the Vicinity of the Cape of Good Hope. 339 



to 29° 14''6 ; or at something less than an average annual rate 

 of 1'. (Cape Magnetic Observations, vol. i. p. Ixv.) Supposing 

 this rate to have continued to 1854, when Captain Klein observed, 

 the variation at the Cape should have been 29° 18', which is still 

 ll'^rea^er than that which is shown by the map (instead of less). 



But we are not without data from the vicinity of the Cape still 

 nearer the epoch at which Captain Klein observed, from which 

 we may obtain a corroboration of the correctness of the variation 

 as given in the map, and its applicability to the present time. 

 Lieutenant Dayman, R.N., formerly of the Magnetic Obsei'va- 

 tory of Van Diemen's Land, whose skill and accuracy as a nau- 

 tical sui'veyor are justly held in the highest estimation, has trans- 

 mitted to the Admiralty the results of determinations in 1853 at 

 eighteen different stations, partly obsei-ved on land, and partly 

 at sea on board H.M. Surveying Ship Hydra, between Capes 

 Hangklip and Aguilhas. By the kindness of the Hydrographer, 

 these were sent to me as soon as they reached the Admiralty. 

 The mean of the eighteen determinations is 29° 24'. It would 

 be difficult to say what result could be more in accordance with 

 the map, allowing for the small difference of latitude and longi- 

 tude. Therefore, had Captain Klein's observations agreed with 

 the latest observations made on land (i. e. at the Cape Ob- 

 servatory and on the coast in its vicinity), they would also have 

 substantially agreed with my map. If they differ from it, they 

 differ also from the observations obtained by many observers of 

 high reputation both on shore and at sea. 



In conclusion, I wish to express the satisfaction with which I 

 view the evidence afforded by Captain Klein's communication of 

 the increasing interest which is taken in these questions, since 

 the attention of the mercantile marine of different nations has 

 been called to their importance ; and I also see with pleasure the 

 freedom with which Captain Klein's remarks and criticisms are 

 given. We may confidently expect that one result of such in- 

 creased attention will be improvement in maps of the magnetic 

 variation in localities where they need improvement : and it may 

 not be amiss to notice, that since, by the increased employment 

 of iron in ships, the compass has been rendered much more 

 subject to deviation than it formerly was, maps of the magnetic 

 variation which can be relied upon, both as to epoch and amount 

 of variation, have, besides their primary use in navigation, this 

 additional value, that a single observation taken of the variation 

 with the ship's head on the course which she may happen to be 

 steering, furnishes, by comparison with the map, the deviation 

 of the compass with the ship's head on that particular point in 

 that part of her voyage ; and thus such a map facilitates the 

 task, which has now become an essential part of correct navigation. 



