in particular Situations. 299 



visible vapour, at common temperatures, can lose any consider- 

 able quantity of heat by radiation ; and, if mist could be formed 

 from such a source, it must always be produced to a great extent 

 upon the ocean in calm weather during the night, particularly 

 under the line, and between the tropics, which the journals of 

 voyages sufficiently prove is not the case. I have myself had an 

 opportunity of making some observations which coincide with 

 this view. During a voyage to and from Pola, I passed the nights 

 of September 3, 5, and 6, off the coast of Istria^ there was very 

 little wind on either of the nights, and from sun-set till nearly 

 midnight it was perfectly calm in all of them. On the 3d it was 

 cloudy, and the lightning was perceived from a distant thunder- 

 storm, and the vessel was never far from the shore : but on the 

 5th and 6th the sky was perfectly clear, and the zodiacal light, 

 after sun-set, wonderfully distinct and brilliant, particularly on 

 the 5th, and we passed by the help of oars from two to eight miles 

 from the shore. The temperature of the sea at sun-set was 76° F. 

 on the 5th, 77° F. on the 6th, that of the atmosphere imme- 

 diately above it 78° F. and 79° F. On the 5th, at midnight, about 

 five miles from the shore, the temperature oftheseawas74°F.and 

 that of the atmosphere 75 " F., and on the 6th, at the same hour, 

 at about four miles from ihe shore, the temperature of the sea 

 was 73° F. and that of the atmosphere 75° F. There was not 

 the slightest appearance of mist on either of these nights on the 

 open sea, or at any distance from the land : but close under the 

 hills of Istria there was a slight line of haze visible before sun- 

 rise, which was thickest under the highest landj and as we ap- 

 proached at sun-rise, on the 7th, the mountains of the Friul, the 

 tops of those nearest to Trieste were seen rising out of a thick 

 white mist, which did not reach a quarter of a mile from the 

 shore. 



After mists have formed above rivers and lakes, their increase 

 seems not only to depend upon the constant operation of the cause 

 which originally produced them, but likewise upon the radiation 

 of heat from the superficial particles of water composing the mist; 

 which produces a descending current of cold air in the very body 

 of the mist, whilst the warm water continually sends up vapour: 

 it is to these circumstances that the phsenomena must be ascribed 

 of mists from a river or lake, sometimes arising considerably above 

 the surrounding hills. I have often witnessed this appearance 

 during the month of October, after very still and very clear nights, 

 in the Campagna of Rome above the Tiber, and on Monte Al- 

 bano over the lakes existing in the ancient craters of this extin- 

 guished volcano; and in one instance, on the 17th of October, 

 before sun-rise, tlierc not being a breath of wind, a dense white 



cloud 



