the intensity of magnetic forces , ^c. 59 
evening, and to have been decreasing from an early hour in 
the morning. 
The general agreement of these intensities with those de- 
duced from the observations made in doors, is as near as 
could be expected, considering that an interval of twenty 
days had elapsed between the two sets of observations. From 
this, and the agreement in the manner in which the westerly 
and easterly points of equilibrium approach and recede from 
the north in the two cases, which I have before pointed out, 
we may conclude, that there is nothing anomalous in the 
action which takes place on the needle under the different 
circumstances of its being placed in doors or in the open air; 
and that the apparent anomaly in the directions of the needle 
in the two cases, which was observed by Mr. Barlow and 
myself, arose from the cause which I have assigned for it in 
my former paper ; namely, the difference in the changes of 
temperature in the magnets when in doors and when in the 
open air. 
The diurnal changes in the terrestrial magnetic intensity 
have been determined by Professor Hansteen, by means of 
the vibrations of a needle delicately suspended. From these 
observations it appears, that in general the time of minimum 
intensity was between ten and eleven o'clock in the morn- 
ing ; that the maximum happened between four and seven 
entirely from the rotation of the iron, and are not produced by any friction on the 
axis. As the effects are not very considerable, to render them conspicuous it is 
necessary to make use of a plate eighteen inches in diameter, and to have its centre 
within sixteen inches of that of the needle. If the needle is under the influence of 
magnets, as in the foregoing observations, the effects produced by the rotation of 
the plate are considerable. 
