INFLUENCE OF TIDES UPON THE EAETfl's ROTATION. 243 



tation 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 sec- 

 ond. 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 earth would be constantly 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 sub- 

 jected. 



I would not have brought forward these conclusions, which 

 aprain plunge us in the most distant future, if they were not 

 unavoidable. Physico-mechanical laws 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 

 planetary system has reference to its future temperature and 

 illumination. 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 affects the question. 

 The quantity of heat falling from the sun during a given time 

 upon a given portion 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 



