Vol. 6, 1920 INTERNATIONAL ASTRONOMICAL UNION 
371 
quent local disturbances of a definite type in the sun's reversing layer, a 
subject still under investigation. 
The formulae of Faye represent satisfactorily the relation of velocity 
to latitude as given by the observations of Duner, Adams, Plaskett and 
DeLury, and Schlesinger, but observations in the higher latitudes should 
be greatly extended both in number and in range of latitude. The failure 
of the Faye formula to represent Hubrecht's observations is probably due 
to the very limited time covered by them. 
The only published results relating to a difference in the rotation law 
in the two hemispheres are those of Hubrecht. His earlier paper (1912) 
referred to in the report at the Bonn meeting showed an excess of the ve- 
locities in the southern over the northern hemispheres, but in the later 
publication (1915) covering the same epoch he comes to the opposite re- 
sult basing his conclusions upon Tunstall's measurement of his plates to 
the exclusion of his own. The question of a difference between the two 
hemispheres is one of great interest and need not wait for solution until 
all discrepancies between observers are settled as it is a relative matter 
for any one observer and instrument. It, therefore, offers an inviting 
field of investigation at the present time, and is on the working program 
at Mount Wilson. 
The different rates of rotation for different levels in the solar atmos- 
phere shown by the Mount Wilson observations of Adams appear again 
in preliminary reductions of the present Mount Wilson series which show 
relatively high velocities for the strong magnesium triplet in the green, 
for 4227 and the H and K lines of calcium, but low values for the lines of 
lanthanum and of the nitrogen (cyanogen) band at 3883. The observa- 
tions of Schlesinger published since the Bonn meeting agree with those of 
Plaskett and DeLury, Storey and Wilson, and Hubrecht in showing no 
positive evidence of differences in rotational velocity. It is to be noted 
that in the Mount Wilson observations lines differing widely in intensity, 
representing great differences in level, have been employed. As differ- 
ences in rotational velocity must be directly related to differences in level, 
such lines, notwithstanding the difficulties of measurement, offer a valuable 
means of detecting it. The great majority of lines of medium intensity 
of the common elements originate in not greatly different levels, and for 
such lines differences in rotation and velocity would be small and corre- 
spondingly difficult to detect. 
Conditions in the earth's atmosphere have been suggested by DeLury 
as possible causes of divergent results in observations for solar rotation. 
He finds evidence of the effect of an over-lapping sky spectrum in some of 
his observations at Ottawa, and attributed to it the large variations in 
rotational velocity at different times and all differences found for different 
elements and lines. 
That a large proportion of superposed skylight might appreciably re- 
