Researches on Solar Physics. 323 
riod, can only be supposed to afford very approximate means of 
estimating the extent of solar activity at that period; while, 
again, if we wish to study the behavior with respect to size of 
each group as it passes over the visible disk, this can only 
be done accurately by the laborious but sure method of meas- 
urement. 
36. Method adopted in measuring Carrington’s groups.—In order 
to accomplish this task, the following method was adopted :—In 
the first place, in order to obtain the apparent area of any group, 
a piece of plate glass had a number of lines etched upon it, by 
means of which it was cut up into squares, the side of each 
Square being ;th of an inch. In order to facilitate reading, 
each fifth line was painted red. 
This piece of glass was then applied (the engraved face toward 
the drawing) to the group whose apparent area it was desired to 
measure, and the number of squares and fractional parts of a 
Square occupied by the umbra, the penumbra, and the whole 
Spot was separately reckoned and noted down. If it was found 
that the number of squares reckoned for the whole spot was 
equal to the sum of those reckoned for the umbra and penum- 
although not used by us in our present research. 
7. But it is evident that after the apparent area of a group 
has thus been correctly estimated, this apparent area will not 
indicate the real size of the group, unless allowance is made for 
the foreshortening occasioned by its angular distance from the 
visual center of the disk. 
[The practical methods by which this allowance for foreshort- 
ening was made are given in detail. The final results of the 
measurements form an extensive table and give the material for 
& graphical representation of the observed spotted area for each 
clear day from the beginning of 1854 to the end of 1860.) 
40. Distribution of Spotted Area over Disk.—Our next inquiry 
has reference to the relative distribution of spotted area over 
different parts of the solar disk. We use the word disk in con- 
tradistinction to surface, because it is evident that, on account of 
the sun’s rotation, the center of his visible disk on one day d 
not represent the same portion of the solar surface as on another 
day; indeed from this cause it is well known that sun-spc 
travel over the visible disk from left to right. It is there ore 
one inquiry to study from day to day the relative distribution 
of spotted area over different parts of the sun’s actual surface, 
and another to study the same from day to day over different 
