200 SCIENCE PROGRESS 



in Surrey from 1 890-1906, and since then as director of the 

 Kodaikanal Observatory. The results of the whole series of 

 his observations and of those taken at Kodaikanal previous to 

 his becoming director are discussed in this memoir. 



It is well known that sun-spots, with few exceptions, occur 

 only between heliographic latitudes 15 and 35 , the mean 

 latitude varying with the phase of the solar cycle. At the 

 commencement of a new cycle of activity, small spots begin 

 to appear in the high latitudes of the spot zones. As the 

 cycle progresses the mean latitude decreases, and towards its 

 end the spots die out near the equator. Rather different 

 results hold for prominences ; there are two distinct belts 

 both north and south of the equator in which prominences 

 occur predominantly. The low-latitude belts are in the same 

 latitudes as the sun-spot zones, and vary in activity in a 

 similar manner, drawing in towards the equator and gradually 

 dying out as the cycle progresses. The high-latitude zones 

 decrease in activity at sun-spot minimum, but do not then 

 disappear, and as the solar activity increases they move to- 

 wards the poles and die out there at about the time of sun-spot 

 maximum. These prominences obviously cannot be con- 

 nected in any way with sun-spots, and it is found that the 

 majority of those in the low-latitude belts also have no con- 

 nection. 



Prominences vary enormously in shape, size and behaviour. 

 Mr. Evershed has classified them into broad groups and finds 

 that groups with certain characteristics are almost invariably 

 associated with sun-spots, whilst those with certain other 

 characteristics rarely ever are. Prominences in the form of 

 rockets, small bright jets and arches, are usually, and metallic 

 prominences (i.e. those whose spectra contain metallic lines) 

 are almost always associated with spots. Young and active 

 spots are most commonly associated with jets ; small old spots 

 are usually unaccompanied by prominences. 



Large massive forms, long groups of prominences, pyramids 

 and columns appear to be associated rarely, if ever, with sun- 

 spots ; these are the prominences which in spectroheliograms 

 taken with calcium or hydrogen light appear as long, narrow, 

 dark " filaments " projected on the disc. They are usually 

 long-lived and may reappear for several solar rotations. It 

 has been doubted whether filaments are actually the projec- 



