OBSERVATIONS FOR TERRESTRIAL MAGNETISM 11 



change of no less than 6° in only 200 miles, — that is to say, in 17 hours' run for 

 a 12 knot ship. 



It will be noticed that the variation at sea off Mauritius is 8° 34' W., while 

 the mean declination for 1905 at the Royal Alfred Observatory, which was distant 

 5 miles from the position of the "swing," was 9° 11' 26" W., and for the time at 

 which the "swing" was made, was 9° 10'1' W. 



This difference is no doubt due to the local magnetic attractions in the grounds 

 of the Observatory, as reported by Mr T. F. Claxton, the Director, in 1905. 



The " Sealark " swing should therefore more nearly indicate the true declination 

 of the locality. 



For the benefit of any future expedition that may visit these localities, more 

 particular statements will now be given both of the magnetic elements, and of the 

 exact positions at which the " Sealark " observations were made on the various islands 

 visited. 



At each island these particulars were made the subject of a letter to the 

 " Administrateur " of the firm working the plantation on it; with a request that 

 the document might be retained in the Archives kept on each island, for the benefit 

 of future investigators ; and were as follows : 



lies Salomon. 



The Magnetic Observation Spot is in Latitude 5° 18' 19" S., Longitude 72° 26' 00" E., 

 namely, on the beach, at the western extreme of the lie de la Passe (see Admiralty 

 Chart No. 4, Principal groups of the Chagos Archipelago), just above high water 

 mark. 



The exact position is marked by a cube of concrete about 15 inches each way, 



and engraved on the top thus 



SEALARK 



A 



, the central hole being the spot of observation. 



This cube was buried about 2 feet in the sand ; but there were unfortunately no 

 near objects of a permanent nature to which its position could be referred. 



On May 27th, 1905, the following magnetic elements were observed at this position: 



Declination. 

 At 1 1 h. 00 m. a.m. 3° 48' 59" W. 



„ 4 h. 05 m. P.M. 3° 45' 20" W. 



Inclination. 

 From 2 h. 51 m. to 3 h. 30 m. p.m., - 30° 45' 20". 



Horizontal Force. 

 From Oh. 37 m. to 1 h. 39 m. P.M. X' = 0-32839 c.g.s. 



Diego Garcia. 

 The Magnetic Observation Spot, of which the geographical position is Lat. 7° 13' 58" S., 

 Long. 72° 26' 00" E., is at the beach end of a road leading to the lagoon, 400 yards to the 



2—2 



