HOW RAIN 18 FORMED. 289 



liquids absorb Ititeut heat when tbey evaporate, but no other knowu 

 li(iuicl requires so uiuch as water. 



Many things fauiiiiar in every one's experience find their explanation 

 iu this absorption of latent heat. For instance, we feel colder with a wet 

 skin than with a dry one, and wet clothes are a fruitful source of chills 

 when the body is in repose; although, so long as it is in active exercise 

 and producing a large amount of heat, since the evaporation only car- 

 ries ofl" the excess, no ill consequence may ensue. Again, if a kettle 

 be tilled with ice-cold water and juit on a gas stove, supi)ose it takes 

 ten minutes to bring it to boil. In that ten minutes the water has ab- 

 sorbed as much heat as raises it from 3'2'^ to 212°, an increase of lS(»o. 

 Now, if it be left boiling, the gas-flame being kept u[) at the same in- 

 tensity, we may assume that iu every succeeding ten minutes the same 

 quantity of heat is being absorbed by the water. But it gets no hotter; 

 it gradually boils away. And it takes nearly an hour, or more than 

 five times as long as it took to heat it, before the whole of the water 

 has boiled away, since all this heat has been used up in converting it 

 into steam. It was by an experiment of this kind that Dr. Black iu 

 the last century discovered the fact of latent heat, and determined its 

 amount ; and it was the knowledge of this fact that led James Watt to 

 his tirst great improvement in the steam-engine. 



One more example I may give, which those who have been in India 

 will be able to appreciate, and which those who intend to go there nuiy 

 some day tind useful to know. Nothing is more grateful iu hot dry 

 weather than a drink of cold water. Now, ice is not always to be had, 

 but wheu a hot wind is blowing nothing is easier than to get cohl water, 

 if you have a pot or bottle of unglazed earthenware, such as are for 

 sale iu every bazaar, or what is better, a leather water-bottle, called a 

 Chha<jal, or a water-skin. All these allow the water to soak through 

 and keep the outside wet, and if auy one of them be tilled with water 

 and hung up iu a hot wind in the course of half an hour or an hour the 

 evaporation from the outside will have takeu away so juuch heat that 

 the contents may be cooled 2(P or 30°, notwithstanding that the ther- 

 mometer may staud at 110° or 115° iu the shade. Sodawater may be 

 cooled in the same way if wrapped iu straw and kept well wetted while 

 exposed to the wind. But it is of little use to do as I have seen natives 

 do sometimes, viz, put the bottles into a tub of water iu a closed room. 

 It is the evaporation that carries off the heat, otherwise the water is 

 no cooler than the air around. 



Now to return to our subject. The atmos[)here always coutains some 

 vapor which the winds have taken up froui the ocean, lakes, rivers, and 

 even from the land, for there are but few regions so dry and devoid of 

 vegetation that there is no moisture to evaporate. The (puintit}' of 

 water thus evaporated from large wat<'r surfaces is a (luestion of some 

 importance to engineers, who have to take account of the loss from 

 reservoirs and irrigation tanks, and a good d(^al of attention has been 

 H. Mis. 22 1 11) 



