horizontal needle when under the influence of magnets. 347 
steel. The form of the needle was that of two segments of 
circles joined by their chords, the sum of the versed sines, or 
the greatest breadth, namely, where the agate was centered, 
being .425 inch, and the chord 6 inches : the extremities were 
hardened and very accurately terminated in very sharp points. 
This I found to have much greater directive force than the 
other ; making, when not under the influence of the magnets, 
twenty-three vibrations in 60 seconds, whereas the other 
made only eighteen vibrations in the same time ; so that the 
directive force of the new needle was to that of the other as 
1.63 to 1. I had also made other needles, of the same steel, 
of greater breadth in the middle, but found that this had con- 
siderably the greatest directive power. With this needle I 
made most of the subsequent observations. 
Having placed the apparatus in a room having two win- 
dows facing W 40° N, and an iron stove at the distance of 
nine feet from the needle in the direction W 20° S, it was fixed 
so that zero of the arc in the compass-box corresponded with 
the magnetic north. On the 4th of April, at io h 35“ P. M. I 
adjusted the magnets, which I had before used, in the line of 
the dip, as I have already described, so that the needle, when 
under their influence, pointed to zero, and made five vibra- 
tions in forty seconds. As previously to applying the mag- 
nets the needle had made thirty vibrations in seventy-eight 
seconds, the directive force was diminished by them nearly 
in the ratio of 1 to .106. I here assume that the forces will 
be as the squares of the number of vibrations in the same 
time, which is not very accurate, since, when the directive 
force was much diminished, I was under the necessity of com- 
mencing the vibrations at 60° from zero, in order to obtain a 
sufficient number to estimate, at all correctly, their duration. 
