i8s Lieut. Foster’s comparison of the diurnal changes 
The above results show, that the mean of all the observed 
times which the horizontal needle required to make one hun- 
dred vibrations was 2092,33 seconds, but that differences 
appear in these times amounting to 94,3 seconds, or — part of 
the interval ; whereas in the dipping needle, in which the mean 
of the times required to perform one hundred vibrations was 
408,65 seconds, the greatest difference is only 5,7 seconds, or 
~ part of the interval, which is a much less proportional 
change than the former. As an additional confirmation, 
however, that the intensity of the earth’s magnetism is not 
subject to much variation, I have given in the following 
Table the results of observations I made on it at the same 
place in November, 1824, January and June, 1825. These 
exhibit the times in which the needle completed one hundred 
vibrations in the magnetic meridian, deduced from the mean 
of the times of its performing four hundred vibrations, with 
the face of the instrument on each side of the vertical, and 
the needle reversed on its axis in the two positions. 
Date. 
Middle Time of 
Observation. 
Mean time in seconds 
of performing 100 
vibrations. 
Temperature. 
Fab*. 
h. m. 
s. 
O 
November 8th 
A. M. IO 20 
404,94 
— i 3 l ; 
January loth 
A. M. II 45 
404,69 
— 22 
June 27th 
A. M. 9 30 
406,50 
+ 47 
These results also show, taking into consideration the dif- 
ferent temperatures under which they have been obtained, 
that little or no change in the intensity took place, notwith- 
standing the observations were made at different hours of the 
day, as well as at different parts of the year. 
