Mists are the vapor near the ground rendered visible by the temperature of the air falling below that of the vapor.  When we see our breath in a cold morning, we see a mist.  Where the surface is comparatively warm and damp, and the air is cooler, we have mists, which, if dense, are called fogs.  These are found plentifully on the banks of Newfoundland; and with icebergs on the one hand and the Gulf Stream on the other, we must always expect to have them.

The distribution of rain, which is one of the offices of the clouds, is another of the more important features of Meteorology.  The amount of water taken up by evaporation into the atmosphere is almost incredible.  It is calculated by Lieutenant Maury that there is annually taken up in the torrid zone a belt of water three thousand miles in breadth and sixteen feet deep.  Rain occurs regularly and irregularly in different parts of the earth.  In some places it may be calculated upon to a day; in others it is quite unknown.  Latitude and longitude may indicate the points of distribution, but the causes are dependent on temperature, winds, locality, and, what may seem a strange assertion, upon the conduct of man himself.  The greatest quantity falls near the equator, diminishing towards the poles.  Much more falls on islands and coasts than in the interior of continents,—­more in the region of the variables and less in that of the trades.  There are, however, tropical countries of great extent where rain is scarcely ever seen.

The influence of man upon rain is seen in the progress of civilization, the destruction of forests, and the drying-up of meres, swamps, and water-courses.

Forests undoubtedly affect the distribution of rain, and the supplies of streams and springs.  Their cooling influence precipitates the vapor passing over them, and the ground beneath them not getting heated does not readily evaporate moisture.  Lands, on the contrary, which are cleared of forests become sooner heated, give off larger quantities of rarefied air, and the passing clouds are borne away to localities of greater atmospheric density.

The Canary Islands, when first discovered, were thickly clothed with forests.  Since these have been destroyed, the climate has been dry.  In Fuerteventura the inhabitants are sometimes obliged to flee to other islands to avoid perishing from thirst.  Similar instances occur in the Cape Verdes.  Parts of Egypt, Syria, and Persia, that once were wooded, are now arid and sterile deserts.

In the temperate zones these results are not so immediately apparent.  It is now much in doubt whether the climate of our country has changed its character within the last two hundred years.  Jefferson and Dr. Rush both contended that it had.  Our oldest inhabitants assert that in their day our winters began nearly two months earlier than they do now.

The general laws laid down in relation to rain are these:—­

1.  It decreases in quantity as we approach the poles.