12 METEOROLOGY. 



the void created by the ascent. On the contrary, the atmosphere of 

 Tapour raised at the ecpiator has a continual tendency to proceed to 

 the poles in the lower strata of the atmosphere : the air tends from 

 the colder to the warnier parts ; the vaj)our from the warmer to the 

 colder. From this circinnstance alone it will be seen that the solution 

 of their actual effects in any locality becomes very difficult, dependent 

 on the predominance of one current or the other, and which currents 

 themselves are modified by the varied distribution of land and water 

 and many other causes. 



In the aggregate such a balanced provision exists, that no excess 

 of min or of drought incompatible with the requirements of animal 

 and vegetable life is possible. This, however, is entirely dependent 

 on the sun's action in producing heat varied according to the 

 different circumstances of the earth's surface afterwards to be noticed : 

 it must be borne in mind that the important elements of weather, 

 clouds, mists, fogs, rain, hail, and snow, result from the sole operation 

 of the two principles before stated viz. -condensation and evaporation : 

 again, the effect of the presence or absence of the sun,, occasioning 

 heat or cold is through the media of absorption and radiation. A 

 reference to actual phenomena will illustrate this : — during the winter 

 months in this country, a rapid fall of the thennometer, or cold 

 weather, succeeds a short continuance of a clear and still atmosphere, 

 a very few days of such a state bringing on a severe frost ; this arises 

 from the unimpeded radiation of previously absorbed heat from the 

 earth's surface into open space. In summer, on the contrary, absorption 

 predominates over radiation, and a consequent higher temperature 

 results. It may be here noticed as a peculiarity of the climate of 

 England, that unless with the continuance of a North or East wind, 

 no frost lasts beyond a few days without intervals of thaw : this arises 

 from our insular position ; the ocean which surrounds us preserves a 

 greater uniformity of temperature than large continents, its smface in 

 summer absorbs less heat and is cooler, and in winter radiates less 

 and is warmer, the capacities of the two surfaces for absorption and 

 radiation, and indeed, the mode of the action of the sun's heat on the 

 two being different : thus at sea the daily range of the thermometer is 

 rarely more than 4° to 6°, whereas, on the continent of Europe it 

 often amounts to 20® to 30<> ; when, therefore, by rapid and con- 

 tinued radiation produced by a cloudless sky, the temperature of this 



