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and that this must happen within a given distance; so that 

 if they be separated at such a distance from the earth that 

 the power of the earth cannot act upon them, they will 

 remain suspended like the earth, and not fall at all. 60 



The following instance of the cross may be adopted. 

 Take a clock moved by leaden weights, 81 and another by 



60 The error in the text arose from Bacon s impression that the earth was 

 immovable. It is evident, since gravitation acts at an infinite distance, that no 

 such point could be found; and even supposing the impossible point of equilib 

 rium discovered, the body could not maintain its position an instant, but would 

 be hurried, at the first movement of the heavenly bodies, in the direction of the 

 dominant gravitating power. Ed. 



61 Fly clocks are referred to in the text, not pendulum clocks, which were 

 not known in England till 1662. The former, though clumsy and rude in their 

 construction, still embodied sound mechanical principles. The comparison of 

 the effect of a spring with that of a weight in producing certain motions in cer 

 tain times on altitudes and in mines, has recently been tried by Professors Airy 

 and Whewell in Dalcoath mine, by means of a pendulum, which is only a 

 weight moved by gravity, and a chronometer balance moved and regulated 

 by a spring. In his thirty-seventh Aphorism, Bacon also speaks of gravity 

 as an incorporeal power, acting at a distance, and requiring time for its trans 

 mission ; a consideration which occurred at a later period to Laplace in one 

 of his most delicate investigations. 



Crucial instances, as Herschel remarks, afford the readiest and securest 

 means of eliminating extraneous causes, and deciding between the claims of 

 rival hypotheses ; especially when these, running parallel to each other, in the 

 explanation of great classes of phenomena, at length come to be placed at issue 

 upon a single fact. A curious example is given by M. Fresnel, as decisive in 

 his mind of the question between the two great theories on the nature of light, 

 which, since the time of Newton and Huyghens, have divided philosophers. 

 &quot;When two very clean glasses are laid one on the other, if they be not perfectly 

 flat, but one or both, in an almost imperceptible degree, convex or prominent, 

 beautiful and vivid colors will be seen between them ; and if these be viewed 

 through a red glass, their appearance will be that of alternate dark and bright 

 stripes. These stripes are formed between the two surfaces in apparent con 

 tact, and being applicable on both theories, are appealed to by their respective 

 supporters as strong confirmatory facts; but there is a difference in one circum 

 stance, according as one or other theory is employed to explain them. In the 

 case of the Huyghenian theory, the intervals between the bright stripes ought 

 to appear absolutely black, when a prism is used for the upper glass, in the 

 other half bright. This curious case of difference was tried, as soon as the op- 



