510 NOVUM OKGANUM. [bOOK It 



let tlicm be set well together, so that one bo ncitlier quicker nor 

 slower than the other; then let the clock moved by weights ha 

 placed on the top of a very high church, and the other be kept 

 below, and let it be well observed, if the former move slower 

 than it did, from the diminished power of the weights. Let the 

 same experiment be made at the bottom of mines worked to a 

 considerable depth, in order to see whether the clock move more 

 quickly from the increased power of the weights. But if this 

 power be found to diminish at a height, and to increase in sub- 

 terraneous places, the attraction of the corporeal mass of the 

 earth may bo taken as the cause of weight. 



Again, let the required nature be the polarity of the steel 

 needle when touched with the magnet. We have these two ways 

 Avitli regard to this nature: — Either the touch of the magnet 

 must communicate polarity to the steel towards the north and 

 south, or else it may only excite and prepare it, whilst the actual 

 motion is occasioned by the presence of the earth, which Gilbert 

 considers to be the case, and endeavours to prove with so much 

 labour. The particulars he has inquired into with such inge- 

 nious zeal amount to this : — 1. An iron bolt placed for a long 

 time towards the north and south acquires polarity from this 

 habit, without the touch of the magnet, as if the earth itself 

 operating but weakly from its distance (for the surface or outer 



a consideration which occurred at a later period to Laplace in one of 

 bis 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 v.-hen 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. 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 colours 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 ai-e formed between the two 

 surfaces in apparent contact, and being applicable on both theories, are 

 appealed to by their respective supportei's as strong confirmatory facts ; 

 but there is a difference in one circumstance, 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 abso- 

 lutely black, when a prism is used for the upper glass, in the other lialf 

 bright. This curious case of difference was tried, as soon as the opposing 

 consequences of the two theories were noted by M. Fresnel, and tha 

 results is stated by him to be decisive in favour of that theory whicL^ 

 rr^c-K hght to con-sist in the vibrations of an elastic medium. £d. 



