312 THE CHEMISTRY OF CREATION. 



It is related by an observer, that if in a still 

 day the atmosphere of London, or some equally 

 great city, were to be carefully observed in calm 

 weather, it would be found that in the morning, 

 streams of fresh air are flowing in from the 

 country round about the metropolis down all 

 its suburban streets, to supply the place of the 

 current which, heated by the vast city, rises up 

 into the air from its centre. Prevailing cur- 

 rents or other accidental circumstances, so con- 

 stantly interfere with this phenomenon, as to 

 make it difficult to verify the observation. 

 A common fire-place furnishes us with an ex- 

 cellent home-illustration of the same laws. 

 A hot ascending current pours up through the 

 chimney, and to take its place a cold horizontal 

 stream sets from under the door, or from open- 

 ings in the floor made for its accommodation. 

 In tropical climates this law produces the inte- 

 resting and vastly -important natural phenomenon 

 of Land and Sea breezes. During the day, under 

 the powerful rays of a vertical sun, the land 

 becomes greatly heated, much more so than the 

 sea; an ascending current consequently arises 

 from the land, and to supply the place thus 

 partially vacated, a body of cool air flows in from 

 seaward, producing the sensible effect of a fresh 

 breeze blowing from the sea, from about nine 

 in the morning until two or three o'clock in the 



