634 
ME. B. STEWAET ON THE NATTJBE OF THE FOECES 
27. It appears from this Table that the proportional value of the declination-element 
is very nearly constant during a disturbance, but that it varies much from one disturb- 
ance to another. The constancy of this element during the same disturbance will 
perhaps be best seen by comparing the values in the declination column with those in 
the column of the horizontal force, as the vertical-force changes being small, any error of 
measurement is very apt to alter the proportion between them and the larger changes 
with which they are compared. 
28. It will also be noticed that the great disturbance of August — September 1859 
seems to consist of two disturbances superimposed, one being of the normal type, but the 
other, as regards the declination-element, being decidedly abnormal. This agrees well 
with the results of Table III. 
29. It would thus appear that in the great majority of cases only one type or group 
of forces operates in producing a disturbance, and that the various individual forces 
which compose this group have a very small range as regards their mode of action upon the 
three elements of the earth’s magnetism at Kew. We may therefore (approximately at 
least) represent a disturbance by a single force ; and for this purpose it is competent for 
us to average for each disturbance the results of Table V. 
30. Now that force which affects the declination is evidently the horizontal component 
of the disturbing magnetic force which acts in a direction perpendicular to the magnetic 
meridian, and its value will be X tan hd, where X denotes the absolute horizontal force, 
and the angular displacement of the declination magnet. But an inch of the declination- 
ordinate represents 22', hence the disturbing force which would cause a change to this 
amount will be =3*8 tan 22'= ‘024, while, as we have abeady seen, an inch of change 
in the horizontal-force ordinate denotes a disturbing force =‘038. Again, decreasing 
ordinates represent increasing westerly declination ; and since (except in one case) the 
three curves are simultaneously affected in the same direction, it follows that (with the same 
exception) a magnetic force acting from magnetic south to north must be compounded 
with one acting from magnetic east to west, and with one acting vertically downwards. 
But, for the anomalous disturbance of August — September 1859, a force acting from 
magnetic south to north must be compounded with one acting from magnetic west to east. 
Applying now the ordinary rules for combining forces, we obtain the following Table, 
in which the astronomical azimuth and dip of the various disturbing forces are given. 
31. It will, however, be first necessary to allude to a peculiarity in this method of de- 
termining the direction of the disturbing force. This is, that our results apply in strict- 
ness only to those small and rapid changes which occur as it were on the surface of the 
great disturbance-wave. 
We might, for instance, be engaged in studying the direction of that force which would 
occasion a small depression which we had observed to occur simultaneously in the three 
curves, while at the same time these curves might be elevated above their normals, and 
not depressed by the main body of the disturbing force. We have, however, in art. 15 
given grounds for supposing that these small and rapid peaks and hollows denote 
