the Royal Society’s Inflruments . 387 
avoided, s and s are two brafs plates, on each of which 
is drawn a line to which the index at the end of the 
needle is to point; there is alfo a line parallel to thefe 
drawn on the bottom of the box ; thefe three lines form 
the line F/in fig. 3. r is a double microfcope intended 
to affift us in judging when the index p points exactly to 
the line f, that is, to the line drawn on the plate s. It is 
placed fo, that a wire w w in its focus appears to coincide 
with this line ; and in obferving, the box is moved till the 
wire appears alfo to coincide with the index p. 
The cap in the center of the needle is made to take on 
and off readily, and to fit on upon either face; fo that 
we may on occafion obferve with the under face of the 
needle uppermoft, as is reprefented in fig. 5. But the 
regular obfervations are always made with the needle in 
its upright pofition, and by the help of the index p only ; 
the intention of the other index and of inverting the 
needle is, to fliew whether the line joining the indices 
p and p, or the line p p as I fh all call it, is parallel to the 
direction of magnetifm in the needle, and thereby to find 
whether, in the ufual method of obferving, the index g 
flrews the true angle which the direction of magnetifm 
, makes with the fide ab. The way of doing this is as 
follows ; having fuffered the needle to fettle, the obferver 
moves the box by means of the adjufting ferew t, till 
the index p coincides with the line f, and reads off the 
angle fhewn by the vernier. He then moves the box till 
the other index p coincides with the line /, which, as the 
pin on which the needle is fufpended is fixed to the brafs 
Vo l. LX VI. Fff frame, 
