AT MUNICH, 1840, 1841, 1842. 605 
Mean Declination, its Yearly Diminution and Monthly 
Fluctuation. 
A high or a low state of the barometer and thermometer 
usually prevails for several days, and often for half a month to- 
gether: but with the magnetic declination (which is otherwise 
subject to as many accidents as atmospheric pressure and tem- 
perature) this is not the case ; the movement on one day may 
differ greatly from that on another, but the daily means do not 
_ on that account differ considerably from each other. 
Table V. contains the mean declination for periods of ten days, 
_ and the yearly decrease of declination obtained by subtracting 
_ each ten-day mean from the corresponding determination of the 
_ previous year. We see that the values found for the yearly de- 
crease present differences of some magnitude, but not appa- 
rently governed by any law. This is also seen in Table VI., 
which exhibits the monthly means. 
Combining the observations in periods of four months, we 
obtain the following general view :— 
West Declination. Diff. 
1840, Aug.—Nov.. . . 16 593) 9.) 
1840-41. Dec.—March . . ... 57°2 1 2-8 
April—July . . . «. 54°4) 9-7 
Aug.—Nov. . . «os Sen 6 
1841-42. Dec—March .. ... 50°1 
April—July . . . «. 48-3} LA 
Aug.—Nov. . . . «+. 45°6 jae 
These numbers give no reason to assume an annual period; 
they show, on the other hand, that the secular decrease is not 
uniform but has periods. Present observations do not yet de- 
termine whether these periods are subject to a law or are acci- 
dental. 
Diurnal Movement of the Horizontal Intensity. 
Table III. shows the march of the horizontal intensity as 
given by the monthly means. In order to eliminate irregulari- 
ties, a second calculation has been made omitting the days of 
disturbance, and the results are exhibited in Table IV. The 
march represented in the accompanying Plate is obtained from 
the combination of periods of sixty days. The leading charac- 
teristics of the intensity curves are,—minimum between 9 and 
