8 Captain Sabine's experiments to determine the 
agate planes, and the adjustments of the zeros of the circle 
were verified in every change of position of the instru- 
ment by the double cone apparatus described in page cxl., 
which proves very convenient in use, and is highly conducive 
to accuracy ; the circle is divided into spaces of twenty mi- 
nutes, but by means of a moveable lens, the arcs to which 
the needle settles can be read off, with tolerable correctness, 
to minutes ; the arcs, in each of the four positions forming 
the elements from whence the dip is deduced, are the arith- 
metical mean of (usually) six observations, half of which 
are with the face of the, circle towards the east, and half with 
the face towards the west ; the needle being lifted by the Y’s 
and lowered gently on its supports between each observa- 
tion ; the arcs indicated by both ends of the needle are also 
read, to correct the errors arising from inequality in the 
divisions, or from the axis of the needle not passing correctly 
through the centre of the circle. 
The experiments were made in the nursery garden in the 
Regent's Park, by permission of Mr. Jenkins, the proprietor. 
The situation is in all respects an eligible one, being far re- 
moved from the neighbourhood of iron. 
The two first experiments are given in detail, that the 
method of observation may be more fully exemplified. 
