1878.] 



Magnetic Declination at Prague. 



399 



and Jupiter come together. If then Jnpiter be at' the same ecliptical 

 longitude as that of the perihelion of Mercury, we should expect 

 prominent oscillations from the effect of superposition of the two 

 periods alone, apart from any increased susceptibility of the sun. 



14." Suppose we now take a group of fifteen periods, embracing 

 nearly four years, arouud each of the epochs when Jupiter is in this 

 position with regard to Mercury. These epochs will be as follows (as 

 far as available sun-spot observations are concerned) : March, 1835, 

 January, 1847, November, 1858. Let us make use of these selected 

 periods to determine the inequality due to the period of Mercury, and 

 also that due to the synodic revolution of Mercury and Jupiter. Even 

 if there be no increase on these occasions of the susceptibility of the 

 sun, we shall have results greater than the mean for each of the in- 

 equalities so determined from the effect of superposition alone. In 

 other words, the apparent Mercury inequality is mixed up with and 

 exaggerated by the superposition of the Mercury and Jupiter in- 

 equality, while the apparent Mercury and Jupiter inequality is, in its 

 turn, mixed up with and exaggerated by the Mercury inequality. Now, 

 on the supposition that the susceptibility of the sun does not alter, we 

 can calculate from knowing the mean inequalities what these apparent 

 inequalities ought to be, because we can calculate the effect of mere 

 superposition of the one upon the other. These we may call the 

 calculated inequalities. Now, if there be an exaltation due, let us 

 imagine to an increased susceptibility of the sun on these occasions 

 (which are also those of numerous sun-spots) the observed planetary 

 inequalities should be greater than the calculated. It will be seen 

 from the following table that this is really the case. 



Table XI. Comparing together the observed and calculated sun-spot 

 inequalities for selected periods. 



Period of Mercury (one revolution). 











O 



Observed. 



Calculated. 



Between 







and 



30 



- 2-31 



- 3-95 



55 



30 



55 



60 



-20 -85 



-10 -63 



55 



60 



55 



90 



-33 -07 



-12 10 



55 



90 





120 



-40 -37 



-12 33 



55 



120 



55 



150 



-37 -50 



-11 -96 



55 



150 



55 



180 



-21 -30 



-10 -13 



55 



180 



55 



210 



- 2-04 



- 5-83 



55 



210 



55 



240 



+ 18 -29 



+ 1-54 



55 



240 



55 



270 



+ 37 -09 



+ 6-99 



55 



270 



55 



300 



+ 47-73 



+ 10 -oo 



55 



300 



55 



330 



+ 43 -55 



+ 8-91 



5) 



330 



J> 



360 



+ 22 -22 



+ 3-63 



