138 THE CANADIAN NATURALIST. [Vol. 



VI. 



For a few years Mayer's theory of solar heat had seemed to 

 me probable ; but I had been led to regard it as no longer tenable, 

 because I had been in the first place driven, by consideration of 

 the very approximate constancy of the Earth's period of revolu- 

 tion round the Sun for the last 2,000 years, to conclude that " the 

 principal source, perhaps the sole appreciably effective source of 

 Sun-heat, is in bodies circulating round the Sun at present inside 

 the Earth's orbit " ; and because Le Verrier's researches on the 

 motion of the planet Mercury, though giving evidence of a sensible 

 influence attributable to matter circulatino; as a great number of 

 small planets within his orbit round the Sun, showed that the 

 amount of matter that could possibly be assumed to circulate at 

 any considerable distance from the Sun must be very small ; and 

 therefore, " if the meteoric influx taking place at j^resent is enough 

 to produce any appreciable portion of the heat radiated away, it 

 must be supposed to be from matter circulating round the Sun, 

 within very short distances of his surface. The density of this 

 meteoric cloud would have to be supposed so great that comets 

 could scarcely have escaped as comets actually have escaped, show- 

 ing no discoverable efi'ects of resistance, after passing his surface 

 within a distance equal to one-eighth of his radius. All things 

 considered, there seems little probability in the hypothesis that 

 solar radiation is compensated to any appreciable degree, by heat 

 generated by meteors falling in, at present ; and, as it can be 

 shown that no chemical theory is tenable, it must be concluded as 

 most probable that the Sun is at present merely an incandescent 

 liquid mass cooling." 



Thus on purely astronomical grounds was I long ago led to 

 abandon as very improbable the hypothesis that the Sun's heat is 

 supplied dynamically from year to year by the influx of meteors. 

 But now spectrum analysis gives proof finally conclusive against 

 it. 



Each meteor circulating round the Sun must fall in along a very 

 gradual spiral path, and before reaching the Sun must have been 

 for a long time exposed to an enormous heating efiect from his 

 radiation when very near, and must thus have been driven into 

 vapour before actually falling into the Sun. Thus, if Mayer's 

 hypothesis is correct, friction between vortices of meteoric vapours 

 and the Sun's atmosphere must be the immediate cause of solar 

 heat; and the velocity with which these vapours circulate round 

 equatorial parts of the Sun must amount to 435 kilometres per 



