PECULIAR FOG IN ICELAND. 163 



times I have found the smell of a cigar, used by a per- 

 son fully a quarter of a mile on, over the road at about 

 the same height as his mouth, nothing -being visible. In 

 these cases have we any thing to look to but the size of the 

 particles? They are so small that their resistance to the 

 atmosphere is diminished to its utmost, as the resistance 

 of the air is increased so much in proportion to the weight 

 that they cannot fall rapidly. 



We can, then, make clouds lie on the air without falling. 

 In some cases these artificial clouds rise ; this is no doubt 

 caused by upward currents, which must exist for various 

 reasons. One very common is that when sailing you come 

 to the end of the current of air or wind very suddenly ; it 

 goes neither to the right nor left. These currents make 

 long serpentine movements in the line of smoke, and abun- 

 dantly explain alternate calms and winds on one line, so 

 often observed by sailors. 



If, however, the particles of the water forming a cloud 

 are large, there is abundant tendency to fall observed, 

 as in other bodies. Dust from a chimney will fall down 

 on a deck at once ; but the smoke will float, as said. Let 

 the particles be large, and the fall will be rapid and 

 evident. If, as in the case of the fog observed at Reik- 

 javik, the particles are inordinately large, the fall is also 

 greater than usual; the water acts exactly as dust in 

 similar cases acts, with due respect to specific gravity 

 and other qualities. One of these is the fact that, unlike 

 dust, the fine vapour-particle will evaporate and become 

 transparent if heat is given. When, then, it falls a little, 

 in certain atmospheres it is resolved into gas, as we may 

 say ; but in this state it may rise and be again cooled, and 

 so on without intermission ; and this may be one of the 

 modes of keeping clouds pretty uniform in the air. It is, 

 however, to be observed that they are never long of the 

 same appearance; they are constantly changing; and no 



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