234 



Dr. B. Stewart oh Observations of Dip 



[Mar. 3, 



nearly all made by Mr. Chambers, and those for the last six years nearly 

 all by Mr. Whipple. 



From the first six years we deduce a mean dip equal to 68° 20'*07, cor- 

 responding to middle epoch April 1, 1860, and from the latter six, a mean dip 

 equal to G8° 6 *62, corresponding to middle epoch April 1, 1866, while the 

 secular change deduced from the first series is 2''00, and that deduced 

 from the last series 1 ' * 9 2, the mean of these two values being l'*96. 



If we apply this mean value of the secular change to the mean result 

 corresponding to the epoch April 1, 1860, in order to bring it to the epoch 

 April 1, 1866, we obtain 



68° 20'-07-ll'v6 = 68° 8'-31, 



whereas that deduced from the second series corresponding to this epoch 

 is 68° 6 -62. 



The former of these is l'-69 higher than the latter, and it may be de- 

 sirable to investigate the cause of this difference. 



7. In the first place, it cannot I think be due to any personal equation 

 in the observer. Of late I have made occasional observations with the 

 circles and needles used by Mr. Whipple, with the view of determining 

 whether there is any personal peculiarity in the dip observations of either 

 of us. 



The mean of 12 such dips taken by me is 68° 3' '95 



while the mean of 12 comparable dips taken by Mr. Whipple is 68° 3''85 

 showing a difference of not more than O'l, which small difference may pro- 

 bably be occasioned by accidental disturbance rather than by personal 

 peculiarity. 



During the time when Mr. Chambers was at Kew no comparative 

 observations were made with this particular object in view, and I cannot 

 find a sufficient number of strictly comparabb dips to determine with 

 certainty what was the mean difference, if any, between his readings and 

 mine. The result would, however, seem to indicate that his reading is 

 rather lower than mine, certainly not more than half a minute, but pro- 

 bably much less. 



There is therefore no reason for supposing that Mr. Whipple reads 

 the dip to an appreciable amount lower than Mr. Chambers, so that the 

 difference of 1 ' 69 cannot be accounted for by this supposition. 



8. Sir E. Sabine has remarked as follows (rPoceedings of the Royal Societv, 

 Nov. 30, 1865, p. 491) :— " The general effect of the disturbances of the 

 inclination at Toronto is to increase what would otherwise be the amount 

 of that element ; therefore if the disturbances have a decennial period, 

 the absolute values of the inclination (if observed with sufficient delicacy) 

 ought to show in their annual means a corresponding decennial variation, 

 of which the minimum should coincide with the year of minimum dis- 

 turbance, and the maximum with the year of maximum disturbance." 

 At Toronto, where the true secular change is very small, the effect of this 



