ON TERRESTRIAL GALVANIC CURRENTS. 
217 
results for the same nominal hour on numerous days was to be used, it was better to 
measure the ordinate (whether of the curve or of the zero formed by breaking connexions) 
by one scale (a scale of inches was in fact used) ; to take the means of all corresponding 
to the same hour ; and then to multiply the means by the two factors obtained from 
the theory explained in the former paper. This being done for the two Galvanic 
Curves, the results were combined in the way explained in the former paper to exhibit 
the Inferred Northerly Force and the Inferred Westerly Force. 
The general multiplier of the geometrical factors used in the former investigation was 
determined tentatively, to satisfy this condition, that on the whole the magnitudes of the 
sudden changes of the large ordinates of the curves representing Inferred Northerly Force 
and Inferred Westerly Force should be sensibly equal to the similar magnitudes in the 
curves given by the Magnetometers. Considering it as proved that the great disturbances 
are really produced by the galvanic currents, it is evident that we have thus a fairly 
accurate scale for converting galvanic indications into magnetic forces (referred, as is 
done all through, to the total horizontal magnetic force as unit), which will also apply 
accurately to the days of tranquil magnetism. Also, the zero-indications being formed 
in the same way for the disturbed and the tranquil days, any error which we may dis- 
cover in the zeros for tranquil days, or in the references of the ordinates to those zeros, 
will apply to the zeros or references of disturbed days. 
I now proceed with the numerical treatment of the observations of the tranquil days. 
The readings in inch-measures of the galvanic ordinates for each nominal hour being 
grouped by months, and, where there were observations in the same months in different 
years, the different years being combined, the means were taken, and were converted 
into Magnetic Forces by the following formulae 
In the scale of Horizontal- Force Photograms, O'Ol of Horizontal Force is represented 
by2'3565 inches (Introduction to Greenwich Magnetical and Meteorological Observations, 
1866 and 1867); and for the graduation of “Scale E for Hartford,” the value - X graduation 
2*3565 in dies 
of Horizontal-Force Photogram (Phil. Trans. 1868, p. 470), or QV 6 go 59 x 0-5437 (P* ^69) 
must be used. This number reduced gives for Scale E for Hartford, 
O'Ol of Horizontal Force = 6'350 inches. 
Similarly, for Scale F for Croydon, 
0-01 of Horizontal Force = 7'768 inches. 
For Scale G for Hartford, 
0-01 of Horizontal Force=5'471 inches. 
For Scale II for Croydon, 
O'Ol of Horizontal Force = 4'901 inches. 
With these elements, Tables were prepared for converting inch-measures into measures 
of Horizontal Force. In the original adaptation to base-lines below the photographic 
curves, the measures with E and F, both used negatively, were to be added, to form 
