276 EVAPORATION. 



ers are formed to replace them on the other side. The 

 reason of the phenomenon is, that the air constituting the 

 wind from the southeast, having passed over the great 

 Southern Ocean, comes charged with as much invisible 

 moisture as its temperature can sustain. In rising up 

 the side of the mountain it is rising in the atmosphere, 

 and is therefore gradually escaping from a part of its 

 former pressure ; and on attaining the summit, it has 

 dilated so much and has consequently become so much 

 colder, that it lets go part of its moisture. This then 

 appears as the cloud now described ; but it no sooner 

 falls over the edge of the mountain, and again descends 

 in the atmosphere to where it is pressed and condensed, 

 and heated as before, than it is re-dissolved and disap- 

 pears ; the magnificent apparition thus dwelling only 

 on the mountain top.' 



When the elevation, to which moisture is suddenly 

 carried, is very great, the fall of temperature is propor- 

 tional, and the separating water becomes snow instead of 

 rain. A sudden reduction of temperature by other 

 means will produce the same effect. This is curiously 

 illustrated by a fountain used in one of the mines of 

 Hungary ; during the play of which the air is so com- 

 pressed, that on being released it expands and cools 

 itself enough to cause the moisture driven out with it to 

 appear, even in summer, as a shower of snow. 



At the time of the great eclipse of the sun in February 

 last, it was observed in several different places, that 

 although the sky was clear at the commencement of the 

 eclipse, clouds began to appear before the middle, and 

 continued to increase till the sun was almost or wholly 

 hid from view. It was also observed that soon after the 

 eclipse ended the clouds began to diminish in density, and 

 that at length they vanished away. It is not difficult to 

 account for these appearances on the principles of evapo- 

 ration. As the heat received from the sun was diminished 

 in consequence of the eclipse, it became insufficient to 

 keep all the vapor contained in the air in an invisible 

 state ; the moisture therefore condensed and formed 

 clouds. When the sun after the eclipse shone on these 

 clouds, the heat thus communicated caused them to dis- 



