HERMANN VON HELMHOLTZ 299 



case as far as astronomical observations reach, for they have never 

 been able to detect any retardation of the planets, such as would occur 

 if they moved in a resisting medium. But on a body of less mass, 

 the comet of Encke, changes are observed of such a nature : this comet 

 describes ellipses 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 exist- 

 ence 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 at- 

 mosphere are observed on the Sun, on Venus, Mars, Jupiter, and 

 Saturn. Signs of water and ice upon Mars ; and our earth has un- 

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

 portion of fluid within it. The motions of the tides, however, pro- 

 duce friction, all friction destroys 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 infinite 

 slowness, still with certainty, diminishes the store of mechanical force 

 of the system ; and as a consequence of this, the rotation of the planets 

 in question round their axes must become more slow; they must 

 therefore approach the sun, or their satellites must approach them. 

 What length of time must pass before the length of our day is 

 diminished one second by the action of the tide 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 Hipparchus, according to which, during a period of 

 2000 years, the duration of the day has not been shortened by the one- 

 three-hundredth part of a second. The final consequence would be, 

 but after millions of years, if in the mean time the ocean did not be- 

 come frozen, that one side of the earth would be constantly turned to- 

 wards the sun, and enjoy a perpetual day, whereas 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 



