INTRODUCTION. 
341 
board frequently ; those on the north, south, east and west points, occasionally ; and 
the dip and horizontal intensity on shore or on the ice, with corresponding observa- 
tions on board, as often as possible. 
“8. Index Correction . — The most convenient mode of employing Mr. Fox’s appara- 
tus at sea being to use it with the face of the circle in one direction only (i. e. east or 
west, I shall here assume it east), the index correction with the face east must be 
sought, by a comparison of the Inclinations observed in that position of the instrument 
on shore and on fixed ice, with the true Inclinations determined with needles whose 
poles may be reversed and a complete observation made with them. As the index 
correction is liable to vary as a function of the Inclination, it should be determined 
in different Inclinations, and for this purpose it will be desirable to obtain at least 
one determination in a high latitude. 
44 When observing on shore or on the ice for the index correction with the face east, 
do not omit to observe with the face west also, as the mean index correction is useful 
in showing the kind of separation which exists between the centre of gravity and the 
point of suspension in the needle for which it is determined. Mr. Fox’s apparatus is 
furnished with three needles ; one to be used when the poles are required to be re- 
versed ; the magnetism of the other two should be preserved from change if possible ; 
it has been found a convenient practice to employ one of the latter always as the 
mounted needle, and the other as a deflector. 
“ 9. Comparison of the PFeights and Deflectors. — Experience has shown that the in- 
tensity may be more conveniently and satisfactorily determined on board ship by the 
use of deflectors than by constant weights. 
“It is necessary however that the 4 equivalent weights’ of the deflectors employed 
should be carefully ascertained. Besides the table which you will form for this pur- 
pose in the manner practised by Mr. Fox, it will be necessary to have comparisons 
between the angles of deflection produced by the deflectors and the constant weights 
at the Cape before and after the voyage, and on any opportunity which you may 
have in a high latitude either on shore or on the ice. You may also get occasional 
comparisons on board in very favourable weather. 
“In the choice of constant weights to be employed during the voyage, use none that 
give a less angle of deflection than 15°. In the observations at the Cape, as your 
base station, make a double series (i. e. the same observations repeated on two sepa- 
rate days) both before and after the voyage. 
44 10. Azimuths. — You will find it a convenient practice to deduce your azimuths 
from the hour angle, instead of from the altitude, which is the more usual custom. 
First take the altitudes which will give you the hour angle corresponding to the time 
by chronometer (at least until you materially change your geographical position) ; and 
as soon as you have completed this observation, take the sun’s azimuth, noting the 
time of observation by chronometer ; the hour angle will then give you the true 
azimuth. Blank forms are sent suited to this mode of observation. 
2 Y 
MDCCCXLVI. 
