III.] RAIN AND DEW. 45 



the latter case, the warm water yields more vapour than the 

 air can retain at the given temperature. The British Isles, 

 washed by warm water on their western shores, are peculiarly 

 subject to fogs; and, of all places, large towns seated on 

 rivers are the most affected, since the artificial heat, coupled 

 with the moisture of the air over the river, produces con- 

 ditions favourable to the formation of fogs whenever the 

 air becomes sufficiently cooled. The proverbial London 

 fog owes its density and darkness to the smoke, or particles 

 of carbonaceous matter, disseminated through the atmosphere 

 and mingled with the partially condensed water. 



As long as water remains in the state of cloud, or fog, its 

 particles are so minute that they hang suspended in the air, 

 or mount upwards on the slightest current. But, when these 

 droplets run together, they produce drops too heavy for 

 suspension in the atmosphere, and are then precipitated to 

 the earth as rain. The rainfall, or amount of rain which 

 falls in any given locality, is a most important element in 

 determining its climate. 



What does a meteorologist mean when he says, in his tech- 

 nical language, that the annual rainfall in London is about 24 

 inches ? By such a statement he means, simply, that if all the 

 rain which falls on any level piece of ground in London during 

 an average year could be collected — none being lost by 

 drying up, none running off the soil, and none soaking into 

 it — then, at the end of the year, it would form a layer- 

 covering that piece of ground to the uniform depth of two 

 feet. The year's accumulation of rain would thus form a 

 vast mass of water. Remembering that an inch of rain 

 represents about 100 tons of water to the acre, it will be 

 found that every acre of land in the metropolis receives 

 during the year, when the year is neither very wet nor very 

 dry, not less than 2,400 tons of rain. 



