CLOUDS AND KIVEKS, ICE AND GLACIERS. 101 



where it stands low ; the change being due, not to any 

 difference in the temperature of the air, but simply to 

 the withdrawal of the thermometer from the direct 

 action of the solar rays. Nay, without shifting the 

 thermometer at all, by interposing a suitable screen, 

 which cuts off the sun's rays, the coldness of the air 

 may be demonstrated. 



258. Look now to the snow upon your house roof. 

 The sun plays upon it, and melts it ; the water trickles 

 to the eave and then drops down. If the eave face the 

 sun the water remains water; but if the eave do not 

 face the sun, the drop, before its quits its parent snow, 

 is already in shadow. Now the shaded space, as we 

 have learnt, may be below the freezing temperature. If 

 so, the drop, instead of falling, congeals, and the 

 rudiment of an icicle is formed. Other drops and 

 driblets succeed, which trickle over the rudiment, con- 

 geal upon it in part and thicken it at the root. But a 

 portion of the water reaches the free end of the icicle, 

 hangs from it, and is there congealed before it escapes. 

 The icicle is thus lengthened. In the Alps, where the 

 liquefaction is copious and the cold of the shaded 

 crevasse intense, the icicles, though produced in the 

 same way, naturally grow to a greater size. The drain- 

 age of the snow after the sun's power is withdrawn 

 also produces icicles. 



259. It is interesting and important that you should 

 be able to explain the formation of an icicle ; but it 13 



