Mr. Harvey on the Formation of Mists. 61 



and for about 300 yards across, the air reposing on the water, 

 preserved the same temperature. Towards the middle of the 

 river, however, the temperature of the air was only 41° ; but on 

 approaching the western shore, it was found gradually to in- 

 crease to 43°. This depression of temperature in the middle 

 of the mist, most strikingly accords with the view Sir Humphry 

 Davy has taken of the increase of mists after their first forma- 

 tion ; — and which he accounts for by supposing, that the in- 

 crease depends not only upon the constant operation of the 

 cause which originally produced them, but likewise upon the 

 radiation of heat from the superficial particles of water com- 

 posing 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. The temperature of the river was 53°, both 

 near its shores, and in the middle. 



The land beyond the western side of the river, is hilly and 

 unequal ; and accordingly the temperature of the air was found 

 to vary from 43° to 39'J°. The air in the fields close to the 

 river was 42°; on higher land it amounted to 43°, and in the 

 valleys and lower grounds, it varied from 41° to 39^°. 



At a quarter past nine, the mist still continued, and so dense, 

 as totally to obscure the sun. The temperature of a rivulet 

 was found to be 51°, being two degrees colder than the water 

 of the river ; and the air above it 40°, also two degrees colder 

 than the medium temperatura of the air reposing on the Tamer. 

 At the same moment, the temperature of a meadow was found 

 to be 44°, and of a ploughed field 46°. At half-past nine the 

 mist suddenly disclosed the sun, when the air above the same 

 meadow was found to be 42°, and the green soil 46°. At 

 noon, the mist had disappeared, and the temperature of the 

 air, both over the land and sea, was 55°, the river preserving 

 the temperature of 53°, the same as early in the morning. 



During the afternoon of the 10th of June, a dense mist had 

 formed, which covered the beautiful hill of Mount Edgcumbe, 

 and also completely concealed from view, the Breakwater, the 

 ships in the Sound, and Hamoaze. Circumstances prevented 



