1892.] President's Address. 307 



the same time, into the sun. Astronomical reasons, however, 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 gravita- 

 tion on the shrinking mass is the true source of the sun's heat, as 

 given out at present, a,nd 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 extraordinarily 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 incandescent fluid rushing 

 up from below, to take the place of matter falling down from, the 

 surface, in consequence of being cooled in the regular regime of solar 

 radiation. It seems, indeed, very improbable that meteors fall in at 

 any time to the sun in sufficient quantity to produce dynamical dis- 

 turbances at his surface at all comparable with the gigantic storms 

 actually produced by hot fluid rushing up from below, and spreading 

 out over the sun's surface. 



But now let us consider for a moment the work which must be 

 done at the sun to produce a terrestrial magnetic storm. Take, for 

 example, the magnetic storm of June 25, 1885, of which Adams 

 gives particulars in his paper of June, 1891 (' Phil. Trans.,' p. 139 

 and PI. 9). We find at eleven places, St. Petersburg, Stonyhurst, 

 Wilhelmshaven, Utrecht, Kew, Vienna, Lisbon, San Fernando, Colaba, 

 Batavia, and Melbourne, the horizontal force increased largely from 

 2 to 2.10 P.M., and fell at all the places from 2.10 to 3 P.M., with 

 some rough ups and downs in the interval. The storm lasted alto- 

 gether from about noon to 8 P.M. At St. Petersburg, Stonyhurst, 

 and Wilhelmshaven, the horizontal force was above par by 0'0007o, 

 0-00088, and 0'00090 (C.G.S. in each case) at 2.10 P.M. ; and below 

 par by 0'0007, 0*00066, 0'00075 at 3 o'clock. The mean value for all 

 the eleven places was nearly 0'0005 above par at 2h. 10m., and 0'0005 

 below par at 3h. The photographic curves show changes of some- 

 what similar amounts following one another very irregularly, but 

 with perfectly simultaneous correspondence at the eleven different 

 stations, through the whole eight hours of the storm. To produce 

 such changes as these by any possible dynamical action within the 

 sun, or in his atmosphere, the agent must have worked at something 

 like 160 million million million million hore-power* (12 x 10 3 * ergs 

 per sec.), which is about 364 times the total horse-power (3*3 X 10 33 

 rgs per sec.) of the solar radiation. Thus, in this eight hours of a 

 * 1 horse power = 7*46 x 10 9 ergs per second. 



