FOUCAULT'S EXPERIMENT. 



be given to ita point of suspension. Thus, if a pendulum suspended 

 in a room and put into vibration in a plane parallel to one of the 

 walls, be carried round a circular table, the plane of its vibration 

 will continually be parallel to the same wall, and will therefore 

 vary constantly in the angle it forms with the radius of the table 

 which is directed to it. 



Now, if a pendulum, suspended anywhere so near the pole of 

 the earth that the circle round the pole may be considered a plane, 

 be put in vibration in a plane passing through the pole, this plane, 

 continuing parallel to its original direction as it is carried round 

 the pole by the earth's rotation, will make a varying angle with 

 the line drawn to the pole from the position it occupies. After 

 being carried through a quarter of a revolution it will make an 

 angle of 90 with the line to the pole, and so on. In fine, the 

 direction of the pole will appear to be carried round the plane of 

 vibration of the pendulum. 



The same effects will be produced at greater distances from the 

 pole, but the rate of variation of the angle under the plane of 

 vibration and the plane of the meridian will be different, owing to 

 the effects of the curvature of the meridian. 



This phenomenon, therefore, being a direct effect of the rotation 

 of the earth, supplies a proof of the existence of that motion, 

 attainable without reference to objects beyond the limits of the 

 globe. 



25. Another evidence of the rotation of the earth upon its axis 

 is derived from the ascertained fact that the planets which hold 

 places in the solar system similar to that of the earth, do revolve 

 on axes, in times not very different from that of the earth's 

 rotation, as has been shown in our tract upon the Planets. 



It may, then, be taken as proved that the earth is not fixed and 

 quiescent, but that it has a rotatory motion round the diameter 

 which passes through its poles, completing a revolution in a day. 



26. Having explained the proofs by which we have arrived at 

 the knowledge of the globular form of the earth, it may occasion 

 some surprise that we shall now have to reconsider and modify 

 that conclusion. In this there is nevertheless nothing unusual. 

 It is quite in harmony with all the labours of those who devote 

 themselves to the discovery of the laws of nature. 



27. It is the condition of man, and probably of all other finite 

 intelligences, to arrive at the possession of knowledge by the slow 

 and laborious process of a sort of system of trial and error. The 

 first conclusions to which, in physical enquiries, observation 

 conducts us, are never better than very rough approximations to 

 the truth. These, being submitted to subsequent comparison with 

 the originals, undergo a first series of corrections, the more 



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