Prof. Helmholtz on the Interaction of Natural Forces. 513 



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 

 wdthin it. The motions of the tides, however, produce 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 infi- 

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

 nical 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 diminished 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 Hipparchus, according to which, during a period 

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

 by the j^^dth part of a second. The final consequence would 

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

 not become frozen, that one side of the eai*th would be con- 

 stantly turned towards 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 as regards their planets ; it 

 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 un- 

 avoidable. Physico-mechanical laws are, as it were, the tele- 

 scopes 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 pla- 

 netary system has reference to its future temperature and illumi- 

 nation. As the internal heat of the earth has but little influence 

 on the temperature of the surface, the heat of the sun is the only 

 thing which essentially aff'ccts the question. The quantity of 

 heat faUing from the sun during a given time upon a given por- 

 tion of the earth's surface may be measured, and from this it 

 can be calculated how much heat in a given time is sent out 

 from the entire sun. Such measurements have been made by 

 the French physicist Pouillet, and it has been found that the 

 sun gives out a quantity of heat per hour equal to that which a 



