NATURAL PHILOSOPHY. 157 



tion by deflecting it from its position of equilibrium and leaving it to 

 itself. A plane of oscillation is thus determined, which, from the per- 

 sistence of the visual impressions, is clearly delineated in space. Now 

 it was remarked that, on turning around with the hand the arbor which 

 formed the support of this vibrating rod, the plane of oscillation was 

 not carried with it, but always retained the same direction in space." 

 From this came the conclusion that a pendulum set in motion will con- 

 tinue in the same plane of vibration, however the point of suspension 

 be rotated ; a fact easily proved by a simple trial with a weight at the 

 end of a cord. The rotation of the point of suspension may make the 

 pendulum revolve on its axis ; but the plane of vibration will remain 

 the same. The reason for this is obvious : the swinging pendulum, 

 when about to return (after an outward swing) from its point of rest, 

 is made to move from that point by gravity alone, and can therefore 

 fall in one direction ; and the momentum acquired by falling carries it 

 beyond this centre in the same direction to the point of rest on the 

 other side ; here again it is in a like condition, and must return under 

 the force of gravity in one and the same line, gravity acting in the 

 same direction whether the point of suspension be rotated or not. Thus 

 the plane of vibration is fixed from the very nature of the forces at 

 work. 



It is evident, therefore, that if a pendulum were swinging at the 

 pole of the earth, the plane of vibration, as it would not change with 

 the revolution of the earth , should mark this revolution by seeming to 

 revolve in the contrary direction, and in 24 hours it would make appar- 

 ently the whole circuit of 360 degrees. But, at the equator, the plane 

 of vibration is carried forward by the revolution of the earth, and so 

 undergoes no change with reference to the meridians. Between the 

 equator and poles, the time required for the pendulum to make 360 

 degrees varies according to the latitude, being greater the farther from 

 the pole. 



The motion of the pendulum at the poles and the motion at the 

 equator is not hard to comprehend; a clear explanation, however, 

 of the motion of the pendulum at stations between the pole and the 

 equator is a matter of no little difficulty. As the pendulum is here 

 influenced by so many varying conditions, a strictly true mechanical 

 conception of its motions may be impossible. 



The peculiar interest of Foucault's experiment has caused it to be 

 repeated in all parts of the world. Every experiment, says the London 

 Philosophical Magazine, which has been properly conducted, under 

 favorable circumstances, has furnished results most striking. Experi- 

 ments made at Colombo, Ceylon, in latitude 6 56' N., with a pendu- 

 lum 66 feet six inches long, give a mean hourly variation of 1.87, which 

 is a very close approximation on the calculated variation, which is 

 1.8111. The law, therefore, seems to hold good even at points very 

 near to the equator. A Mr. Bunt, of Bristol, England, in a set of long 

 experiments, obtained a mean motion of 11. 750 per hour instead of 

 11.7631, as per theory, the latitude being 51 27'. In another set of 

 experiments, undertaken at a different latitude, he obtained correspond- 

 ing results, which, he thinks, not only confirm the truth of Foucault's 



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