ASTRONOMY. 



sufficient to state, that the atmosphere has the power of absorbing, and 

 holding in solution, great quantities of water. It has been calculated, 

 that assuming the surface of all the water on the earth to be 130,000,000 

 geographical miles, that about 60,000 cubit miles of water will be 

 changed into vapour every year. Now, the whole of the water is preci- 

 pitated to the earth again in the form of rain, and the cause of this, 

 according to Dr. Plutton, will, perhaps, be understood by describing a 

 simple experiment, which every one can perform himself. 



Take two beer tumblers, and into one put a little water, just enough 

 to wet the sides, then hold the glass near the fire, till the air in it, having 

 become heated, has absorbed the water. If the edges of the other 

 tumbler, which has been kept cool, be placed upon the one that has been 

 warmed, the sides of the latter will become covered with moisture. Dr. 

 Hutton explains this as follows: " According," says he, "to the tempera- 

 ture of the atmosphere so is its power of containing fluids in a state of 

 vapour. A warm atmosphere will absorb a greater quantity of water 

 than a cool atmosphere ; and if, therefore, a current of warm air mixes 

 with a current of cold air, a part of the vapour held in solution by the 

 former will be precipitated to the earth in the form of rain; because the 

 temperature of the warm air will be reduced by its mixture with the cold, 

 and its capacity or power of containing vapour be thereby diminished. 

 When, therefore, in the experiment above described, we place the tumbler 

 containing cold air upon that containing warm air, holding water in 

 solution, the temperature of the air is reduced, its power of holding so 

 much water in solution is diminished ; and hence the water is precipated, 

 and forms a moisture on the sides of the glass." 



Some philosophers have imagined that rain is the result of an electrical 

 action of the clouds upon each other ; and there is Very probably some 

 truth in this theory, because we know how suddenly, and with what 

 violence, rain is generally precipitated during a thunder storm. But there 

 can be no doubt but that it is principally to the changes in the tempera- 

 ture of the atmosphere we must attribute the cause of rain. 



DEW AND HOAR FROST. 



The phenomena of dew were first satisfactorily explained by the late Dr. 

 Wells ; who showed, by the most decisive experiments, that, apparently, 

 all these phenomena were owing to the effects of the radiation of heat from 

 the earth's surface during the absence of the sun. It is admitted that, 



