2O2 METHODS IN TEACHING 



determine the order of the rapidity of evaporation. Satur- 

 ate some cotton with the liquid that evaporates most rapidly, 

 wrap the cotton around the bulb of a thermometer, having 

 first noted the temperature indicated by the dry bulb. 

 Swing the thermometer in the air to hasten the evapora- 

 tion, and note the rapid fall of the mercury. Bring out 

 in discussion that evaporation cools the surface from which 

 it occurs. Why does moistening the skin even with warm 

 water make it feel cooler when exposed to the air? Why 

 does it feel cooler when moistened with alcohol or ether 

 instead of water? Why are wet cloths put around jugs 

 and casks of drinking water? What is the effect of the 

 evaporation of perspiration upon the temperature of the 

 body? Where would this effect be greater, in regions 

 where the air is moist or dry? Why are sunstrokes more 

 common in moist climates than in dry ones? 



Discuss the formation of dew, fog, clouds, rain, and 

 snow. Have the class see the clouds growing larger on 



days when there is a falling barometer, and 

 Condensation 



smaller when the weather is clearing. Dis- 

 tinguish between true vapor, which is invisible, and fogs 

 and clouds, which are visible. The immense, flat-bottomed 

 cloud masses that fill the sky on spring days, and the great 

 cumulus clouds commonly called " thunder heads " are. 

 really parts of vapor masses reaching to the earth ; only the^ 

 tops are visible, however, because these lie in currents 

 of air sufficiently cold to condense them into fog. The 

 lower and warmer layer may be from half a mile to a mile 

 in depth. Often two layers of air can be made out above 

 this, when the clouds at different heights float in different 



