218 
THE ASTRONOMER ROFAL 
Northerly Magnetic Tendency; and the measures with G taken positively and H taken 
negatively were to be added, to form Westerly Magnetic Tendency. In the present 
operations, in which all measures were taken from the photographic base-lines, which 
reversed the direction of measure for Croydon, E was to be applied to the complement 
of actual measure of ordinate to 5 inches, and F, G, H to the actual measures of ordi- 
nates. I have entered into these minutiae with the view of facilitating any future refe- 
rence to the calculations* preserved in the Eoyal Observatory. 
The reading for the zero of each curve (or the indication when the galvanic communi- 
cations are broken) is found by taking the mean of a group of such zeros, as far as 
there appears to be no probability of instrumental change ; and these readings are 
treated in the same way as those for the ordinates of the curves. Subtracting these 
mean zeros from the mean monthly ordinates at each nominal hour, the first Tables 
of Hourly indications of the Magnetic Effect of Galvanic Currents are formed. It is im- 
portant to observe that all the numbers have the positive sign. 
It will be remarked that, in a few instances, the register has been defective at 
one or two hours ; in those cases it has been thought best still to use the registered 
hours, though imperfect in number, in the formation of means. The number of days 
employed to form the means for different hours is thus somewhat variable, as is indicated 
by the list at the bottom of the Tables. The “Means for the whole day” will appear 
again, in a following Table, and will explain an important difficulty. 
* la the original calculations, by a mistake, E was applied to the actual measures of ordinates, and E, G, H 
to the complements of measure to 5 inches. The error was corrected by changing the signs of all the ultimate 
results. 
