ASTRO-PHYSICS 297 



predominant, which are revolving round large 

 bodies in orbits with high speeds, this retarda- 

 tion becomes important, and will eventually cause 

 the meteorites to gravitate towards the centre. 

 In this way it is possible that the sun may clear 

 the neighbouring space of meteoritic dust, which 

 would otherwise move round him in permanent 

 orbits ; and the earth would draw back to herself 

 any particles shot out by volcanic eruptions, 

 such as that of Krakatoa, when the velocities 

 impressed may have been great enough to carry 

 them beyond the atmosphere, and in the right 

 direction to set them moving as satellites. 



The theory of radiation also enables us to 

 solve many other interesting problems connected 

 with the solar system. By means of a thermo- 

 dynamic proof it has been shown that the total 

 radiation from a source should vary as the fourth 

 power of the absolute temperature T, that is, 

 as T *. By experimental investigation it is 

 possible to establish a numerical relation, and, if 

 R be the energy radiated per square centimetre 

 per second by a full radiator such as lamp- 

 black, the constant k in the theoretical equation 

 R = kT ^ has been found by Kurlbaum to be 

 about 5.32 io~^ erg.^ 



Now we can calculate the total energy 

 radiated from the sun per second by measuring 

 the amount received at the surface of the earth, 

 and estimating the amount lost by reflection 

 and absorption by the atmosphere. These con- 

 siderations lead directly to the effective tempera- 



^ The erg is the French unit of work or energy. About an erg 

 of work is done when the thousandth part of a gram is raised 

 through one centimetre. 



