330 THE CHEMISTRY OF CREATION. 



undeservedly complain. During the prevalence 

 of cholera, this was 'actually noticed by the 

 meteorological observers at Greenwich. On 

 many days when a strong breeze was blowing 

 on the top of the Observatory and over Black- 

 heath, there was not the slightest motion in the 

 air near the banks of the Thames ; and this 

 remarkable calm continued for some days to- 

 gether. On September 11 and 12, however, 

 the whole mass of air at all places was in 

 motion, and for the first time for nearly three 

 weeks the hills at Hampstead and Highgate were 

 seen clearly from Greenwich. These capricious 

 currents carry away the smoke of cities, and roll 

 the masses of aerial impurities hither and thither 

 until they become diluted indefinitely, and ulti- 

 mately entirely lost. Again they return, bearing 

 from the fields and woodlands the pure air, in the 

 words of Spenser, so 



" Gently attempered and disposed so well, 

 That still it breathes forth sweet spirit and wholesome 

 smell." 



Thus by their ceaseless changes they so agi- 

 tate and intermingle the atmosphere of our 

 country, as to preserve in it a degree of purity 

 and freshness which could in no other way be 

 attained. In tropical countries the population 

 is not so large in proportion to the area occu- 

 pied, and manufacturing processes are scarcely 



