AQUEOUS ATMOSPHERIC PHENOMENA. 



465 



cases this is the general rule. It sometimes survives throughout the entire day, or 

 maintains a successful conflict with the solar beams to an advanced period of the morning, 

 accumulating first in heaps, then separating from the earth, and losing its continuity, 



/x before retiring from the field. The 



/ effect is striking, when from an 

 eminence which commands a view of 

 an extensive plain or valley, we see 

 this gossamer curtain of the night 

 resting upon the surface, gradually 

 rent and torn by the action of the 

 sun's rays, reflecting as it disappears 

 their golden hue. Many of the most 

 felicitous images of poetry are derived 

 from this source, as in Ossian : 

 " The soul of Nathos was sad, like 

 the sun in a day of mist, when his 

 face looks watery and dim ; " and 

 again, when two contending factions 

 are silenced by Cathmor : " They 

 sunk from the king on either side, 

 like two columns of morning mist, when the sun rises between them on the glittering 

 rocks." 



The stratus is occasionally seen under peculiar and striking circumstances, extending 

 over the surface of a sheet of water, without passing the boundary of its banks. Thus a 

 lake or river will exhibit a white cloud of visible vapour resting upon it, from which the 

 adjacent land is perfectly free. When in the neighbourhood of Loch Achray, well known 

 to the readers of Scott, 



" The minstrel came once more to view 

 The eastern ridge of Ben-venue, 

 For, ere he parted, he would say 

 Farewell to lovely Loch Achray 

 Where shall he find, in foreign land, 

 So lone a lake, so sweet a strand ? " 



at the close of a calm and warm September day, the expanse was nearly covered with a 

 beautiful stratum of mist, while the atmosphere of its borders presented no trace of visible 

 vapour. Mr. Harvey repeatedly observed a similar cloud hovering over the stream which 

 supplies Plymouth with water, whose boundaries on a calm night would exactly coincide 

 with the banks of the stream, however winding and irregular its outline. Sir Humphrey 

 Davy thus explains this curious phenomenon : " All persons who have been accustomed 

 to the observation of nature, must have frequently witnessed the formation of mists over 

 the beds of rivers and lakes in calm and clear weather after sunset ; and whoever has 

 considered these phenomena in relation to the radiation and communication of heat and 

 the nature of vapour, since the publications of MM. Rumford, Leslie, Dalton, and Wells, can 

 hardly have failed to discover the true cause of them. As, however, I am not aware that 

 any work has yet been published in which this cause is fully discussed, and as it involves 

 rather complicated principles, I shall make no apology for offering a few remarks on the 

 subject to the Royal Society. As soon as the sun has disappeared from any part of the 

 globe, the surface begins to lose heat by radiation, and in greater proportions as the sky 

 is clearer ; but the land and water are cooled by this operation in a very different manner : 



the impression of cooling on the land is limited to the surface, and very slowly transmitted 



H H 



