AT MUNICH, 1840, 1841, 1842. 613 
of two vertical bars in the plane perpendicular to the middle of 
the free magnet, one deflecting on one side with the south pole, 
the other on the other side with the north pole. My first experi- 
ments were noways Satisfactory ; the magnetism of the bar in- 
creased so steadily and uniformly, that it could not but be expected 
that it would come toaconstant condition. At the same time the 
influence of temperature was considerable, but not always equal. 
It was only by degrees that I recognised the conditions to be 
attended to, and the method by which the object would be most 
surely attained. If it is desired to use an iron bar for a differ- 
ential instrument for the inclination, it must be plunged fifteen 
or twenty times alternately into hot and cold water, without 
ever being brought out of its vertical position; the bar thus ac- 
quires a constant amount of permanent magnetism. Its relation 
to temperature is then examined. I have found that some bars 
increase and others diminish their magnetism with increasing 
temperature; my experiments are not yet sufficiently numerous 
to show on what conditions this depends, but the circumstance 
is a very advantageous one, because the bars may be so chosen 
that the influences of temperature may compensate each other. 
The hourly observations of the 1st of February 1843, were 
commenced with an instrument constructed in this manner*. 
From the first it was striking how exactly the march of the in- 
clination accorded with that of the intensity; and this relation 
continued unaltered in considerable disturbances. I concluded 
from hence, as M. Kreil had already done from his observations 
* The instrument consists of a small needle weighing hardly a half gramme, 
and two iron bars 8 inches long, 6:4 lines broad, and 1°7 thick; the nar- 
rower side is turned towards the magnet, the distance is 33 lines, and 
the deflection is 44°; the needle carries a mirror 8 lines square, and vibrates 
in 2'"-7. It is so narrowly enclosed on all sides that it has only room to move 
just so far as the measurements require, which circumstance naturally causes 
the vibrations to diminish much more rapidly than would be the case if the 
needle was merely suspended under a bell-glass; the decrease, however, is 
only =4,th of the are of vibration for each oscillation. With the small magnets 
used in this observatory for measurements of absolute intensity, and which are 
suspended under bell-glasses when determining their time of vibration, I have 
never determined the decrease of the arc of vibration; the small amount of the 
resistance of the air may, however, be inferred from the circumstance, that in 
the measurements before-mentioned the are of vibration was about 20! at the 
commencement, and that after 400 vibrations it was still sufficiently large for 
the passage of the needle to be observed with facility. In the registers of 1842 
eases occur in which 800 and even 1000 vibrations were observed. I mention 
this because a singular opinion has sometimes been expressed, to the effect 
that small magnets are quieted by the resistance of the air, and that it is owing 
to this kind of quieting that instruments of the construction in use in this ob- 
servatory remain free from oscillation. : ' 
