154: 



VARIATION. 



variation. It must be observed, however, on both 

 coasts of Africa, within 6° — 7° of the Line, this 

 instrument requires especial study for nautical 

 purposes. Here it is an imperfect indicator, 

 because, affected from great distances, it rises 

 without fine weather and it falls without foul. 

 At Zanzibar the case of a whaling captain is 

 quoted for wasting in vain precautions nearly 

 two months. Moreover, sufficient observations 

 have not yet been accumulated in the south- 

 ern hemisphere. Where there is so little ex- 

 pansion in the mercurial column the convexity 

 and concavity of the column-head must be care- 

 fully examined with a magnify in g-glass, and by 

 a reflecting instrument the smallest change 

 could be correctly measured. The trembling of 

 the aneroid needle, sometimes ranging through 

 a whole inch during the gusts of the highly elec- 

 trical tornado, also calls for observation. The 

 sympiesometer is held to be even more sensitive 

 than the mercurial barometer, especially before 

 storms, and ignorance of its peculiarity has often 

 ' frightened a reef in ' at unseasonable times. 

 The same was found to be the case, in high lati- 

 tudes, by Lieut. Robertson, H.N., when sailing 

 under Captain Ross (1818), between N. lat. 51° 

 39' and 76° 50'. 



