66 H. I. JENSEN. 



(2) The attraction of the planets ; a theory which has 

 in its favour the fact that the sunspot cycle is of approxim- 

 ately the same length as the period of revolution of the 

 planet Jupiter. 



(3) Olouds of cooling metallic vapour and carbon in a fine 

 state of division, a hypothesis obviously related to Dr. 

 Siemen's theory of solar heat. 



(4) There is also a theory advanced by Sir Norman Lockyer 

 which has the merit of accounting at the same time for 

 coronas, prominences and snnspots. From the time of a sun- 

 spot maximum masses of vapour and solid matter in a fine 

 state of division rise from the sun's photosphere by centri- 

 fugal force often to the immense distance of ten millions of 

 miles. They keep rising and expanding until we have a 

 sunspot minimum, when the density of these masses obscure 

 photospheric phenomena. We then have a magnificent 

 equatorial corona such as was seen in 1868, 1878, 1889, 

 and 1900-01. Then through cooling the vapour masses 

 liquify or solidify, and fall with a splash into the photosphere, 

 causing renewed solar activity and the production of great 

 heat. The constant association of sunspots with protuber- 

 ances of the eruptive type and with faculaB gives a sunspot 

 very much the appearance of having been caused by a fall 

 of darker and cooler matter into the more luminous and 

 ]] otter atmosphere, throwing the photospheric matter up 

 in long streamers. 



(5) Finally we have Herr J. Halm's theory, 1 which accepts 

 Helmholtz's theory of solar contraction creating heat to 

 compensate for radiation into space. The sun is a star 

 whose loss of heat by radiation is greater than gain of heat 

 produced by contraction. The result is that the tempera- 

 ture of the layer of maximum incandescence must decrease. 

 Hence it sinks to a level where the temperature is sufficient 



1 Astr. Nachr. Nos. 3723 and 3724 ; or Nature. Vol. lxv., Feb. 13, 1902. 





