NATURAL PHILOSOPHY. 107 



the sun, and that the existence of any such operation seems improba- 

 ble. It has been suggested that the sun may be supplied with fuel 

 by comets or other bodies falling into it. 



Another element of destruction probably exists in the form of a 

 very rare resisting medium in space, whose existence, however, has 

 not yet been demonstrated with certainty. Yet there is evidence in its 

 favor. There is no reason a priori why we should suppose the plan- 

 etary spaces to be perfectly void ; on the contrary, the general analo- 

 gies of nature would lead us to suppose that they still contain some- 

 thing material. Now, there are two classes of phenomena which 

 point to the existence of an ether, filling all space, and possessing 

 t he property of inertia. These are as follows : The phenomena of 

 light and heat. These seem to be due to a vibratory or oscillatory 

 motion among the molecules of an ethereal medium. By the heat- 

 vibrations, force may be communicated from one body to another, 

 distant and having no material connection with it ; it is, therefore, 

 concluded that ether is possessed of the property of inertia. In our 

 ignorance of the exact nature of its motion, and the amplitude of its 

 vibrations or oscillations, we have not sufficient data for determining 

 the density of the hypothetical medium. But this density, however 

 small, must be appreciable, and therefore retard the motions of all 

 bodies moving through it. 



The observations of Encke's comet made during the last thirty or 

 forty years show that its motion is continually undergoing accelera- 

 tion 1 from some cause, and that, if this continues, it will in a few 

 centuries fall into the sun. This comet, being a small nebulous mass 

 of excessive tenuity, is precisely the object which would be most 

 affected by a resisting medium, and Encke attributes its acceleration 

 to this cause. His view is controverted by other astronomers, some of 

 whom attribute the anomalies of the comet to the repellant action of 

 the sun in driving off the comet's tail a subject to which we shall 

 presently revert. In view of these controverted points, it will be 

 hardly fair to consider it certain that the motions of the planets will 

 ever be affected by the ether, especially as it is possible that, even if 

 the ether exists, it may not affect their motions. 



Yet another cause, slowly producing an entire change in our earth, 

 is found in the mutual action of the moon and the tide-wave. As the 

 latter glides over the oceans, and rushes into the numerous indenta- 

 tions of the coast, the motions which it produces in the waters neces- 

 sarily involve an expenditure of power, or vis viva, in overcoming the 

 effects of friction. This vis viva thus expended must be drawn from 

 the set of machinery which produces the motions, that is, from the 

 motion of revolution of the moon and the motion of rotation of the 

 earth. .It cannot be returned to this machinery, because all that is 

 not spent in triturating the sand or other material which forms the 

 bed of the ocean is turned into heat, and radiated off into space. Its 

 loss will manifest itself in exactly the same way in which a resisting 



1 It may appear paradoxical that a resisting medium should cause the motion 

 of a body moving through it to be accelerated. It produces this efl'ect indirectly. 

 If the medium retards the body by the smallest amount, the latter will fall 

 slightly toward the centre of attraction, and the increase of velocity caused by 

 this fall will more than compensate for the retardation which produced it. 



