480 E. D. Preston — Magnetic and Gravity 



found the result to be less than the number of oscillations, 

 actually counted at the sea-level. As some recent observations 

 on island mountains give results at variance with this, it was 

 desirable to repeat the Ascension work with modern instru- 

 ments. Indeed, it was to connect with Foster, and to verify 

 his result for Ascension, that an extension of our gravity work 

 was proposed. The entire voyage lasted eight months, of 

 which 123 days were spent on shipboard. The area covered 

 by the stations extends from Washington on the north and 

 west, to Capetown on the south and east, making a range of 

 73° in latitude and 96° in longitude. It so happens that the 

 most northern station is also the most western, and the most 

 southern the most eastern. The elevations range from 7 feet 

 at Bermuda to 2,250 at Ascension. The magnetic observations 

 at Azores. Cape Verde Islands, Freetown and Elmina were 

 shortened by lack of time, one, or at most two days being de- 

 voted to each place ; but at all other stations the declination, 

 dip, and horizontal force, were determined on each of three 

 consecutive days, besides, at a few. stations, making hourly ob- 

 servations on several other days. At Cabiri the needle re- 

 mained suspended during the total eclipse, but no abnormal 

 change was noticed. For the gravity work, at every station 

 about 30 swings were made with each pendulum — using them 

 in both positions, and continuing the observations through the 

 entire 24 hours. The pendulums used were Nos. 2 and 3, of 

 the Peirce pattern, being of the invariable reversible type. 

 The length of the former between the knives is one meter, 

 that of the latter a yard. Besides having been employed at 

 numerous home stations, No. 3 has now been swung at 13 for- 

 eign ones ; including several in the Pacific. 



Of the 14 magnetic stations all but one have been occupied 

 by earlier observers. Determinations were made in the Azores 

 between 1497 and 1829 ; in the Cape Verde Islands between 

 1841 and 1853; in Sierra Leone between 1826 and 1842; at 

 Freetown, by Sabine, in 1822; at Cape Coast Castle, seven 

 miles from Elmina, in 1838 and 1841. St. Paul de Loanda 

 has a magnetic observatory, and issues published reports con- 

 taining results for declination, dip and intensity. The work at 

 the Cape of Good Hope between 1840 and 1850, is well known, 

 and Sabine's account of five years' observations at St. Helena 

 from 1841 to 1845, is one of our classic magnetic volumes. 

 The station at Sister's Walk in Jamestown, was selected by 

 Sir James Ross in 1840, but as the values at this point are not 

 normal the Coast Survey station was chosen some distance 

 back from the mountain and midway between it and Ladder 

 Hill. Sister's Walk is close against the foot cliffs of Rupert's 

 Hill. The dip was measured at Ascension in 1822, and all 



