272 PHYSICAL GEOGllAPHY OF THE SEA, AND ITS METEOROLOGY. 



of sea fogs. The falling of this dust in the form of fog is no 

 doubt owing to those influences (§ 331), the effects of which are 

 so often observable morning and ievening in the settling smoke 

 from neighbouring chimneys. The fogs which at early dawn 

 are discovered hovering over our cities or skirting the base of the 

 hills near by are of the same sort. The " black fogs " of London 

 may be taken as the type of them. These particles of dust, like 

 the atoms of smoke, are brought into conditions favourable for 

 radiation on occasions when the air in which they are floating 

 happens to have a high dew-point. Thus each one of these in- 

 numerable little atoms of smoke and microscopic particles of sea- 

 dust become loaded with dew, and, being made visible, have the 

 appearance of fog. Red fogs, therefore, do not properly come 

 under our classification of sea fogs. 



506. Cloudless regions and height of clouds at sea. — On the 

 polar side of 40^ at sea the weather is for the most part cloudy. 

 On the equatorial side, and especially within the trade-wind 

 region, it is for the most part clear until we approach the 

 cloud-ring, where clouds again indicate the normal state of the 

 sky at sea. What is the height of the cloud region at sea ? for 

 vapour plane it can scarce be called. As yet our sailor obsei-vers 

 have not turned their attention either to the height or the 

 velocity of clouds. It is to be hoped that they will. Observa- 

 tions here are to be made rather under the direction of the com- 

 mander of a fleet or squadron than of a single ship, and it is 

 hoped that some of the distinguished admirals and brave old 

 commodores who cruise about the world, with willing hearts and 

 ready hands for the cause we advocate, may signalize their flag 

 by contributing, for the advancement of human knowledge 

 touching the physics of the sea and the machinery of the air, a 

 series of well-conducted observations upon the force of the trade- 

 winds,* upon the height and velocity of the clouds, the height 

 and velocity of the waves, etc., in different parts of the ocean. 



507. Height and velocity of ivaves — ;2^?«n for determining. — 

 Commodore Wiillerstorf, of the Austrian frigate Novara, made an 

 interesting series of observations upon the height and velocity of 

 the waves during his cruise in that vessel upon his last scientific 

 mission. These, no doubt, will be published with the other im- 

 portant results of that admirably conducted expedition. The 



* See Maury's Sailing Directions, vol. ii., "Average Force of the Trade- 

 wind b." 



