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 evening in the settling smoke from neighboring 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 innumerable 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 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 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,[1] 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 waves—plan for determining.—Commodore Wüllerstorf, 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 important results of that admirably conducted expedition. The
- ↑ See Maury's Sailing Directions, vol. ii., "Average Force of the Trade-winds."
