1886.] Sun-spot Areas and Diurnal Declination-ranges. 231 



activity would probably alter the quality as well as the quantity of 

 the solar rays, so as to bring in a greater proportion of those which 

 are absorbed in the upper regions of the atmosphere. We might pro- 

 bably thus expect a set of terrestrial actions following promptly after 

 the solar outbreak. This is similar to what we have more especially in 

 the magnetic Inequalities around 24 days. 



On the other hand, if the Inequalities around 26 days are due to the 

 earth's being placed in a favourable position for receiving the solar 

 influence, we shall have a state of things physically different from that 

 which we imagine to characterise the Inequalities around 24 days, and 

 in our ignorance of the exact way in which the sun influences the 

 magnetism of the earth, we cannot assert that the Inequality pro- 

 duced in the one case will be necessarily the same as that produced in 

 the other. 



Apparent Progress of Magnetic Weather. 



12. In order to prevent ambiguity, it is desirable to define what we 

 mean by the apparent progress of magnetic weather. If a particular 

 state of declination diurnal range — a maximum for instance — be 

 found to occur at Prague four days after it occurs at Toronto, and if 

 there is reason to believe that this difference in time depends upon the 

 distance between the stations, we should characterise the phenomenon 

 by terming it an apparent progress of magnetic weather from west to 

 east. But this phrase must not be regarded as implying any theoreti- 

 cal explanation of the observed fact, or as asserting that it is an actual 

 progress of matter in the direction from west to east which gives rise 

 to the phenomenon. 



It is obvious that if such a progression exists it will be most readily 

 seen in the undisturbed observations, for it is one of the characteris- 

 tics of a disturbance to occur simultaneously or nearly so at stations 

 far apart, while it is another characteristic to exalt the daily range. 

 Hence if disturbances possess periodicity, the maxima of their 

 periods might be expected to occur simultaneously or nearly so at 

 stations far apart. Magnetical weather is, however, something 

 different from disturbances, and denotes, as we have used the term, a 

 particular state or value of undisturbed diurnal magnetic range, just 

 as a particular state or value of diurnal temperature-range may be said 

 to denote a particular kind of meteorological weather. Again, in cer- 

 tain preliminary investigations evidence has been given by one of us 

 tending to show that there is possibly a progress of magnetic weather 

 from west to east. But it is clear that in making a comparison of this 

 nature not only must we get rid of disturbances as much as possible, 

 bnt we must likewise limit our comparison to Inequalities of the same 

 type or nearly so. Now both of these conditions are possessed by the 

 eries of Table V, for in the first place we may imagine that they are 



b 2 



