ON THE MAGNETIC SURVEY OP ENGLAND. 273 



two surveys. We have no means oF estimating with precision the operation 

 of the secular change in this interval, inasmuch as we have no absolute 

 measures of the force at so early a date as that of the survey in 1837. The 

 only satisfactory determinations which we now possess, from which an 

 approximate value of the secular change for one particular station, and for a 

 part of the interval, may be inferred, are the absolute measures made monthly 

 at the Kew Observatory since April 1857. From these the force appears to 

 have increased between April 1857 and March 1862 at an average rate of '00125 

 annually. If we assume the same rate of increase to have taken place in the 

 same years at the central position, which is not far distant from Kew, and if 

 we extend the assumption so as to include the whole interval between the 

 two surveys, the value of the fundamental line of the survey of 1837 would 

 be 10-303 instead of 10-332 ; and the isodynamics for 10-200, 10-300, 10-400, 

 10-500, and 10-600 for 1837 in Plate ii., computed by the formula 



10-332 + -00052 a + -00074 h 



(a and b being the distances in longitude and latitude in geographical miles 

 from the central position), and represented in the Plate by broken lines, 

 would each require to be diminished by 0-029. It is obvious, however, from 

 the increase in the value of r (viz. -00091 in the earlier survey, and -00106 

 in the later), that the secular increase of the force must have been greater 

 in the northern parts of England than at Kew, or generally those in the 

 southern parts of the kingdom. We must recognize also the operation of 

 the increase in the value o( u from 54° 54' to 57° 35''5 in producing a small 

 diminution in western longitudes of the secular increase observed at Kew, 

 and which has been inferred to have been still greater in the northern and 

 eastern parts. It is to be hoped that the series of monthly determinations at 

 Kew, which appear to give a satisfactory approximate measure of the secular 

 change of this element during the last five years, may be continued until the 

 survey be repeated at the expiration of a third interval ; and that in the 

 mean time determinations similar to those at Kew, and equally satisfactory, 

 may be made in other parts of the British Islands ; the present conclusions 

 regarding the secular change of the force in the interval between 1837 and 

 1860 must be necessarily imperfect. 



Division III. — Declitiation. 



[Contributed by Frederick John Evans, Esq., R.N., F.R.S., Superintendent of the Compass 



Department of the Royal Navy.] 



Plate X. exhibits a comparative view of the isogonic lines, or lines of 

 equal magnetic declination, corresponding to the epochs 1837 and 1857. 

 The lines corresponding to the first of these epochs have been drawn in con- 

 formity with a map of the isogonic lines crossing the British Islands in 1840, 

 published in plate 23 of ' Johnston's Physical Atlas ' (2nd edition), contributed 

 to that work by Major-General Sabine. The authorities on which the lines 

 for 1840 were drawn may be found in a memoir in the Philosophical 

 Transactions for 1849, art. xii. The small corrections required to reduce 

 these lines to the epoch of 1837 have been made. 



The isogonics corresponding to the later epoch (1857) rest on the autho- 

 rity of the observations contained in the subjoined Table (No. XVL). The 

 instruments chiefly used were either the Admiralty Standard Compass, or 

 Kater's Azimuth Compass ; all of which had undergone previous examina- 

 tion and adjustment at the Compass Observatory at Woolwich. 



1861. T 



