HOW THE SUN WARMS THE EARTH — ABBOT 



171 



this interesting course, that the order of the two polarities is opposite 

 in the north and south solar hemispheres and that tlie order con- 

 tinues unchanged through each 11-year sun-spot period, but reverses 

 at the beginning of the next period (fig. 14). Thus it requires two 

 11-year periods to bring the sun through a full cycle of magnetic 

 changes. 



N 



r''iGr"RK 11. — Dlitsriim of reversal of sun-spot magnetic polarity which O'liiis at t1 

 beginiiini; of a new sun-spot cycle, of 11 years. 



SUN SPOTS AND WEATHER 



For many ^ctirs we have known that though the solar constant is a 

 little higher when sun spots are most numerous, paradoxically the 

 weather at most stations is a little cooler. The effect is only about 

 1° F. at maximum. Recently I Iiave found a more important effect 

 than this of sun spots on weather. Figure 15 gives one of the many 

 instances Avhich illustrate it. At Bismarck, N.Dak., I find the solar 

 periodicities plainly shown in the departures from normal tempera- 



