Observations on the West Coast of Africa. 481 



three elements were determined by the Challenger Expedition 

 in 1876. Observations have been made eight times at the Bar- 

 bados between 1726 and 1844 and six times at Bermuda be- 

 tween 1831 and 1876. The Coast Survey determinations at 

 the latter place were made on Nonsuch Island at the extreme 

 eastern end of the group. They are, undoubtedly, the only 

 magnetic observations ever made at this place, and it is more 

 than probable that the station will never be re- occupied. This 

 is to be regretted ; but it was necessary to utilize the ten days 

 spent in quarantine in order to make steamer connection. The 

 work had to be done here, if it was to be done at all in Ber- 

 muda. 



The gravity stations in common with other observers were 

 Capetown, St. Helena and Ascension. Foster's celebrated 

 series includes all of these. Sabine determined gravity at 

 Ascension in 1822 and deFreycinet observed at the Cape in 

 his voyage around the world in 1819. Besides the idea of 

 verifying Foster's result, that Ascension Island is too light, it 

 was highly desirable to connect his service with our own, which 

 now includes island as well as continental stations. But it was 

 assumed sufficient to have the series exactly coincident at two 

 points. Lemon Yalley was therefore not re-occuj)ied at St. 

 Helena. Moreover, the Ascension stations are practically iden- 

 tical in the two series, and St. Helena was occupied at the sea 

 as well as at the summit, which gives a third connection with 

 Foster, and supplies a check on his Ascension result. 



In order that an approximate estimate might be made 

 for the matter lying above the sea-level at St. Helena, many 

 heights were determined barometrically by Professor Abbe of 

 the U. S. Signal Service. By using these, some idea of the at- 

 traction of the mountain may be had, and it will then be seen, 

 whether the islands in the Atlantic and Pacific differ essentially 

 as regards internal structure. Rock specimens were brought 

 from both St. Helena and Ascension. Their densities may 

 give an indication of what we should look for in the gravity 

 results, providing that both islands were subject to the same 

 laws of formation. 



The " Pensacola " staid at St. Helena but sixteen days. Dur- 

 ing this time two stations were selected, the pendulums were 

 swung through six consecutive days and nights at each place, 

 magnetic observations were made on six different days and the 

 instrumental outfit transjDorted to and from the mountain top. 

 Equally rapid progress was made at Ascension, where the con- 

 ditions were similar in many respects. This amount of work 

 could not have been accomplished, however, without the able 

 and generous assistance of the naval cadets attached to the 

 " Pensacola," and of Professor Bigelow of the eclipse party, to 

 all of whom I wish here to express my obligations. I wish 



