236 Royal Society. 



Toronto, as deduced from the monthly determinations. In doing so 

 he remarked that " the general effect of the disturbances of the in- 

 clination at Toronto is to increase what would otherwise be the 

 amount of that element; therefore, if the disturbances have a decen- 

 nial period, the absolute values of the inclination (if observed with 

 sufficient delicacy) ought to show in their annual means a corre- 

 sponding decennial variation, of which the minimum should coincide 

 with the year of minimum disturbance, and the maximum with the 

 year of maximum disturbance." At Toronto, where the true secular 

 change is very small, the effect of this superimposed variation is 

 very visible, so that the yearly values of the inclination appear to 

 increase up to the period of maximum disturbance and to decrease 

 after it. At Kcw the general effect of disturbances is probably the 

 same as at Toronto — that is to say, tending to increase the inclina- 

 tion ; but the secular change being considerable, and tending to de- 

 crease the inclination, the joint effect of the secular change and the 

 superposed variation might be expected to appear in a diminution 

 of the yearly secular change for those years during which the dis- 

 turbances are increasing from their minimum to their maximum 

 value, and in an increase of the yearly secular change for those years 

 during which the disturbances are decreasing from their maximum 

 to their minimum. 



The Kew records appear to exhibit a variation of this nature. 

 Observations of dip were commenced at the Kew Observatory in 

 1S54 ; and by comparing a good number of observations taken during 

 the latter months of 1854, with two circles and four needles, with 

 observations taken with the same circles and needles during the same 

 months of 1855, we obtain a yearly secular change of 2'*24. 



During the years from 1856 to 1859 inclusive, monthly observa- 

 tions 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. 



1856. 



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 observations 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 satis- 

 factory j and accordingly since this date Barrow's circle No. 33 has 



