PREFACE. 



THE Essays in the present volume have been selected 

 from my contributions to serial literature during the 

 past three or four years. Although I have for some 

 time been urged to publish such a volume, I think 

 I should not have ventured to do so but for the 

 kindness with which my " Other "Worlds " and " The 

 Sun " have been received, both by the press and the 

 public. 



In preparing these Essays, my chief object has been 

 to present scientific truths in a light and readable 

 form clearly and simply, but with an exact adherence 

 to the facts as I see them. I have followed here and 

 always the rule of trying to explain my meaning 

 precisely as I should wish others to explain, to my- 

 self, matters with which I was unfamiliar. Hence I 

 have avoided that excessive simplicity which some 

 seem to consider absolutely essential in scientific essays 

 intended for general perusal, but which is often even 

 more perplexing than a too technical style. The chief 



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