298 Some Observations on the Formation of Mists 



warmer and the river colder, the temperature of the one being 

 from 66 " F. to 75 ° F. during the day, and that of the river, where 

 I examined it, from 59" to 60° F. 



July 1 1th. I examined the temperature of the Raab near Ker- 

 mond in Hungary, at 7 o'clock P.M. and found it 65" F. that 

 of the atmosphere being 72° F. During the whole evening there 

 were some thin fleecy clouds in the western sky, which being 

 lighted up by the setting sun, greatly interfered with the cooling 

 by radiation from the earth, and at half past nine the thermome- 

 ter, in the atmosphere, was still 69 ° F. and at half past ten 67 ' F. 

 when there was not the slightest appearance of mist. In the 

 morning, before sun- rise, the temperature of the atmosphere on 

 the banks was 61 " F., that of the river 64° F., and now the bed 

 of the river was filled with a white thin mist, which entirely dis- 

 appeared half an hour after sun-rise. 



I made similar observations on the Save in Carniola, in the end 

 of August ; on the Izonzo in the Friul, in the middle of Septem- 

 ber ; on the Po near Ferrara, in the end of September ; and re- 

 peatedly on the Tiber and on the small lakes in the Campagna 

 of Rome in the beginning of October; and I have never in any in- 

 stance observed the formation of mist on a river or lake, when the 

 temperature of the water has been lower than that of the atmo- 

 sphere, even when the atmosphere was saturated with vapour. 



It might at first view be supposed, that whether the cooling 

 cause existed in the water or the land, the same consequences 

 ought to result ; but the peculiar properties of water, to which 

 I referred in the beginning of the paper, render this impossible. 

 Water in abstracting heat from the atmosphere becomes lighter, 

 and the warmer stratum rests on the surface, and its operation in 

 cooling the atmosphere is extremely slow : besides, the cooled 

 atmospheric stratum remains in contact with it, and water can- 

 not be" deposited from vapour, when that vapour is rising into an 

 atmosphere of a higher temperature than its own ; and the law 

 holds good, however great the difference of temperature. Thus, 

 August 26, at sun-set, the day after a heavy fall of rain, and when 

 the atmosphere was exceedingly moist, I ascertained the tem- 

 perature of theDrave near Spital inCarinthia, and though it was 

 14° F. below that of the air, yet the atmosphere above the river 

 was perfectly transparent. 



It may be imagined, that without any reference to the cooling 

 agencies of air from the land, mist may form upon rivers and lakes, 

 merely from the loss of heat by radiation from the air, or the va- 

 pour itself immediately above the water; and that the pheenome- 

 non is merely one of the formation of vapour, the source of heat 

 being in the water ; and its deposition, the source of cold, being 

 in the atmosphere j but it is extremely improbable, that air or in- 

 visible 



