FORMAL ASTRONOMY. 53 



matured. It occurs iu Lib. ii. Aphorism xxxvi., in which lie i> .-i 

 ing of Prerogative Instances, of which he gives twenty-seven species. 

 In the passage now referred to, he is speaking of a kind of Prerogative 

 Instances, better known to ordinary readers than most of the kinds by 

 name, the Instantia Crucis : though probably the metaphor from 

 which this name is derived is commonly wrongly apprehended. Ba- 

 con's meaning is Guide- Post Instances : and the Crux which he alludes 

 to is not a Cross, but a Guide-Post at Cross-roads. And among the 



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cases to which such Instances may be applied, he mentions the diurnal 

 motion of the heavens from east to west, and the special motion of the 

 particular heavenly bodies from west to east. And he suggests what 

 he conceives may be an Instantia Crucis in each case. If, he says, we 

 find any motion from east to west in the bodies which surround the 

 earth, slow in the ocean, quicker in the air, quicker still in comets, 

 gradually quicker in planets according to their greater distance from 

 the earth ; then we may suppose that there is a cosmical diurnal mo- 

 tion, and the motion of the earth must be denied. 



With regard to the special motions of the heavenly bodies, he first 

 remarks that each body not coming quite so far westwards as before, 

 after one revolution of the heavens, and going to the north or the 

 south, does not imply any special motion ; since it may be accounted 

 for by a modification of the diurnal motion in each, which produces 

 a defect of the return, and a spiral path ; and he says that if we look 

 at the matter as common people 2 and disregard the devices of astron- 

 omers, the motion is really so to the senses ; and that he has made an 

 imitation of it by means of wires. The instantia crucis which he here 

 suggests is, to see if we can find in any credible history an account of 

 any comet which did not share in the diurnal revolution of the skies. 



On his assertion that the motion of each separate planet is, to sense, 

 a spiral, we may remark that it is certainly true ; but that the business 

 of science, here, as elsewhere, consists in resolving the complex phe- 

 nomenon into simple phenomena ; the complex spiral motion into sim- 

 ple circular motions. 



With regard to the diurnal motion of the earth, it would seem as if 



o r 



Bacon himself had a leaning to believe it Avhen he wrote this passage ; 

 for neither is he himself, ncr are any of the Anticoperuicans, accus- 



2 Et certissimum est si paulisper pro plebeiis nos geramus (missis astrouomorum 

 it scholse commends, quibus ilhul in more cst, ut sensui in multis irnmerito vim 

 fuciant et obscuriora malint) talem esse motum istuin ad scnsum qualem diximus. 



