Mr. Chariot Chrpp 



[March 4, 



inequality was only 10° "0 F. Similarly, the mean absolute daily 

 range of declination at Kew derived from the 11 years 1890 to 

 1900 was 13' "6, while the corresponding range of the regular diurnal 

 inequality was only 8'*0. Analogous results hold for the other 

 magnetic elements. 



§ 4. The range of the regular diurnal inequality varies with the 

 season of the year. Table I. shows its amplitude in the case of the 

 declination at Kew (51° 28' North lat.), Batavia (6" 11' S.) and the 

 ' Discovery's ' winter quarters {jT 51' S.). 



, Table I. — Eange of Regular Diurnal Inequality (Declination). 



Eemembering that in the Southern hemisphere June represents 

 midwinter, it will be seen that the range is in all cases larger in 

 summer than in winter. So far as is known, this is true all over 

 the earth. 



It will be noticed that the range in the Antarctic is much larger 

 than that at Kew, which in its turn much exceeds that at Batavia. 

 Allowance must, however, be made for the fact that the distur]:)ing 

 force required to dis])lace the needle 1' out of the magnetic meridian 

 is proportional to tlie intensity of the horizontal component H of 

 the local magnetic force. Now the values of H in C.G.S. measure, 

 at the epochs to which the data refer, were 0'18o at Kew, O'^^jGT at 

 Batavia, and only • 065 at the Antarctic station . Thus the disturbing 

 force required to produce a range of 1' at Batavia would produce a 

 range of 2' at Kew and of nearly 6' at the ' Discovery's ' winter 

 quarters. But even allowing for this, the Antarctic range is much 

 the largest of the three. The large size of the range is of course 

 not peculiar to the ' Discovery's ' winter quarters. The same 

 phenomenon has been observed at all Arctic stations. 



§ ^. The great increase apparent in the amplitude of the regular 

 diurnal changes as we pass from temperate to Arctic or Antarctic 

 latitudes is even more conspicuous in the case of the irregular move- 

 ments, which when sufficiently pronounced are known as magnetic 

 storms. This is illustrated by Table II. 



