SCIENTIFIC SUMMARY. 
83 
The authors began by establishing the trustworthiness of Carrington’s 
sun-pictures, and then measured for each group the amount of spotted area, 
inasmuch as the method hitherto employed, namely, the mere statement of 
the number of sun-spots occurring at any period, 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 behaviour 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 measurement. 
One of the enquiries has reference to the relative distribution of spotted 
area over different parts of the solar disk. The word disk is used in contra- 
distinction to surface , because it is evident that, on account of the sun’s 
rotation, the centre of his visible disk on one day does not represent the 
same portion of the solar surface as on another day ; indeed from this cause 
it is well known that sun-spots travel over the visible disk from left to right. 
It is therefore one enquiry 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 parts of his apparent 
disk. 
It follows from the investigations— for details of which we must refer to 
the paper itself — that, in the first place, during the time embraced in a series, 
the amount of spotted area which crosses one ecliptical longitude is different 
from that which crosses another — that is to say, the average size of a spot 
varies with the ecliptical longitude. 
In the second place , there seems to be a periodical recurrence of the same 
sort of behaviour, the period of recurrence of the same behaviour appear- 
ing to be nineteen or twenty months. 
In the third place, in all these recurrences the progress of the maximum 
is from left to right, not right to left. The authors continue : — 
O y C 
“We cannot see that these phenomena can possibly be explained unless 
it be admitted that the behaviour of sun-spots is subject to some external 
influence, the nature of which will best be determined by the order of 
recurrence and length of period of the phenomena in question. 
11 In the first place, it is clear that the influence is not stationary, other- 
wise its period would be one year, that being the time in which the earth 
(which must be regarded as the standpoint from which these phenomena 
are viewed) accomplishes one revolution round the sun. Again, since the 
march of the phenomena is from the left to the right of the earth, this would 
seem to identify the influence with one of the inferior planets which passes 
over the sun’s disk in this direction, the superior planets going the opposite 
way. 
“ The period of twenty months will now enable us to determine which of 
the inferior planets exercises the predominant influence on sun-spots. We 
have to ask which of the two inferior planets takes twenty months to return 
to the same position with respect to the earth. This evidently points to 
Venus, for which the synodical period is 583 days, or between nineteen and 
twenty months. We may remark that, apart from all observation, if we 
suppose the various planets to affect the behaviour of sun-spots, the influence 
of Venus should be very great, on account of its nearness to the sun com- 
bined with its very considerable size The average size of a spot 
YOL. YI. — NO. XXII. 
II 
