TERRESTRIAL HEAT. 



situation which is exposed to the clear sky and sheltered from 

 currents of air. Evaporation is promoted by the porous quality 

 of the pans which become soaked with water, and radiation takes 

 place at the same time both from the water and the pans. Both 

 these causes combine in lowering the temperature of the water in 

 the pans, which congeals when it falls below 32. 



67. When the steam issuing from the surface of warm water 

 ascends into air which is at a lower temperature, it is con- 

 densed, but the particles of water formed by such condensation are 

 so minute, that they float in the air as would the minute particles 

 of an extremely fine dust. These particles lose their transparency by 

 reason of their minuteness, according to a general law of physical 

 optics. The vapour of water is transparent and colourless. It is 

 only when it loses the character and qualities of true vapour, that 

 it acquires the cloudy and semi-opaque appearance just mentioned. 



Fogs are nothing more than such condensed vapour produced 

 from the surface of seas, lakes, or rivers, when the water has a 

 higher temperature than the stratum of air which rests upon it. 

 These fogs are more thick and frequent when the air, besides 

 having a lower temperature than the water, is already saturated 

 with vapour, because in that case all the vapour developed must 

 be immediately condensed, whereas, if the air be not saturated, it 

 will absorb more or less of the vapour which rises from the water. 



Clouds are nothing but fogs suspended in the more elevated 

 strata of the atmosphere. Clouds are most frequently produced 

 by the intermixture of two strata of air, having different tempera- 

 tures and differently charged with vapour, the mixture being 

 supersaturated, and therefore being attended with partial conden- 

 sation as already explained. 



68. When condensation of vapour takes place in the upper 

 strata of the atmosphere, a fog or mist is first produced, after 

 which the aqueous particles coalescing form themselves in virtue of 

 the attraction of cohesion into spherules, and fall by their gravity 

 to the earth, producing the phenomenon of rain. 



69. The quantity of rain which falls in a given time at a given 

 place, is expressed by stating the depth which it would have if it 

 were received upon a plane and level surface, into which no part 

 of it would penetrate. 



At Paris, the average annual quantity of rain which falls, 

 obtained from observations continued for thirty years at the 

 Observatory,' is 23*6 inches. There is, however, considerable 

 variation in the quantities from which this average is deduced ; 

 the smallest quantity observed being 16 -9 inches, and the greatest 

 27-9 inches. 



The greatest annual fall of rain is that observed at Maranliam, 

 94 



