PRESIDENTIAL ADDRESSES. 519 



the earth are due. We may, indeed, knowing that 

 meteorites do fall into the earth, assume without 

 doubt that much more of them fall, in the same 

 time, into the sun. Astronomical reasons, how- 

 ever, led me long ago to conclude that their 

 quantity annually, or per century, or per thousand 

 years, is much too small to supply the energy 

 given out by the sun in heat and light radiated 

 through space, and led me to adopt unqualifiedly 

 Helmholtz's theory, that work done by gravitation 

 on the shrinking mass is the true source of the 

 sun's heat, as given out at present, and has been 

 so for several hundred thousand years, or several 

 million years. It is just possible, however, that 

 the outburst of brightness described by Lord 

 Armstrong may have been due to an extra- 

 ordinarily great and sudden falling in of meteoric 

 matter, whether direct from extra-planetary space 

 or from orbital circulation round the sun. But it 

 seems to me much more probable that it was due 

 to a refreshed brightness produced over a larger 

 area of the surface than usual by brilliantly incan- 

 descent fluid rushing up from below, to take the 



