150 Royal Society, 



"At the March equinox the commencement of the change is 

 equally definite : no trace of change can be discovered in the mean 

 from the Ist to the 20th of March, when compared with the mean 

 of the six months from the 22nd of September to the 20th of March ; 

 the change then commences, but from some cause not yet apparent, 

 the conversion from the phaenomena of the one half-year to those of 

 the other is effected less rapidly at this than at the September 

 equinox. The mean of the month of April retain* the distinct traces 

 of the group which it has quitted, and is in fact a month of transition 

 between the two groups, but in May the conversion is quite com- 

 plete ; the phaenomena of that month have no characteristic distin- 

 guishable from those of June, July and August. 



•• From what has been stated in the preceding paragraphs, it will 

 be evident that the epochs of the sun's passage of the equator have a 

 very marked influence on the phaenomena under consideration, and 

 that the influence is the same and produces similar effects whether 

 the station itself be north or south of the equator, and however di- 

 verse may be its climatic or magnetic conditions. The semiannual 

 characteristics continue unchanged up to the days of the respective 

 equinoxes ; these form the epochs when the transition from the cha- 

 racters of the one semiannual group to those of the other commences, 

 the transition being completed a very few days after the September 

 equinox, but somewhat less rapidly after the March equinox. Like the 

 changes in the induced magnetism of ships, which follow immediately 

 the changes in the terrestrial magnetism corresponding to the ship's 

 altered geographical position, but complete the change only after in- 

 tervals of time of greater or less duration, so the changes which we 

 are here considering appear to commence at the equinoxial epochs, 

 but to require a greater or less interval of time for their completion." 



The divergence of the semiannual groups at the different hours 

 from a mean march in the year has been shown in figs. 1, 2 and 3 by 

 their comparison with the latter projected as straight lines, because 

 the accordance of the divergence at the three stations is seen thereby 

 in its simplest form. In another diagram the lines thus projected as 

 straight lines were exhibited in their true Declination values, and com- 

 pared with a Zero-line representing at each station the mean Decli- 

 nation in the year. " In the previous comparison of the annual varia- 

 tions at the three stations with each other, it was shown that there 

 is no inversion, or contrariety, between the phaenomena at Toronto and 

 Hobkrton as representativesof opposite hemispheres, the same semi- 

 annual group diverging (during the hours of the day when the cha- 

 racters are most marked) in the same direction at the same hours at 

 both stations. But markedly opposite characteristics are shown when 

 we compare the divergences of the mean diurnal variation in the 

 year from the zero-line at different stations ; these divergences, so 

 far from according with each- other at the two stations, present a 

 strong contrast throughout; the divergence at Toronto being to the 

 east at the hours when at Hobarton it is to the west, and vice versd. 

 St. Helena, moreover, which agrees with both the other stations in 

 the divergences of the semiannual groups, differs from both in those 



