Page:A Voyage to Terra Australis Volume 2.djvu/530

Observations taken at different dips of the needle in the Northern magnetic Hemisphere, from which are deduced the errors of variation at the Investigator's binnacle when the ship's head was at East or West.

 

Time 1801.	Lat. N.	Lon. W.	Az. or Amp.	Comps.	Observers	Ship's Hd.	Variat. Observ. West.	Variat. suppᵈ true.	Errors of observⁿ.		Errors at 8 points.		Dip Nᵒ.	Remarks.
July	°  ′	° ′					°  ′	°  ′	° ′			°  ′	°
p.m. 20	49 50	3 52	Az.	No. 1,2	T	West.	29 34	25 40	3 54	−	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	3 52	72	Off the Start.
—	49 48	4  2	Amp.	No.   1	—	—	29 30	—	3 50	−
August a.m. 29	5 38	16 50	Az.	No. 1,2	F	S.E. by S.	12 18	13 15	0 57	+		1 43	32	Off the African Coast.
Sept. a.m.  5	2 21	14 15	—	—	T	W.S.W.	14 54	13 40	1 14	−		1 20	26

 

At dip	72°,	error for 8 points	3°	52′,	or in parts of the dip	,0537
	29 ,	. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	1	31½	. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	,0526
Mean error for eight points in parts of the dip						,0531

 

These are all the observations taken in the northern hemisphere, where there was any certainty of knowing the true variation; for not being then aware of the difference which changing the direction of the ship's head produced, I did not seek to multiply the observations, nor to take them at opposite points.^[1] In the southern hemisphere, the survey of Terra Australis required the variation to be frequently observed, and it will be seen in the following table what considerable differences arose on the direction of the ship's head being changed; it will also be remarked, that the errors were of an opposite nature to those in the northern hemisphere, and decreased with the dip of the needle.

 

1. The mean of two variations or bearings taken with the head in directly opposite points, will be the correct variation or bearing, however much the attraction in the ship may make them differ from each other.