PREFACE. 



19 



them through their strict scientific details, and to those whose 

 object is to obtain a profound knowledge of them would 

 have assuredly led to the production of a work which would 

 have been useless to both classes. It would have been unin- 

 telligible to the popular reader, and insufficient for the scien- 

 tific student. 



Mathematical reasoning and technical phraseology have, 

 therefore, been almost if not altogether excluded from these 

 essays. Instead of the rigid demonstrations of which the 

 propositions and principles are susceptible by the aid of the 

 language and symbols of the pure mathematics, other proofs 

 are substituted, expressed in ordinary language, based on or- 

 dinary notions, and coming within ordinary comprehension. 

 Illustrations which would be inadmissible in strictly scientific 

 essays, are here freely used, and even profusely resorted to. 

 The same position, where it presents any difficulty or ab- 

 struseness, is presented to the reader successively under dif- 

 ferent aspects, and elucidated by different illustrations; so that 

 understanding, which may not be reached by one, will proba- 

 bly be struck by another. Subjects also are occasionally 

 selected for discussion, such, foj example, as the plurality of 

 worlds, which, though quite admissible here, would scarcely 

 find a fit place in a strictly scientific work. 



Great pains have been taken by me, and no expense has 

 been spared by the publishers, in supplying these volumes 

 with instructive and useful diagrams. Those which I used in 

 my public lectures, have been reduced in scale, and engraved 

 for this purpose. The telescopic views of the planets have 

 been taken from the drawings of the observers of highest 

 reputation; and some of the views of the lunar surface, copied 

 from Madler's drawings, now appear for the first time (so far 

 as I am informed) in this country. 



