Sun-spot Areas and Diurnal Temperature-Ranges. 313 



between brightness and blackness. On the other hand, the meridian 

 on the opposite side of this would pass before the earth always bright. 

 There would thus be a period representing the synodical period of one 

 solar rotation with respect to the earth, and this might be termed a 

 solar Inequality of the second kind. 



There seems some reason, from the observations of Broun and Horn- 

 stein, to think that peculiar districts, if not a peculiar hemisphere, of 

 the sun may be affected by spots in the way we have mentioned, the 

 result of which would be that we should have amongst the Inequalities 

 of sun-spots one representing the synodical period of revolution of the 

 sun -with respect to the earth. Professor Stokes has likewise sug- 

 gested the desirability of seeing whether there may not be such a 

 period. Now it seems to us that the proof in favour of such a period 

 would not be invalidated by the occurrence of several periods around 

 26 days ; inasmuch as observations founded on sun-spots might pre- 

 sent the same variety of period, when treated as we have treated 

 them, which they presented when treated in another way by Car- 

 rington, who found that the spots in one solar latitude had a different 

 period of rotation from those in another. It seems, therefore, quite 

 possible that the periods around 26 days may denote the synodicai 

 period with respect to the earth of certain spot-producing solar 

 centres not far distant from the equatorial regions, where, according 

 to Carrington and others, the rotation is quickest. 



The Inequalities around 24 days have, however, too short a period 

 to admit of the same interpretation being put upon them, inasmuch 

 as they correspond more nearly with the time of rotation of the sun 

 as regards a fixed point in space than with his time of rotation as 

 regards the earth ; and here we come to a fact which is worthy of 

 being recorded, although we cannot pretend to discuss it exhaustively 

 in our present paper. It is that the Inequalities around 26 days 

 represent with much accuracy what would be the synodic periods 

 with respect to the earth of the Inequalities around 24 days. In 

 other words, the two sets of Inequalities appear to be related to each 

 other, as if the 24-day series represented in time not merely the period 

 of a variable state of things, but likewise one revolution with regard 

 to a fixed line of space ; while the second or 26-day series represents 

 one revolution of the same phenomenon with respect to the earth 

 regarded as the point of view. This would seem to indicate, if con- 

 firmed by further inquiry, that the Inequalities around 24 days have 

 reference to something without the sun,* inasmuch as we may imagine 

 the maxima and the minima of the 24-day Inequalities to take place 

 when certain districts of the sun come opposite certain fixed positions 

 of space. 



* This peculiar correspondence is perhaps best indicated by the Toronto Tempe- 

 rature-range observations which we think may offer the most trustworthy means of 



