EVAPORATION. 



lion which succeeds a shower in summer, causes that grate- 

 ful coolness, which is so generally felt after such showers 

 Though the heat, by which vapor is raised, is given out 

 again, when that vapor is condensed into rain ; yet it is not 

 generally feJt by us, as it remains mostly in that region of 

 the air where the condensation causing the rain took place. 



A room may be made quite comfortable even in the 

 hottest weather, by keeping the floor wet. The water 

 will rapidly evaporate, and by evaporating absorb so much 

 heat, as to render the room comfortably cool. In India, 

 by an application of this principle, they even produce 

 such a degree of cold as to freeze water, though at the 

 same time the temperature of the air is several degrees 

 above the freezing point. Water is exposed to the air 

 during the night in large shallow dishes, and by its 

 evaporation carries off so much heat, that what remains 

 in the morning is found covered with a thin coat of ice. 



It is well known that islands and places near the ocean 

 are warmer in winter, and cooler in summer than others. 

 For an explanation of this fact, we must refer to the 

 principles of evaporation. In summer, much of the heat 

 is employed in converting the water of the ocean into 

 vapor, and consequently the air is cooled. In winter, 

 the ocean retaining a portion of the heat which it ab- 

 sorbed in the summer, and having parted with it less 

 rapidly than the land, gives it off by degrees, and thus 

 moderates the severity of the cold. The desert of Sa- 

 hara has long been distinguished for its wide extent of 

 barren and inhospitable sands. In the torrid zone, in 

 which this desert is principally situated, the wind blows 

 constantly from the east, or some point not far from east. 

 The vapors brought by it from the ocean are arrested by 

 the mountains of Abyssinia, and there descend in tor- 

 rents of rain, which go to swell the waters of the Nile 

 -and fertilize the fields of Egypt. As there are few or no 

 inland seas in Africa, there are no sources from which 

 vapor can be obtained to furnish rain for the desert. 

 Hence plants find no nourishment, and consequently 

 cease to grow, and the whole region becomes a desolate 

 and barren waste, except where some spring, fed by sub- 

 terranean streams or reservoirs.diffuse its vivifying influ- 

 ence around for a few rods, and supplies the plants which 



