51 
the intensity of magnetic forces, ^c. 
From the mean obtained here, it appears that the terres- 
trial magnetic intensity was the least between lo and ii 
o'clock in the morning, the time, nearly, when the sun was 
on the magnetic meridian ; that it increased from this time 
until between 9 and 1 o'clock hi the evening; after which it 
decreased, and continued decreasing during the morning 
until the time of the minimum. 
Having by this reduction of the observations made within 
doors, determined the nature of the changes in the direction 
of the needle in that situation, independent of the changes 
which took place in the temperature of the magnets, and 
thence deduced the diurnal changes in the intensity of the 
terrestrial forces acting upon the needle, I shall now detail 
similar observations which I made in the open air, for the 
purpose of comparing with them, when these had also been 
cleared of the effects due to changes in the temperature of 
the magnets, in order to determine how far there was any 
thing anomalous in the directions of the needle when in doors 
and when in the open air. I have already mentioned that, 
for the purpose of making these observations, the apparatus 
was placed on a table fixed firmly in my garden, the mag- 
nets being placed in earthen pans containing water. The 
observations were made in the same manner as those in 
doors, excepting that, as the magnets were here liable to 
greater changes of temperature, their temperatures were 
noticed at the beginning, and also at at the conclusion of each 
of the observations ; they are contained in the following table, 
where the time set down is that at which the observation 
commenced, the time occupied in making the whole of each 
being from four to six minutes. 
