MAJOR SABINE ON TERRESTRIAL MAGNETISM. 
155 
Postscript. — Aprils, 1840. 
Since the above communication was read to the Society, I have received letters from 
Captain James Clark Ross, commanding the Antarctic Expedition, dated from 
St. Helena, February 9, 1840, stating that he finds the observations of the magnetic 
dip and intensity made at sea with Mr. Fox’s instrument succeed far beyond his 
most sanguine expectation. I venture to lay before the Society the following extract 
from one of his letters, because it is likely to convey, more strongly than any com- 
ments of my own could do, a conviction of the value of a class of observations, which 
it was one of the principal objects of this paper to recommend to more general prac- 
tice. 
“ Our Fox’s needles are doing their work admirably in both ships: you would be 
somewhat surprised to see the interchange of signals between the two ships when the 
dip comes to the same minute, and it is seldom more than a few minutes apart when 
corrected for the deviation due to each vessel ; and even this small difference partly 
occurs from our only at present knowing but imperfectly the values of the correc- 
tions. We have however ample means for their determination, and we shall even- 
tually be able to make all the necessary corrections. I feel satisfied that on board 
ship much of the disturbing influence can be done away with, and the exact value of 
the remainder can be determined ; can we say the same of observations on shore, 
even under the most favourable circumstances ?” 
