IN NATURAL PHILOSOPHY. 47 



67. How are hailstones formed? 



There are two separate currents of air, one hot and charged 

 with moisture, the other cold. The former is displaced by 

 the latter and driven up in the atmosphere. There its vapor 

 is condensed at the centre of the cloud into snow, and at the 

 extremities into ice-cold water. In this cloud there is a whirl- 

 ing motion which collects the snow into little balls, each of 

 which is the nucleus of a hailstone. Each of these is carried, 

 alternately, by the whirling currents, into the snow-cloud at 

 the centre, and the ice-cold water outside. Both give it a 

 coating, one of snow-like, spongy ice, and the other of trans- 

 parent ice. This is done with great rapidity, until at last its 

 weight overcomes the violent upward motion which sustains it 

 in the air, and the hailstone falls to the ground. When a 

 hailstone is carefully examined we can see this nucleus, and 

 these concentric layers, like the coats of an onion. * 



68. Why do we have hailstorms in summer, and not in 

 winter? 



The small spongy hail or sleet of winter has the same origin 

 as hailstones in summer, but there is not enough vapor in the 

 cold air to give them the size of summer hailstones. 



69. Is the sweating of a pitcher a sign of rain ? 

 See question 32. 



70. Where should ice be applied, to cool water? 

 At the top, because cold water falls. 



71. Why is evaporation hastened in a vacuum? 

 Because the pressure of the air is removed. 



72. Is a dusty boot hotter to the foot than a polished one ? 

 It is, because it is a better absorber of heat. 



* The above theory is that advanced by Prof. Loomis, in his "Treatise on 

 Meteorology." Th > teacher will find thii work invaluable on all raeteorologi- 



cal questions. 



