54 PROF. BALFOUR STEWART AND MR. W. DODGSON ON 



VII. An Analysis of the recorded Diurnal Ranges of Mag- 

 netic Declination, with the view of ascertaining if these 

 are composed of Inequalities which exhibit a true 

 Periodicity. By Balfour Stewart, M.A., LL.D., 

 F.R.S., Professor of Natural Philosophy at the Owens 

 College, Manchester, and William Dodgson, Esq. 



Eead March 8th, 1881. 



i . It is well known that Professor Rudolph Wolf has en- 

 deavoured to render observations of sun-spots made at 

 different times and by different observers comparable with 

 each other, and has thus formed a list exhibiting approxi- 

 mately the relative sun-spot activity for each year. This 

 list extends baek into the seventeenth century, and is un- 

 questionably of much value. Nevertheless it must be 

 borne in mind that we possess no sun-spot data sufficiently 

 accurate for a discussion in a complete manner of questions 

 relating to solar periodicity before the time when Schwabe 

 had finally matured his system of solar observations, which 

 was not until the year 1832. 



We have, however, a much longer series of the Diurnal 

 ranges of Magnetic Declination. Now these are already 

 well known to follow very closely all the variations of sun- 

 spot frequency, being greatest when there are most, and 

 least when there are fewest spots; and it may even be 

 imagined that such ranges give us a better estimate of 

 true solar activity than that which can be derived from 

 the direct measurement of spotted areas. 



The long-period inequalities of the diurnal range of 

 magnetic declination are thus, we may imagine, precisely 



