628 
ME. B. STEWAET ON THE NATURE OF THE FOECES 
Table III. — Great disturbance extending from August 28 to September 7, 1859. The 
behaviour of peaks and hollows compared vsdth the general appearance of the curves. 
Time. 
Declination movements compared 
with those of the horizontal force. 
G-eneral character of the disturbance. 
Corre- 
spondence. 
Non-corre- 
spondence. 
Doubtful. 
1859. Augustus, 10 A.M. to 9 p.m. 
10 
0 
1 
The great disturbance had not yet com- 
menced. 
August 28, 10.30 p.M. to 
August 29, 9.52 a.m. 
0 
6 
0 
The great disturbance had now com- 
menced depressing the declination and 
raising both elements of the force. 
September 2, 4.50 a.m. to 
September 3, 11.50 a.m. 
0 
20 
2 
A second great disturbance commenced 
about September 2, 4.50 a.m., de- 
September 3, 1.40 p.m. to 
September 4, 10.30 a.m. 
9 
13 
3 
pressing the declination and raising 
both elements of the force, which 
September 4, 12.30 p.m. to 
September 6, 9-30 a.m. 
16 
3 
7 
changed into or was succeeded by one 
which seemed to impress all the ele- 
ments in the same manner. 
14. Before discussing the results in this Table, I ought to mention that in this great 
disturbance the rapid nature of the motion makes the comparison of simultaneous peaks 
and hollows a matter of some little uncertainty. On the other hand, these comparisons 
have been much facilitated by the exceediugly good definition which the labours of the 
late Mr. Welsh secured for the Kew curves, without which, indeed, an investigation of 
this nature would have been impossible. On the whole, I am well persuaded that the 
results in Table III. represent the truth. 
15. To recapitulate. It appears from Table I. that, studying merely the general result 
of a disturbance, there is a decided tendency to raise or lower the curves of all the elements 
simultaneously, this being stronger between the horizontal and the vertical-force curves 
than between either of these and the declination. 
From Table II. it appears that, if we leave out of account the great disturbance of 
August — September 1859, a peak of declination corresponds to a peak of horizontal 
force, and a hollow of the one to a hollow of the other, while the same correspondence 
holds still more strongly between the horizontal and vertical forces. 
Again, from Table III. it appears that the great disturbance of August — September 
1859 may be broken up into two. In one of these the general result was to lower the 
declination and raise both elements of the force, while in the second the result was to 
raise or lower all the three curves simultaneously. It also appears that, while the first 
of these disturbances prevailed, a declination hollow corresponded to a peak of either 
force, and that, on the other hand, while the second prevailed, a declination hollow corre- 
sponded to a hollow of either force. 
This very marked correspondence between the behaviour of the peaks and hollows 
and that of the general disturbing force in all cases, leaves, I think, little doubt that the 
former represent sharp and sudden changes in the intensity of the latter. 
