440 M. A. Secchi on the Periodical Variations 



that the oscillation of the needle has always a double period, 

 diurnal and nocturnal, but their respective lengths vary with the 

 season. The diurnal period, considerable in summer, is con- 

 tracted in winter, and the nocturnal period, short and hardly 

 discernible in summer, is greatly developed in winter; in this 

 will be seen the fact, elsewhere noticed, that the absolute noc- 

 turnal minimum is greater than the diurnal minimum, and 

 hence we see the cause of the error of those who consider that 

 there is a single period in winter. But two things are particu- 

 larly to be observed in these curves. 1st. The nocturnal loop is 

 always in diametical opposition to the point of noon ; from this 

 it appears that the pbases succeed each other near the lower 

 meridian with the same march as near the upper. 



2nd. That the magnitude of each loop in the opposite seasons, 

 diurnal as well as nocturnal, is in a constant proportion to the 

 one diametrically opposed to it, viz. between yth and ^th ; thus, 

 for example, the loop of the diurnal curve for December beco- 

 ming the nocturnal loop in June, is diminished to about £th. In 

 like manner the diurnal loop of June, when it becomes the noc- 

 turnal loop in December, is diminished to about ^th. This con- 

 stant proportion, which is observed in all the months, must not 

 be overlooked ; and physically considered, it must depend on the 

 manner in which the influence of the solar magnetism operates 

 across the earth. 



3rd. The appearance of these curves is that which would 

 arise from the superposition of two circular spirals with different 

 moduli, the one having a simple, the other a double period. 

 The curves which are seen in Wheatstone's undulation machine, 

 when two spirals are superposed, the one half the length of the 

 other (in which case the projection of the resultant at right 

 angles to the axis of the spirals forms a kind of c), are evidently 

 of the same kind as the present. 



Mr. Broun has given analogous curves for Makerstoun, and a 

 glance at these, as in the case of Hobarton, will show the same 

 law, though somewhat more complicated from having grouped 

 too many months together, and from the higher latitude and 

 more frequent disturbances. 



Among the points to be remarked in these curves' is the fol- 

 lowing : — " Tracing in them the direction of the magnetic me- 

 ridian (that is to say, noting the hour at which the sun passes it), 

 it is seen that the greatest velocity of the needle occurs when 

 the sun passes through this plane, and that the centre of the 

 nocturnal loop is to be found in the same line, or very near to 

 it, and that the movements of the needle in inclination are com- 

 plementary (but with a distance of 3 h ) to those of the declina- 

 tion." 



