23G Royal Society .— 



And the clips corresponding to every tenth year within the period 

 speciticd are as follows : — 



1820.0 /0 07-3 



1830.0 69 39-6 



1840.0 69 11-9 



1850.0 G8 45-9 



1860.0 68 19-9 



The progressive diminution of the Dip in London during the last 

 forty years has thus been traced and followed by the observations 

 recorded and discussed in this paper ; and the further progress of the 

 research will now devolve on the systematic observations which are 

 made for that purpose monthly at the Observatory at Kew. 



The rate of diminution in the last forty years does not appear to 

 differ materially from the mean rate in the preceding hundred years. 

 The experiments of Mr. George Graham between March and May, 

 1723, recorded in the Philosophical Transactions for 1/25, No. 389, 

 give a mean dip in London at that epoch of " nearly " 74° 40'. Com- 

 paring this with 69° 11''95 in 1840.0, we have a difference of 5° 28'- 1 

 in 11 6* 7 years, equivalent to a uniform diminution of 2'" 81 annually ; 

 or if the formula 



0=69° ll'-95-2'-713 (£-* ) + 0'-00056 (t-t f 

 be employed, it gives the dip in March 1 723.3 equal to 74° 36'" 1, being 

 a difference of less than 4' from the result of Mr. Graham's experi- 

 ments ; which difference is doubtless less than the probable error 

 of that gentleman's determination with the instruments then in use. 



An expectation appears to have prevailed in some quarters that the 

 decrease of the Dip in London should have ceased, and its subsequent 

 increase have commenced, contemporaneously with the alteration 

 which took place in the secular change of the Declination in the 

 early part of this centur} r , when the increase of west declination, 

 which had been continuous in the British Islands for about two cen- 

 turies, ceased, and was succeeded by a decrease of the same. But 

 this supposition is by no means in accordance with that general 

 view and interpretation of the phenomena of terrestrial magnetism 

 for which we are indebted to Dr. Halley, and which, since its pro- 

 mulgation in 1683, has received so much confirmation in various and 

 distant parts of the globe. In accordance with that hypothesis, the 

 diminution of the Dip in London might be expected to continue until 

 the epoch should arrive when, by the easterly movement of translation 

 of the minor magnetic system in the northern hemisphere, the dis- 

 parity of the magnetic force prevailing in the European and American 

 portions of the hemisphere should have attained its maximum : — which 

 is certainly not yet the case. 



Is there then, in the secular change of the Dip, no feature in 

 which, in conformity with the Halleian hypothesis, an alteration 

 might be expected to synchronize with the reversal in the direction 

 of the secular change of the declination? Assuredly there is ; and 

 the facts which recent investigations have brought to our knowledge 



