220 
MR. BARLOW ON THE ERRORS IN THE COURSE 
It has been stated that the maximum attraction or deflection is much less 
(all other things being the same) with a small dip than with a large one ; the 
proportion being, “ that the tangents of the angles of deflections are inversely 
as the cosines of the dip now, the dip at Rio Janeiro being about 22°, and 
in London 69^°, we have 
cos 22° : cos 69§° : : tan 9° 30' : tan 3° 33'. 
That is, the Gloucester leaving Rio under similar circumstances to the Thetis, 
would on any course about the S.E. or N.E. (the former being stated as the 
course of the latter vessel on the 4th, and the other her probable course after 
tacking on the 5th of December), be constantly deflected about 3^ 0# out of her 
supposed course, and as the sine of 3° 33' is about xcth part of the radius, it is 
obvious, taking only the error due to the 5th of December, and reckoning the 
distance run at eighty miles, that the ship would pass five miles nearer to Cape 
Frio than her reckoning, an error quite sufficient to account for the fatal cata- 
strophe which has occurred to the Thetis ; for it appears that a distance of 
only so many fathoms would have nearly carried her clear of the land. I do 
not include the error due to the first day, because its tendency would only be 
to carry the vessel to the southward about the same quantity, which of itself 
could have produced no evil. 
After all, let it be remembered that this is a supposititious case, and that my 
object in stating it is merely to show what might happen if the deflection from 
local attraction were disregarded, and thereby to prove the propriety and ne- 
cessity of ascertaining whether in the case of the Thetis, and in all similar 
cases, the proper correction was made, before the apology of currents can be 
admitted. 
I urge this the more particularly, because I fear this source of error is too 
much disregarded, and as I think it probable that in the several investigations 
which have been held to inquire into cases of vessels lost in a similar way for 
the last ten years, since I have been interested on this subject, no question has 
been asked whether or not the error of the compass had been corrected ; and 
thus vessel after vessel is lost, and current after current is imagined to ac- 
* 1 say about 3^°, because much depends upon the direction of the centre of attraction in the 
vessel. 
