NATURAL PHILOSOPHY. 185 



in proportion to the speed will be the pressure against the side of the 

 rails, which, at a high velocity, will give the engine a tendency to 

 clinib the right hand rail in each direction. Could the journey be 

 performed in two hours between London and Liverpool, this lateral 

 movement of rotative velocity of the locomotive would have to be 

 increased or diminished at the rate of nearly one-quarter of a mile 

 per minute, and that entirely by side pressure on the rail, which if not 

 sufficient to cause the engine to leave the line, would be quite sufficient 

 to produce violent and dangerous oscillation. It may be observed, in 

 conclusion, that as the cause above alluded to will be inoperative 

 while we travel along the parallels of latitude, it clearly follows that a 

 higher degree of speed may he attained with safety on a railway run- 

 ning east and west, than on one whicl^runs north and south." 



VISIBLE ROTATION OF THE EARTH. 



M. FOUCAULT, the originator of the ingenious pendulum experi- 

 ment, for affording ocular "proof of the rotation of the earth upon its 

 axis, has recently presented to the French Academy an account of 

 another method for establishing his theory, which is reported in the 

 Journal des Debats for October, 1852. M. Foucault says : 



" Nobody now questions the apparent deviation of the pendulum, 

 and few hesitate to regard that deviation as a demonstration of the 



^^ 



earth's rotation. But, when we try to explain the experiment, diffi- 

 culties arise in the minds of many persons who cannot comprehend 

 how it is that the plane of oscillation is said to be fixed. Since the 

 point of attachment travels with the earth's surface, since a line drawn 

 perpendicularly to the horizon of the place of operation must change 

 its inclination in space every moment, how can the plane of oscillation 

 remain fixed, or even preserve its original direction ? Here is the 

 stumbling-block of all who, without taking into account the decompo- 

 sition of the movements of rotation, confidently advance into the 

 domain of mechanical science, relying only on the illuminations of 

 common sense. The error of such persons arises from their taking 

 as an absolute fact that fixity of the plane of oscillation which is only 

 a fact relatively to the vertical line of the place of operation. 



" But if in the experiment we substitute for the plane of vibration 

 of the pendulum, the plane of rotation of a body freely suspended by 

 its centre of gravity, we rid ourselves of this embarrassing relative 

 fixity of plane, and have only to consider a plane physically defined, 

 which really enjoys an absolute fixity of direction. If, at the moment 

 of putting it in rotation, the axis of this body points to a given star 

 in the sky, then during the whole time of the rotation the axis will 

 continue to point towards the same point of the firmament, and this 

 in virtue of the inertia of matter ; or, for this simple reason, that it is 

 unable to displace itself, or, so to speak, incapable of un-east-ering 

 itself alone. If, then, we select a star, or if we fix upon one of those 

 points in the heavens which seem to have the swiftest movement, the 

 axis of rotation of our freely suspended body, when attentively exam- 



