horizontal needle when under the influence of magnets. 383 
the same circumstances with respect to the magnets, of the 
intensity of the terrestrial forces. 
Previous to giving the observations, I have considered what 
would be the case if, at the same time that the direction of 
the force deviated towards the west, its intensity increased ; 
but from the observations of Hansteen this does not appear 
to be precisely the case. He found the minimum intensity to 
take place about ten o'clock in the morning, that is, about two 
hours and a half after the westerly deviation had commenced ; 
and the maximum intensity about the same time after the re- 
turn towards the east. This increase of the intensity during 
the latter part of the westerly deviation will account for the 
general character of the changes ; and the difference in the 
times at which the changes of intensity and direction take 
place, may possibly be pointed out by some of the apparent 
anomalies in the various changes observed at different points. 
Having arrived at these conclusions from my observations, I 
intended to close them for the present, with those I have last 
given, but as I had not removed the compasses, I again observ- 
ed the directions on the following day, and I soon found that 
changes took place to an extent I had never before noticed. 
In consequence of this I continued to observe during the day, 
and felt much gratified that I still had the opportunity, as by 
this means I discovered that I had not noticed a circumstance, 
which must in all cases affect the extent of the deviations, 
and must be allowed for when the effects arising solely from 
the changes in the terrestrial forces are to be considered. In 
my first observations I had not noticed the temperature of 
the room in which they were made; and although I had 
noticed that the state of the weather appeared to have an 
