PART I. ON MAGNETIC STORMS. CHAP. III. 275 



There are two circumstances in connection with this relative motion, that must here be considered. 

 This is easily seen by looking at the conditions at the point of intersection of the magnetic axis with, 

 for instance, the northern hemisphere. In the first place, the sun's azimuth will increase, in the course 

 of the day, more or less evenly by 360 in a westerly direction; and in the second place, the height 

 of the sun above the astronomical horizon of this place during the same period, will vary periodically 

 with an amplitude of about 23 20'. 



If we now look at these two components of the motion separately, we must in the first place 

 assume, as regards the change of azimuth, that this by itself will cause the systems to move right round 

 the earth in a westerly direction in the course of the twenty-four hours. 



The alteration of altitude will cause a displacement of the systems in a manner characteristic of 

 this condition; and it is quite conceivable, that this displacement may sometimes be the reverse of that 

 due to the variation in azimuth. It is therefore probable at the outset that the displacement of the 

 systems would be somewhat different at different times of day. When the sun is near the meridian of 

 the magnetic axis, and the variation in altitude is therefore very slight, it might be supposed that the west- 

 ward movement of the systems, caused by the variation in azimuth, will most frequently predominate. 



At times when the alteration in altitude is comparatively great however, we might possibly expect 

 to find comparatively greater effects from this second component of the motion; and it would then be 

 natural that the conditions became rather more complicated. Nor does it appear to be impossible for 

 the displacement due to the alteration in altitude to be sometimes greater than that due to the variation 

 in azimuth. 



We now find, when we look at these two perturbations, that the time at which we considered 

 the conditions in this respect on the 313! October, was just about that at which the sun passed the 

 above-mentioned meridian. There, too, we found a displacement of the systems westwards with the 

 sun; whereas in this perturbation we are just at a time when the alteration in altitude is very great; and 

 we find that the conditions are actually now developing somewhat differently. 



It might not be out of place here, as an analogy to these conditions, to compare them with those 

 found by Stormer's calculations. This cannot, of course, be regarded as anything more than an analogy, 

 at any rate here; for a number of circumstances have been set aside in the calculations, which would 

 certainly exert no small influence. In this connection we need only look at fig. 76, p. 160, to obtain 

 a general idea of the conditions. 



To every altitude, tp, of the sun above the magnetic equator, there are one or more corresponding 

 fields of precipitation, whose positions are determined by the corresponding value of <. If we imagine 

 the sun to sink, for instance, from </> = iotoi/; = 10, we should find a field of precipitation for 

 the negative rays that would move during this period from about <P = 37 to 53, or eastwards on 

 the post-meridian side. The next system, which appears on the evening and night-side, will have a 

 westward motion from about <P = 157 to -- 121 and thus changes place with almost double 

 the rapidity. The third system again, will undergo an eastward displacement, from about <P = 218 

 to 259, that is to say with a rapidity even greater than that of the preceding one. We thus see that 

 the displacement, on account of the alteration in the sun's altitude, of the systems of precipitation, con- 

 sidered from the place mentioned above, is sometimes in one direction, sometimes in the other. 



In this case, that is when the sun is sinking as indicated, in the first and third systems of preci- 

 pitation the two components of the motion will move the systems to opposite sides, and they will thus 

 counteract one another. The alteration of altitude will moreover have the greatest significance for the 

 system on the night-side. In the case of the second system, the two components will move the system 

 in the same direction. 



