300 On the Formation of Mists (ti particular Situations. 



cloud of a pyramidal form was seen on the site of Alban lake, and 

 rising far above the highest peak of the mountain, its form gra- 

 dually changed after sim-rise, its apex first disappeared, and its 

 body, as it were, melted away in the sun-beams. 



Where rivers rise from great sources in the interior of rocks or 

 strata, as they have the mean temperature of the climate, mists 

 can rarely form upon tliem except in winter, or late in autumn, 

 or early in spring. In passing across the Apennines, October 1st, 

 2d, and 3d, 181 S, there having been much rain for some davs 

 preceding, and the nights being very clear, I observed the beds 

 of all the rivers in the valleys filled with mist, morning and even- 

 ing, except tiiat of the Clitumnus near its source, in which there 

 was no mist, and this river rises at once from a lime-stone bed, 

 and when I examined it, at half past six o'clock A.M. October 3, 

 was 7y° lower than the atmosphere. 



Great dryness in the air, or a current of dry air passing across 

 a river, will prevent the formation of mist, even when the tem- 

 perature of the water is much higher than that of the atmosphere : 

 thus on the I4th of June, near Mautern, though the Danube at 

 five in the morning was 61° F. and the air only 54°, yet there 

 was no mist ; but a strong easterly wind blew, and from the ra- 

 pidity with which water evaporated it, it was evident that this 

 wind was in a state of extreme dryness. 



The Tiber has furnished me with a number of still more strik- 

 ing examples. October 13th, the night having been very clear, 

 on arriving at the Ponte Molle, at half-past six in the morning, 

 I found no mist on the river, yet the temperature of the air im- 

 mediately al)ove it was 48° F. and that of the river 56° F., a 

 strong north wind blew, which indicated, by the hygrometer, a 

 degree of dryness of 55", and this part of the river was exposed 

 to it ; but tlie valley above, where the river was sheltered from 

 the wind, was full of mist, and the mist in rising to the exposed 

 level might be seen, as it were, dissolving, presenting thin striae 

 which never reached above a certain elevation, and many of which 

 disappeared a few seconds after they rose. From the 13th to the 

 25th of October, during which time the tramontane or north wind 

 blew, I witnessed repeatedly the same phEenomenon; and in the 

 whole of this time there was only one morning when there was 

 no mist in the sheltered valleys, and the cause was perfectly ob- 

 vious ; the night had been very cloudy, and the thermometer, be- 

 fore sun-rise, indicated a difference of only one degree in the at- 

 mosphere below that of the river. 



It is not my intention to discuss the general subject of the de- 

 position of water from the atmosphere, in this paper ; but merely 

 to describe a local cause of considerable extent and variety in its 



modifications : 



