CHAP, iv.] APPLICATIONS OF THE LAWS OF HEAT. 



381 



and which is fixed so that its longest side may be in a direction perpen- 

 dicular to that of the prevailing wind. The water thus trickles down 

 over the branches and little twigs of the fagots so that it presents a 

 large surface to the air ; evaporation is then very rapidly effected, 

 and the water in the basin becomes much more concentrated than that 

 of the supplies. It is drawn up again a second or a third time by the 

 pumps P, P', until a sufficient degree of concentration is effected 

 and the evaporation is completed in the boilers. 



FIG. 262. Salt pits in the west of France. 



The porous vessels to which the name of water-coolers 1 is given, 

 and which serve to keep the water cool in summer, are known to all. 

 The property which this kind of bottle ' possesses is due to the 

 cold resulting from the evaporation of the water from the outer 

 surface. The water which soaks through the sides, and evaporates all 

 the faster when the air is warm and less saturated with vapour, is 

 constantly replaced from the water within. The decrease of tempe- 

 rature resulting from this evaporation prevents the water in the 



1 Called in Spain alcarraza, a word derived from, the Arabic cd quraz, a jug. 



