XXX11 INTRODUCTION. 



ether may still further attenuate itself, or it may grow 

 denser in some regions and rarer in others, thus possibly 

 allowing a freer or a void space for the earth's unimpeded 

 movement. Or better still, and more likely, the resisting 

 medium, as eminent astronomers now begin to think, 

 may have no existence, or, if any, only within a safe and 

 limited region, which does not concern the earth. The 

 whole disturbing prediction may be only a false alarm, 

 founded on the eccentric behaviour of Encke's comet, 

 whose slight anticipation of its calculated time of return 

 led, as is well known, the perplexed astronomers to 

 this hypothesis of a resisting medium. But even if the 

 ether does exist, as Herbert Spencer and Professors Tait 

 and Balfour Stewart believe, we might still escape the 

 apprehended disastrous consequences. Something might 

 always turn up in the long chapter of chances to save 

 the earth. Our present laws of Nature, as Mill suggests, 

 may not keep quite steady and invariable so long. Or, 

 on the other hand, they may ; and the earth, which has 

 gone on so many millions of years in spite of the efforts 

 of the ether, may still contrive to keep off from the sun. 

 But the best encouragement of our scepticism comes from 

 the fact that a shade of it, at moments, appears to cross 

 the mind, at other times so confident, of our physical 

 speculators themselves. Thus, Professor Balfour Stewart, 

 after describing the chaotic beginning of the visible 

 universe to which " our modern knowledge enables us 

 to look back with almost certitude," and after predicting 

 the inevitable end with equal certainty, lets fall the 

 significant sentence, not unsuggestive of scepticism in its 

 author, " It ought, however, to be borne in mind that 

 our knowledge of the laws of matter is in reality very 

 limited." 



In like manner, though to a less degree, we may 



