262 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



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 con- 

 ditions favom^able for radiation on occasions when the air in which 

 they are floating happens to have a high devf-point. Thus each one 

 of these innumerable little atoms of smoke and microscopic particles 

 of sea-dust become loaded with dew, and being made visible, have 

 the appearance of fog. Eed fogs, therefore, do not properly come 

 imder our classification of sea fogs. 



506. On the polar side of 40^ at sea the weather is for the most 

 Cloudless retrions and part cloudy. Ou tlic equatorial side, and especially 

 height of clouds at sea. ^^il-l^jj^ the"^ tradc-Aviud 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 j;?(X?ie it can scarce be called. As 

 yet our sailor observers have not turned their attention either to 

 the height or the velocity of clouds. It is to be hoped that they 

 will. Observations here are to be made rather under the direction 

 of the commander of a fleet or squadi'on than of a single ship, and 

 it is hoped that some of the distinguished admirals and brave old 

 commodores who cruise about the vv'orld, with veiling 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. Commodore Wiillerstorf, of the Austrian frigate No vara, 

 Slvavct-"^ kin"fo*^ made an interesting series of observations upon the 

 determining. height and vclocity of the waves dming his cruise 

 in that vessel upon his last scientific mission. These no doubt 

 will be published with the other important results of that admira- 

 hlj conducted expedition. The most simple plan for determining 

 the velocity of waves, and it may be homdy practised on board of 

 every vessel, is the plan which is followed by Captain Grinn, of the 

 American ship John Knox, one of our co-operators. When he 

 heaves the log with the seas following, instead of hauling in the line 

 immediately, he leaves the chip to tow, watching till he observes it 

 on the crest of a wave ; he then tm-ns the glass, or notes his watch, 

 and marks the time it takes the wave to reach the ship. The usual 



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



