492 METEOROLOGY. — RAIN. 



cooled during bright nights, and to judge from the influence which 

 forests exert in lowering the temperature of a country, it is enough 

 to recollect with M. de Hunboldt that by reason of the vast multi- 

 plicity of leaves, a tree, the crown of which does not present a hori- 

 zontal section of more than about 120 or 130 square feet, actually 

 influences the cooling of the atmosphere by an extent of surface 

 several thousand times more extensive than this section. 



The proportion of watery vapor which a gas will retain in solu- 

 tion is by so much the greater as the tempe^rature of the air is 

 higher. All the causes which cool air saturated with watery vapor 

 occasion, as we have already seen, the precipitation of a certain 

 quantity of moisture. 



When this condensation takes place in the midst of a gaseous 

 mass, the precipitated water collects into small floating vesicles, 

 which trouble the transparency of the medium that momentarily 

 holds them in suspension. Mists, fogs, and clouds are collections 

 of these vesicles ; a fog, as a celebrated naturalist said, is a cloud 

 in which one is, and a cloud is a fog in which one is not. 



The vesicles of clouds tend towards the earth, like all heavy bo- 

 dies, but by reason of their specific lightness the resistance of the 

 air which they displace lessens the rapidity of their descent. When 

 they are of larger size they coalesce and form drops of water which 

 fall with greater celerity. When these drops pass through strata 

 of very dry air they undergo partial evaporation, and this is the 

 reason wherefore there is sometimes less rain upon plains than upon 

 mountains. In opposite circumstances it is the inverse phenomenon 

 that is observed ; the drops increase in size in passing through the 

 inferior strata of an atmosphore saturated with moisture, condensing 

 vapor in their course. This is what happens most generally. 



In taking a survey of a large amount of observations, meteorolo- 

 gists have inferred that the annual quantity of rain varies with the 

 latitude ; that, greatest at the equator, it gradually lessens as higher 

 northern and southern latitudes are attained ; this is as much as 

 saying that the quantity of rain is greater as the temperature of the 

 climate is higher. But to this rule there are numerous exceptions ; 

 for instance, under the line at Payta on the sea-coast it only rains 

 very rarely ; a shower of rain is an event, and when I visited the 

 country eighteen years had elapsed since they had had any thing of 

 a fall of rain. Local causes have the greatest influence upon the 

 fall of rain, so that countries on the same parallel of latitude are far 

 from being equally distinguished by dryness or humidity. 



It is believed that in Europe it rains more heavily and more fre- 

 quently in the day than in the night. In the equinoctial regions, at 

 least in those parts that I have visited, it would seem that the op- 

 posite rule held good. Every one in South America allows that it 

 rains principally during the night, and the observations which I 

 made in the neighborhood of Marmato enable me to state that of 

 7.874 inches of rain which fell in the month of October, 1.336 inches 

 fell in the day, 5.638 inches in the night ; of 8.881 inches which 

 C?ll in the month of November, 0.707 inches came dow;j ir the day, 



