138 ANNUAL OF SCIENTIFIC DISCOVERY. 



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 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 La- 

 place 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 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 subjected. 



I would not haA r e brought forward these conclusions, which again plunge 

 us in the most distant future, if they were not unavoidable. Physico-me- 

 chanical 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 sys- 

 tem has reference to its future temperature and illumination. As the in- 

 ternal 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 en- 

 tire 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 layer of the densest coal ten feet thick would give 

 out by its combustion ; and hence in a year a quantity equal to the com- 

 bustion of a layer of seventeen miles. If this heat were drawn uniformly 

 from the entire mass of the sun, its temperature would only be diminished 

 thereby one and one third of a degree centigrade per year, assuming its 

 capacity for heat to be equal to that of water. These results can give us an 

 idea of the magnitude of the emission, in relation to the surface and mass of 

 the sun ; but they cannot inform us whether the sun radiates heat as a glow- 

 ing body, which since its formation has its heat accumulated within it, or 

 whether a new generation of heat by chemical processes takes place at the 

 sun's surface. At all events the law of the conservation of force teaches us 

 that no process analogous to those known at the surface of the erri:h, can 

 supply for eternity an inexhaustible amount of light and heat to the sun. 

 But the same law also teaches that the store of force at present existing, as 

 heat, or as what may become heat, is sufficient for an immeasurable time. 

 With regard to tbe store of chemical force in the sun, we can form no con- 

 jecture, and the store of heat there existing can only be determined by very 

 uncertain estimations. If, however, we adopt the very probable view, that 

 the remarkably small density of so large a body is caused by its high tern- 



