390 



HYDROGRAPHY. 



rest, over the whole surface, and the pressure would be equal to the 

 elastic force of vapor at its constant temperature. This pressure 

 would prevent the formation or increase of vapor. No precipitation 

 would take place, and the whole mass would be clear and transpa- 

 rent. If, however, the temperature should be changed, and become 

 unequal, and the increase to take place at the Equator, the tension of 

 vapor would be that of the minimum temperature ; but as the evapo- 

 ration would be in proportion, and go on continually, the condensa- 

 tion in like manner would be equally as rapid and as great, and the 

 vapor would flow from the Equator towards the Poles, where the con- 

 densation would take place, the heat be given out, and the precipita- 

 tion flow back to the Equator. 



Now, this cannot take place so freely where the vapor is combined 

 with the atmosphere, for it offers great resistance to the free passage 

 of vapor, which is compelled, as it were, to filter through the air ; 

 but, notwithstanding it is retarded, the interchange takes' place, and 

 evaporation and condensation goes on. 



The temperature of an aerial atmosphere diminishes much more 

 rapidly with increasing elevation than one of vapor ; as the relations 

 of air to heat under different pressures varies from that of vapor in 

 the higher regions, a new cause of precipitation and an increased 

 evaporation beneath would be created. If the earth were covered 

 with water, perpetual clouds would be experienced everywhere but on 

 the Equator. The earth's surface, however, is divided into one-fourth 

 land and three-fourths water ; and as the land supplies but a small 

 quantity of vapor, it produces a great change. Vapor tends to distri- 

 bute itself according to temperature and tension over the whole sur- 

 face. Vapor formed over the ocean will therefore press towards the 

 laYid, and towards the colder parallels, in order to obtain an equili- 

 brium. Now, if this vapor, in its progress, be influenced by the rotary 

 motion of the earth, it must distribute itself more freely and in greater 

 quantity to the west, in opposition to rotation ; or it will flow, so to 

 speak, to the west, without interruption ; or, which is the same thing, 

 the earth glides under it and leaves it behind. When it encounters 

 the land, if the temperature is sufficiently low, it will be condensed ; 

 but if it should be higher, then it will be expanded and pass over the 

 land until it meets with a change of temperature. It will then be 

 condensed and precipitated. This generally occurs when it comes in 

 contact with any high mountain range. 



