PART II. POLAR MAGNETIC PHENOMENA ASH TERREI.LA EXPERIMENTS. CHAP. III. 



519 



of the monthly period of polar storms. Now it has been found that different parts of the sun rotate 

 with a different angular velocity. The least synodic period of rotation is about 26.04 da y s > which is the 

 period of facula near the equator; the period, however, becomes longer as we get deeper into the sun's 

 layers, or towards its pole. In the table below is given the synodic period for faculae, for sun-spots, and 

 for the photosphere. 



TABLE XCI. 



The numbers in table XCI, are taken from ARRHENIUS' Cosmical Physics ('). They indicate that for 

 equal heliographic latitudes the period of rotation increases towards the interior. According to PRINGSHEIM 

 the angular velocity of faculae, photosphere and sun-spots( 2 ) should be the same for the same latitude. 

 However this may be, it is commonly assumed that the angular velocity decreases from the photosphere 

 towards the interior. 



We notice that the period found for the storminess cannot be explained merely by the time of 

 rotation of the sun-spots. The greatest number of sun-spots are found betwen 15 and 20 heliographic 

 latitude. From this we should expect a period corresponding to that latitude, or about 27.3 days. This 

 is a_bout the period found by MAUNDER and HARVEY. Such a period of disturbance may well exist, but 

 it is too small to explain the essential feature of the variation of storminess in our case. If the period 

 of polar storminess is to be explained by the rotation of the sun, we shall either have to go to points 

 deep down in the sun's layers, or to points near the poles, for the source of magnetic storms. 



As both the moon and the sun give rise to a period such as that found for the magnetic storminess, 

 the problem of finding out by exact methods the cause of the period becomes a rather difficult one; and 

 it is hardly possible, by means of purely statistical methods, to decide from which of the^two sources 

 the monthly period originates. At any rate, if a statistical method could give any answer to this question, 

 we should have records covering a long period. 1 think, however, we can get a step further by utilising 

 our knowledge about the physical conditions which might produce the observed changes of storminess. 



ON THE POSSIBLE INFLUENCE OF THE MOON UPON MAGNETIC STORMS. 



96. There are two main sources of influence to consider : 



1 1 1 The moon is the seat of a magnetic field. 



(2) The moon is the source of primary or secondary "electric radiation". 



It is well known that the direct influence of the moon's magnetic field must at any rate be extre- 

 mely small, and would cause variations of quite another type than those considered, in the magnetic 

 storms. But there is still a possibility of an indirect influence, as the presence of the moon's magnetic 



i'l ARRHENIUS: Lehrbtich der kosmischen Physik p. 125. 

 r-'i K. PKIXGSHEIM: Physik der Sonne p. 61. 



