OBSERVATIONS OF THE MAGNETIC INCLINATION. 
323 
§ 12. Observations of the Inclination. 
With Mr. Fox’s apparatus. — The Inclinations obtained with needle F. A. of Mr. 
Fox’s apparatus were observed with the face of the circle successively to the east and 
to the west ; the mean of the arcs read in the two positions and at both ends of the 
needle is the result entered in the Table. The observations were made either direct, 
i. e. without the employment of deflectors, or with a deflector placed successively at 
the same angle on either side of the Inclination, and deflecting the needle to the 
opposite side : half the sum of the arcs read in the two deflected positions is the 
result in the Table. The poles of the needle F. A. were at no time reversed. When 
received in England from the maker in the spring of 1842, the mean of the two arcs 
with the face east and face west were found to give the true Inclination without sen- 
sible index error. At Sorel, in September of the same year, the Inclination obtained 
with needle F. C. of the same apparatus, of which the poles were on that occasion 
reversed, was found to agree with that shown by needle F. A. within the usual limits 
of observation error; and at Toronto in October 1842, the Inclination observed with 
needle F. A. agreed within the same limits with that observed with Gambey’s needles 
of which the poles were reversed. This needle is therefore considered to have had 
no index error. 
With Gambey’s Inclinometer . — This instrument is the property of Captain Robert 
FitzRoy of the Royal Navy, by whom it has been kindly lent for general magnetic 
service. It is the same instrument which was employed by that officer in his voyage 
of circumnavigation, and was afterwards used by myself in the magnetic survey of 
the British Islands. It has since travelled with Lieut. Lefroy over the continent of 
America to the Arctic Circle and back, having been used at more than 100 stations 
during that journey; and it should be recorded, to the credit of the excellent artist 
by whom it was made, that it is still in use apparently quite unimpaired. 
The observations made with the needles of this instrument in different azimuths, 
and recorded in the Memoir of the Magnetic Survey of the British Islands *, have 
shown that the curvature of the axles is without sensible fault, and consequently 
that there is no index error, whatever may be the Inclination. 
The Inclination with Gambey’s circle and needles entered in the Tables are the 
mean of the arcs in the sixteen positions of the circle and needle ; viz. eight positions 
with the poles direct, and the same number with the poles reversed. 
* Report of the British Association, 1838, pages 59 and 60. 
