64 PHYSICAL SCIENCE BK. n 



according to the variety of impact in the clouds ; 

 the larger cavity in some clouds, the smaller in 

 others account for the variety. That air violently 

 driven out is fire, which is called sheet lightning 

 when it forms a fitful flame of no great violence. 

 We see the flash before we can hear the sound : 

 eyesight is swifter than hearing, and far outstrips it. 



XIII 



1 THE mistakenness of the opinion that the fire is 

 stored up in the clouds may be inferred from many 

 considerations. For example, if the fire merely 

 falls from the sky, why does it not do so every day 

 from the glowing mass that is constantly up there ? 

 Then, again, the theory gives no explanation of the 

 downward course of the fire, an element which 

 naturally rises. Fires on earth from which embers 

 fall belong to a different category ; the embers 

 possess a certain amount of weight, which carries 

 them down. Fire cannot descend in the same way, 



2 but must be forced or conducted down. Nothing 

 analogous to a terrestrial fire can take place in that 

 pure ethereal fire which contains nothing that can 

 carry it down to earth. Otherwise , if any portion 

 of it fall down, the whole is endangered ; for 

 anything susceptible of gradual diminution piece 

 meal may evidently also fall in a mass. Besides, if 

 an element whose lightness habitually prevents its 

 fall contain any weight in its hidden depths, how 

 could it maintain itself in the place whence it fell ? 

 But, it is urged, are not certain forms of fire wont 

 to descend into the lower parts of air very much 

 like these bolts of lightning that we are investi- 



