4 Captain Sabine's experiments to determine the 
return of a needle in successive trials to the same division of 
the limb, is one of the principal qualifications of a good 
dipping needle. 
Having been convinced by the trial of several needles, that 
the disagreement in their results was chiefly to be attributed 
to the various causes of inaccuracy in the motion of the axis, 
I requested Mr. Dollond to make a needle on a construc- 
tion suggested, from similar experience, by Professor J. 
Tobias Meyer, in his treatise “ de usu accuratiori acus in- 
clinatorise Magneticae,” published in the Transactions of the 
Royal Society of Sciences at Gottingen for 1814. The ex- 
periments which are now submitted to the Royal Society were 
made with this needle, which, for simplicity of construction, 
convenience in use, and consistency of results, appears to 
deserve a preference over those which have been hitherto 
employed. 
As the needle which Mr. Dollond made, differed in some 
few particulars from the construction recommended by Pro- 
fessor Meyer, it may be proper to prefix a short description 
of it, as well as of the mode of observation. 
The needle is a parallelopipedon of eleven inches and a half 
in length, four tenths in breadth, and one twentieth in thick- 
ness ; the ends are rounded ; and a line marked on the face of 
the needle passing through the centre to the extremities, 
answers the purpose of an index. 
The cylindrical axis on which the needle revolves, is of 
bell metal, terminated, where it rests on the agate planes, 
by cylinders of less diameter ; the finer these terminations 
are made, so long as they do not bend with the weight of 
the needle, the more accurate will be the oscillations ; small 
