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significance of the drop of rain. We can well understand how 

 it is that "the clouds drop fatness on the earth/' when we 

 estimate the powers expended in their genesis. All the coal 

 which could be raised by man from the earth in thousands of 

 years, would not give out heat enough to produce by evapora- 

 tion the earth's rain-supply for one single year ! The sun — ■ 

 whose influence is often contrasted with that of the rain- 

 shower — is the agent in producing that shower as well as in 

 pouring out his direct heat on the soil with such apparently 

 contrasted effect. 



The actual process of the production of rain has not yet 

 been completely explained. We are, in fact, doubtful as to 

 the true nature of clouds, fogs, and mist, and, therefore, it is 

 intelligible that some difficulty should surround the explanation 

 of a phenomenon of which these meteors are, so to speak, the 

 parents. 



It is generally supposed that clouds consist mainly of hollow 

 vesicles of water, and not of minute drops. Yet meteorologists 

 are far from being agreed on this point. On the one hand we 

 have the evidence of De Saussure and Kratzenstein, who 

 actually saw, or supposed they saw, the constituent vesicles of 

 clouds and fogs. De Saussure, indeed, tells us how we may 

 see the vesicles for ourselves. If a cup of coffee, or of water 

 tinctured with Indian ink be placed in the sun, minute vesicles 

 of various thickness will be seen to ascend from the surface of 

 the liquid. He adds that those vesicles which rise differ so 

 much in appearance from those which fall, that it is impossible 

 to doubt that the former are hollow. Kamtz, also, made mea- 

 surements of the vesicles of fogs in Central Germany and in 

 Switzerland; and in his valuable work on Meteorology, gives 

 a table and a figure, showing the law according to which the 

 dimensions of the vesicles vary in the course of the year. 



Despite this evidence, Sir John Herschel holds a contrary 

 opinion. He points out that the observations of De Saussure 

 and Kratzenstein may be readily referred to the effects of 

 optical illusion. The strongest argument put forward by 

 Kratzenstein is founded on the fact that rainbows are never 

 formed on clouds or fogs, as they would be (according to the 

 undulatory theory of light) if these meteors were composed of 

 globules of water. Sir John Herschel, a higher authority on 

 optical questions than either De Saussure or Kratzenstein, is of 

 opinion, on the contrary, that it is possible a re-examination of 

 the very difficult point in question would give a different account 

 than that usually accepted. 



Herschel points out the difficulty of understanding in what 

 manner the condensation of true vapour should result in the 

 formation of a hollow vesicle. Tyndall points out, on the 



