in considerable mass, and has a temperature nearly equal to 

 that of the land, or only a few degrees below it, and above 40 Fahrenheit at sunset, its 

 surface during the night, in calm and clear weather, will be warmer than that of the con- 

 tiguous land ; and the air above the land will necessarily be colder than that above the 

 water ; and when they both contain their due proportion of aqueous vapour, and the 

 situation of the ground is such as to permit the cold air from the land to mix with the 

 warmer air above the water, mist or fog will be the result." He thus accounts for the 

 formation of mists over water, by the difference in the rate of cooling, in the absence of 

 the sun in fluid and in solid bodies. The atmosphere reposing on the water continues 

 warmer after sunset on a clear night than the air of the adjoining land. It obtains also a 

 greater supply of moisture from its 'position over an aqueous surface, which is condensed 

 into visible vapour by the colder air of the land intermingling with it. On descending 



Durrenstein on the Danube. 



the Danube during the three nights of June 9th, 10th, and llth, 1818, Sir H. Davy 

 observed, that the mist regularly appeared over the river in the evening, when the tem- 

 perature of the air on its banks was from 3 to 6 lower than that of the stream, and that 

 it as regularly disappeared when the temperature of the air on the banks surpassed that 

 of the river. Below Passau, where the milky blue waters of the Inn, and the perfectly 

 pellucid Ilz join the green current of the Danube, he found their temperature and that of 

 the atmosphere on shore, with the appearance of the rivers, as follows, at six o'clock in 

 the; morning : 



Temp, of the Air 

 on the. Banks. 



54 



Temp, of the Rivers. 

 Danube, 62 

 Inn, 561 

 Ilz, 56 



State of the Rivers 



Thick fog on the whole breadth. 

 Slight mist. 

 Haziness. 



