34 PLEASANT WAYS IN SCIENCE. 



spots, he attains again a similar condition. After this the 

 spots begin to return, gradually attain to a maximum, then 

 gradually diminish, until after eleven more years have 

 elapsed few or none are seen. It must not be supposed 

 that the sun is always free from spots at the time of minimum 

 spot frequency, or that he always shows many and large 

 spots at the time of maximum spot frequency. Occasionally 

 several very large spots, and sometimes singularly large spots, 

 have been seen in the very heart of the minimum spot season, 

 and again there have been occasions when scarcely any spots 

 have been seen for several days in the very heart of the 

 maximum spot season. But, taking the average of each 

 year, the progression of the spots in number and frequency 

 from minimum to maximum, and their decline from maxi- 

 mum to minimum, are quite unmistakeable. 



Now there are some terrestrial phenomena which we 

 might expect to respond in greater or less degree to the 

 sun's changes of condition with respect to spots. We can- 

 not doubt that the emission both of light and of heat is 

 affected by the presence of spots. It is not altogether clear 

 in what way the emission is affected. We cannot at once 

 assume that because the spots are dark the quantity of sun- 

 light must be less when the spots are numerous ; for it may 

 well be that the rest of the sun's surface may at such times be 

 notably brighter than usual, and the total emission of light 

 may be greater on the whole instead of less. Similiarly of 

 the emission of heat It is certain that when there are many 

 spots the surface of the sun is far less uniform in brightness 

 than at other times. The increase of brightness all round 

 the spots is obvious to the eye when the sun's image, duly 

 enlarged, is received upon a screen in a darkened room. 

 Whether the total emission of light is increased or diminished 

 has not yet been put to the test. Professor Langley, of the 

 Alleghany Observatory, near Pittsburg, U.S., has carefully 

 measured the diminution of the sun's emission of light and 

 heat on the assumption that the portion of the surface not 

 marked by spots remains unchanged in lustre. But unttf 



