424 
ME. B. STEAVAET ON THE GEEAT MAGNETIC DISTUEBANCE 
they afford the means of obtaining a continuous photographic register of the state of 
the three elements of the earth’s magnetic force — ^iiamely, the declination, and the hori- 
zontal and vertical intensity. Reduced representations of the traces furnished by these 
instruments during the great disturbance under discussion accompany this paper ; and it 
will now be necessary to give a short description of these. 
In the originals we have for each element, for each day, a straight line and a curved 
one. The straight or zero-line serves as a line of abscissae, along which the time is 
reckoned ; and if from any point of this line denoting a certain time an ordinate be 
drawn to the corresponding point of the curved line, the length of this ordinate will 
represent the state of the magnetic element at this time. 
The register is taken from 10 a.m. of one day to the same hour of the next, and the 
curve proceeds (in point of time) from the left to the right of the paper. In full size, 
the length of the zero-line is about 18 inches for twenty-four hours, so that three-quai ters 
of an inch denote one hour ; but on the reduced scale appended to this paper, three- 
tenths of an inch denote one hour. The exact Kew mean time corresponding to the 
commencement and end of each curve is stated at the conclusion of this paper. Increasing 
ordinates denote decreasing westerly declination, decreasing horizontal, and decreasing 
vertical force. In the reduced scale which accompanies this paper, a change of one inch 
in the ordinate represents a change of 55' in the declination, while for the horizontal 
force it denotes a change equal to 0237 of the whole, and for the vertical force a change 
equal to -006 of the whole. 
Referring to the curves, it will be seen that the first disturbance commenced about 
half-past ten on the evening of the 28th of August, affecting all the elements simulta- 
neously. It will also be observed that for the early part of this day, before the disturb- 
ance commenced, the curves present a peculiar serrated appearance. This is a pheno- 
menon which often precedes and follows large disturbances. 
At about 7^ P.M., August 29, the violence of the disturbance had somewhat abated, 
and things remained nearly in this state until 5 A.M., September 2, about which time 
another very abrupt disturbance simultaneously affected all the elements, continuing 
with great violence until about 4 p.m. of the same day, when it became somewhat less. 
The elements, notwithstanding, remained in a state of considerable disturbance until 
September 5, and scarcely attained their normal state even on September 7 or 8. 
A graphical representation of the amount of disturbance is furnished by the dotted 
line, or line of normals, which accompanies each curve. 
These normals have been furnished through the kindness of General Sabine, by whom 
they were calculated. They denote the probable position which the curve correspond- 
ing to each element would have occupied, had not disturbance supervened. 
The normal for the declination for any hour is a mean of the daily observations at 
that hour during August and September, after the omission of all disturbed observations 
which differ from the final normal by an amount equal to or exceeding 3'’ 3. 
The normals for the horizontal and vertical force have been obtained in a somewhat 
