212 H. Helmholtz on the Interaction of Natural Forces. 



changes are observed of sucli a nature: this comet describes 

 elhpses round the sun which are becoming gradually smaller. 

 If this kind of motion, which certainly corresponds to that 

 through a resisting medium, be actually due to the existence of 

 such a medium, a time will come when the comet will strike the 

 sun; and a similar end threatens all the planets, although after 

 a time, the length of which baffles our imagination to conceive 

 of it. But even should the existence of a resisting medium 

 appear doubtful to us, there is no doubt that the planets are not 

 wholly composed of solid materials which are inseparably bound 

 together. Signs of the existence of an atmosphere are observed 

 on the Sun, on Venus, Mars, Jupiter, and Saturn. Signs of 

 water and ice upon Mars ; and our earth has undoubtedly a fluid 

 portion on its surface, and perhaps a still greater portion of fluid 

 within it. The motions of the tides, however, produce frictioUj 

 all friction destrovs vis viva, and the loss in this case can only 

 affect the vis viva of the planetary system. We come thereby to 

 the unavoidable conclusion, that every tide, although with mfi- 

 nite slowness, still with certainty, diminishes the store of mechan- 

 ical force of the system ; and as a consequence of this, the rota- 

 tion of the planets in question round their axes must become 

 more slow, they must therefore approach the sun, or their satel- 

 lites must approach them. What length of time must pass 

 before the length of our day is dirainislied one second by the 

 action of the tides cannot be calculated, until the height and 

 time of the tide in all portions of the ocean are known. This 

 alteration, however, takes place with extreme slowness, as is 

 known by the consequences which Laplace has deduced from the 

 observations of Ilipparchus, according to which, during a period 

 of 2000 years, the duration of the day has not been shortened 

 by the 3-^o^h part of a second. The final consequence would 

 be, but after millions of years, if in the mean time the ocean did 

 not become froisen, that one side of the earth would be con- 

 stantly turned towards the sun, and enjoy a perpetual day, 

 ■w^hereas the opposite side would be involved in eternal night. 

 Such a position we observe in our moon with regard to the earth, 

 and also in the case of the satellites as regards their planets ; it 1 



is, perhaps, due to the action, of the mighty ebb and flow to ^ 



which these bodies, in the time of their fiery fluid condition, 

 were subjected. 



I would not have brought forward these conclusions, which . 

 again plunge us in the most distant future, if they were not una- 

 voidable.^ Physico-mechanicallaws are, as it were, the telescopes 

 of our spiritual eye, which can penetrate into the deepest night 

 of time, past and to come. 



. Another essential question as regards the future of our plan- 

 etary system has reference to its future temperature and illumi- 



