20 



VARIATION OF THE MAGNETIC DECLINATION. 



the frequency of the sohxr spots directly derived from observation and (3 the am]iH- 

 tude of the diurnal variation. He finds a very close correspondence between the 

 computed and observed values of (3, and gives in a table Dr. Lamont's and his own 

 results between the years 1835 and 1850. He also reaffirms his former value for 

 the average length of the solar spot period., viz., 11.11 years + 0.04 years, the 

 limits of variation being 8 and 16 years. This period is deduced from observations 

 of maxima and minima since 1626. 



For Philadelphia we have (3 = 7'. 080 + 0'.039 a representing the observed 

 amplitudes as follows : — 



The coiTcspondence between the observed diurnal amplitude and the same 

 derived from observations of the solar spots is farther exhibited by Fig. 5, the 

 heavy line representing the magnetic, the other the solar amplitude curve. The 

 dotted curve is from the Toronto magnetic observations, merely multiplied by | to 

 reduce (approximately) to the Philadelphia scale. The next maximum amplitude, 

 according to the solar spot observations, Avould be in 1848, amounting to ll'.OO; 

 and the whole range of the inequality in the amplitude of the diurnal motion 

 Avould, therefore, be 1 1'.OO — 7'.-16 = 3'. 5 4. The last quantity, it must be observed, 

 is slightly variable with each period ; thus, according to the solar spot observations, 

 the year 1837 Avas a maximum, amplitude 11'.41 ; and the year 1856 a minimum, 

 amplitude 7'.24, the diff"erence being 4'. 17. 



1840 ,18|41|1^43 |18|45, 18,44| |1S|45| J^84l? , ia|.17 , 1^4^ , 18|49| IgSQ 1 18|51 



It is much to be desired that this interesting branch of physical inquiry should 

 be further studied, as it forms one of the links connecting terrestrial with cosmical 

 phenomena. 



