1866.] Secular Change of Magnetic Dip at Kew. 



9 



During the years from 185G to 1859 inclusive, monthly observations 

 were made with a circle known as the Kew circle, two needles being 

 always used, and the mean of the two results taken as the true value of 

 the dip. 



From this circle we have the following results : — 



Year. Mean dip. Yearly secular change. 



185G. 68° 27-67 



1857. 24-36 3-31 



1858. 22-80 1-56 



1859. 20-73 2-07 



If we take the mean of these three values of yearly secular change, and 

 also include that between 1854 and 1855, we have a mean value of yearly 

 secular change, for the period between 1854 and 1859, amounting to 

 2' -29, and this value will not be sensibly altered if we omit the observa- 

 tions between 1854 and 1855. 



In 1859 it was resolved to substitute another circle for the Kew circle, 

 as the action of the latter was not considered to be quite satisfactory ; and 

 accordingly since this date Barrow's circle No. 33 has been employed, 

 and monthly observations have been made with it, generally in the after- 

 noon — two needles being used, as before. 



Trom this circle we have the following results : — 



Year. Mean dip. Yearly secular change. 



1860. 68 20-21 



1861. 18-21 2-00 



1862. 15-58 2-63 



1863. 12-66 2-92 



1864. 9-88 2-78 



exhibiting between 1860 and 1864 a mean secular change of 2'-58. 



It m\\ be noticed from this, that the mean yearly secular change of 

 dip at Kew appears to be greater from 1860 to 1864, a period of increasing 

 disturbances, than from 1854 to 1859, a period of decreasing disturbances. 

 Possibly the yearly decrement of dip has again begun to diminish, since 

 the change from 1864 to 1865 is only r-32. It is, however, premature 

 to assert that this is the case, and it can only be decided by continuing 

 the monthly observations. At all events the Kew observations agree with 

 those at Toronto in indicating that the yearly change of dip contains the 

 combined result of two things — namely, the true secular change and the 

 change due to disturbance ; and this ought to be borne in mind by future 

 observers of this magnetic element. 



VOL. XV. 



c 



