15 
magnesium, sodium, and calcium play a prominent part, is pro- 
jected into the relatively cooler atmosphere of incandescent 
hydrogen which envelopes the sun. These masses of vapour 
absorb the light which comes from the photosphere, and, carried 
round by the rotation of the sun, appear to the eye of the 
observer as dense black spots. Again, these vapours are both 
heavier and hotter than the surrounding hydrogen into which 
they have been projected. They will accordingly condense and 
fall by their own weight, and, sinking into the photosphere, will 
produce in it a cavity filled with a light-absorbing mass. Hence 
we have the appearance of a hollow recognised in sun-spots. 
Spots therefore, are but a secondary phenomenon, serving to 
inform us of the violent crises and upheavals taking place in the 
interior of the sun. Facule, according to Secchi, are caused by 
those eruptions which consist of hydrogen alone. 
Other theories have been proposed by several eminent 
observers, as, for example, the ‘‘ Cyclonic theory,” of M. Faye. 
However, as observation, not merely with the telescope, but more 
especially with the spectroscope, opens out our knowledge of solar 
phenomena, these theories will, no doubt, have to be modified, 
and possibly the true explanation of what is observed will be 
found in a judicious combination of them all. 
Forty-two photographs were thrown upon the screen, which 
served fully to illustrate the various points treated of in the 
lecture. 
The discussion which followed turned mainly upon the 
connection between sun-spots and terrestrial phenomena, and 
the lecturer’s replies to the questions asked may be thus 
summarised : 
(1) Sun-spots and terrestrial magnetism. The eleven- 
year period of sun-spots corresponds to a similar period of 
magnetic disturbances. Maximum and minimum ranges of 
the declination magnet (or mariner’s compass), correspond 
to years of maximum and minimum spots. Great spots are 
accompanied by magnetic storms ; witness the example of 
November, 1882. The latter remark is probably true also of 
auroral displays. 
(2) Sun-spots and rainfall. That rainfall is greater in 
maximum than in minimum years of spots has been established 
for Mauritius, Adelaide, and Brisbane by Dr. Meldrum, and for 
the Cape and Madras by Mr. Lockyer. 
(3) Sun-spots and trade. Dr. Hunter, Director-General of 
Statistics in India, has stated that the years of minimum rainfall 
and of consequent famines, approach to years of minimum sun- 
spots. Likewise the curve showing the variation in the price of 
wheat follows somewhat that of sun-spots. This is, perhaps, 
