( 290 ) 



turned towards tlie Earlli : on llie 5''' of March llio ?]artli moves 

 through that point of ifs orbit which lies farthest from the phme of 

 the Sun's eqnator. In oui' diagram the hitter might therefore l)e 

 repi'esented b}' a line running almost parallel to the ecliptic E, at a 

 little over 7° lieliograpliic latitude to the north of it. (The radius of 

 the Earth is imly 8", 8). The prolongations of the planes of disconti- 

 nuity at 7^ south of the Sun's e(|uator being still almost parallel to it, 

 their position may be indicated by ihe dotted lines '/ which, on tiie 

 21*' of March, are only slightly inclined to A'. 



In Fig. 2, h we see the illnminated hemis|)here of the Earth on 

 the 2P' of June. A shoi-t time before, on the 4^'' of -lune, the Earth 

 passed through the nodal line of the Sun's equator and the ecliptic, 

 so that, on the 21^*' of .lime, the planes of discontinuity <l may still 

 be represented by lines with an inclination of about 7^ to the ecliptic. 



Fig. 2, c shows the position on the 22^^ of September ; at that date 

 the Sun's equator lies south of the Earth. 



Fig. 2, d represents the |)osition of the Earth on the 21*' of 

 December. 



From these diagrams it is plain that about the time of the equi- 

 noxes any point on the strongly illuminated |)arts of the Earth (we 

 except those places where the Sun stands low) in its diurnal rotation 

 always moves in the same sense with regard to the planes of dis- 

 continuity, making with them rathei- large angles (aitout 23''). 

 Uut soon after the solstices, in the beginning of -bdy and January, 

 at midday the said point will uionc about parallel to the planes of 

 discontinuity; in the morning and in the afternoon its movement with 

 regard to these i)lanes is in contrary directions. 



Now, as in the system of the surfaces of discontinuity the most 

 rapid changes of density occur in a direction perj)en(licular to the 

 surfaces, it follows that any |)oinl on the Earth, in its diurnal move- 

 ment, will pass through a greater variety of conditions in spring 

 and autumn than in winter and snmmei'. 



Besides it is evident that the variations in the said conditions will 

 be less marked in the wiiiter than in the summer solstice, because 

 in the former seasoji the days are so much shorter. 



We may therefore expect an annual variation in the anq)litude of 

 certain daily ijiequalities shoAving the following periodicity : 



maximum eml of Miirch 



minimum beji'imiing of Juh' 



maxinmm end of Se})teml)er 



minimum beginning of January 



