ASTRONOMY: HALE AND LUCKEY 
387 
in the same direction, would tend to retain a columnar form, and the 
second spot of the group would not appear. Other conditions favorable 
to the production of single spots are low angular velocity of the vortex 
(similar in effect to the above case), a large ratio of diameter to length, 
and the presence within the sun of persistent eruptive centers, such as 
are suggested by the frequent recurrence of spots in the same region of 
the photosphere. It should be noted that convection may be a factor 
in prolonging the Hfe of single spots. 
If bipolar spots are semi-circular vortices, they should exhibit the 
proper motion characteristic of vortex rings. This motion, as already 
remarked, is at right angles to the plane of the ring, and in a direction 
corresponding with that of its inner edge. In a paper read before the 
Academy at its last autumn meeting,^ it was shown that high and low 
latitude bipolar spot groups rotate in opposite directions. These di- 
rections are such that low latitude groups should move toward the equa- 
tor, while high latitude groups should move toward the poles. 
An investigation of the motion of spots in latitude was made many 
years ago by Carrington, who says: 
In the above table it will be remarked that there is more distinctly a trace 
of motion in latitude, the signs being on the whole + for latitudes higher N. 
or S. than 20°, though the daily polar motion between 20° and 40° of latitude 
on an average does not exceed 2', a quantity which could only be deduced 
from the totality of a large number of single results. Between the parallels 
of 10° to 20° the motion in latitude is evidently very small; but the signs are 
generally negative and a feeble tendency towards the equator of about 1' 
per diem is indicated. Within 10° of the Equator on either side no reliable 
motion in latitude appears to exist, the signs varying much and the mean 
results being of less weight.^ 
These results relate to all spots, both single and multiple, whose mo- 
tions were observed by Carrington. His conclusion as to the reversed 
directions of motion of high and low latitude spots has recently been con- 
firmed by Dyson and Maunder, who discussed the extensive observa- 
tional material comprised in the Greenwich sun-spot measures from 1874 
to 1912. They remark: 
"A shght general tendency is suggested for high-latitude spots in 
either hemisphere to move away from the equator, but for those in lower 
latitudes to move toward it."^ 
This apparent confirmation of the hypothesis should not be taken 
too seriously, as it remains to be seen whether a closer comparison, ex- 
cluding single spots and grouping the results in strict conformity with 
the observed polarities of bipolar pairs, would be equally favorable. 
