PLANETARY MOTIONS 39 



must presuppose a pre-existent order, or a realm of pres- 

 ent nature far removed from us, whence to derive such a 

 miracle. Admit, for the sake of further argument, that 

 there once existed a time when miracles were the rule and 

 natural order the exception, or that such a condition exists 

 to-day in the sky above us ; then what possible use is it for 

 us to try to solve the dynamical problems of the celestial 

 universe at all, since we cannot sanely hope to differ- 

 entiate what is miracle from w r hat is natural, or consist- 

 ently fit one to the other! Once assert that mechanical 

 effects, such as the rectilinear motions of stars and 

 planets undoubtedly are, exist independently of physical 

 causation, and you commit four grave follies; (1) You 

 deprive physical science of the only ground she has to 

 stand upon, namely, the law of physical cause and effect, 

 the sole key to the interpretation of nature; (2) You 

 throw wide the door to the perpetration of all sorts of 

 other pious frauds ; for if Nature departs from her laws 

 in one case, why may she not do so in any other, accord- 

 ing to the exigency of theory or the whim of any of her 

 would-be interpreters? (3) You commit yourself ir- 

 revocably to the hopeless task of trying to correlate facts 

 with miracles, instead of facts with cognate facts; and 

 (4) You require of the present order of Nature not only 

 to regulate itself, but likewise to overcome and discipline 

 the erratic elements left over or inherited from a pre- 

 vious state of chaos. 



Again, continuing our experiment; suppose that in- 

 stead of letting the ball go, I should fire it from a gun ; 

 could I do so, practically or theoretically, with such ve- 

 locity as would carry it around your hand, in a near circle, 

 back to the identical starting point? Or, suppose that 

 having started the ball whirling in the ordinary way until 

 you had it going with a uniform motion ; could you, by a 

 sudden and doubly violent swing, cause the ball to make 

 two uniform turns in succession instead of but the one? 

 Astronomers virtually say both these things are possible 

 in the case of the moon, because the ether of space is non- 

 resisting, whereas the air offers considerable resistance. 

 That they are wrong, the experimenter may see for him- 



