AT MUNICH, 1840, 1841, 1842, 607 
of magnetism in the deflecting magnets) still require some cor- 
rections, but these would not materially alter the final result. 
In the month of May some particular disturbance appears to 
have taken place. It was probably instrumental, and, for the 
present at least, I think it better to make no use of the results 
of that month. 
The comparison of November and December 1841 with the 
_ same months in 1842, givesa yearly increase of horizontal inten- 
sity of 00048, which, supposing the total intensity to remain 
_ unaltered, would correspond to a decrease of about 4' in the in- 
clination. 
Applying this increase of intensity to the numbers in Table 
_ VII., and leaving out the month of May, we obtain the intensity 
reduced to the middle of 1842 as follows :— 
1842. Jan—March . . . . . 1°9326 
April—June. . . . . . 1°9311 
July—September. . . . 1°9298 
October—December. . . 1:9317 
An annual period appears clearly indicated, having its maxi- 
mum in March and its minimum in September. 
The movements of the intensity differ from those of the de- 
clination, in so far that the mean intensity (in analogy with the 
height of the barometer) continues for several days together 
greater, and then again less, while the mean declination on se- 
_ yeral successive days never seems to remain constantly above or 
below the true mean. 
Term Observations. 
It has been thought hitherto that term observations may be 
employed 
a. For determining the diurnal movement. 
b. For determining the mean declination. 
ce. For determining the mode and manner in which disturb- 
ances display themselves simultaneously at different places. 
The last-named object has not been attained by this class 
of observations, because no great disturbance has yet occurred 
during a term, and indeed the probability of such a coincidence 
is but small *, 
[* At Toronto one of the greatest disturbances which have been yet ob- 
served took place during the term-day of May 1840: simultaneous observations 
made by Captain Ross’s expedition at Kerguelen Island show that it extended 
VOL, III, PART XII, 2s 
