MISCELLANEOUS PAPERS. 403 



winter in regions surrounded by lakes or other large bodies of water. 

 This is a case where the air, which is deficient in natural heat, bor- 

 rows in the fall and winter large quantities of heat which it must 

 repay next spring and summer from its surplus stock of heat. Does 

 it repay any interest on the borrowed heat ? I think not ; there ap- 

 pears to be a mutual benefit. 



The changing of vapor into rain or snow always causes a large 

 amount of heat to be set free in the air. That explains one of the 

 reasons why, during a snow-storm, with the wind in a damp quarter, 

 as east or northeast, the thermometer never records so low a tempera- 

 ture as it does when the wind is in a dry quarter, as west or north- 

 west. 



Snow is a beneficent thing. While in itself it is cold to the ordi- 

 nary senses, it is an excellent non-conductor of heat and prevents to a 

 large degree the passage of heat from and cold into the underlying 

 earth. It also melts slowly and thus the water becomes absorbed by 

 the soil instead of running off to the sea. 



The third natural form of water is as vapor or steam. Steam is the 

 true gaseous condition. Above a temperature of 212° F., liquid water 

 cannot exist except under an increased pressure. Water in being con- 

 verted into steam absorbs an enormous amount of heat. The lieat 

 that water at 212' F. absorbs, in being converted into steam at the 

 same temperature, is sufficient to raise five and one-half times the 

 same quantity of water from the freezing-point to the boiling-point. 



Steam is converted into vapor simply by loss of heat on its escape 

 from confinement into the open air. The condition of vapor depends 

 on the amount of heat combined with it. The greater the amount of 

 heat combined with it, the smaller the particles. The amount of heat 

 combined with water accords with the amount of surface. In steam, 

 which contains the greatest possible amount of heat in its composi- 

 tion, the particles are probably reduced to molecules, thus having the 

 greatest possible amount of surface. When steam is superheated — 

 heated above 212° F. — the repulsion of the particles for each other 

 and the distance of the molecules apart are both greatly increased. 



On the other hand, as heat is given out in the open air, the particles 

 unite and thus surface is reduced. When much of the heat has been 

 parted with, the vapor particles are large enough to become visible 

 and clouds are formed. With the loss of more heat, mist is formed. 

 When the surplus heat is about all gone, rain is formed. 



Water may be converted into vapor at all temperatures below the 

 boiling-point. The amount of heat absorbed by water on conversion 

 into vapor is always great. The greatest evaporation is in the hottest 

 regions and where there is the greatest water surface. This vapor 

 with its contained heat is borne by the winds to cooler regions, where 



