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V.—Spectroscopic Observations of the Rotation of the Sun. By Dr J. Halm, 
Assistant Astronomer at the Royal Observatory, and Lecturer in Astronomy at 
the University, Edinburgh. Communicated by Tur AsrronomMER Roya. FOR 
ScorTLAND. 
(MS. received February 19, 1904. Read March 21, 1904. Issued separately May 4, 1904.) 
The causes of the peculiarities of the solar rotation exhibited by the superficial layers 
of the sun’s body must still be considered unknown, notwithstanding recent interesting 
attempts at explanation. This is no doubt partly due to the difficulties of the hydro- 
dynamical problem placed before the mathematician in an investigation of such complexity 
as the movements of particles in a rotating fluid subject to energetic convection. But it 
must also be conceded that the observational data available for a basis of mathematical, 
or even speculative, research are still so scanty, that for this reason alone we may 
perhaps not feel surprised at the failure so far of theoretical attempts. 
The first empirical demonstration of the peculiar law which apparently governs the 
rotation of our luminary was given by CaRRINGTON, whose important investigations were 
taken up and extended by Sporrer. Their observations brought to light the main 
character of the peculiarity of solar rotation, viz., the decrease of the angular velocity 
from the equator towards the poles. ~ An objection has, however, been raised against 
the value of their results with regard to the general rotation of the sun’s photosphere, 
on the ground that they were obtained from the observed movements of solar spots. 
It was justly urged that spots have “proper motions” which preclude their adoption as 
points of reference. Besides, we must remember that these spots were visible only 
within an equatorial zone of about +50° or 40° latitude, and that therefore the polar 
regions remained inaccessible by this method. This dithiculty and limitation was over- 
come by the ingenious application of the spectroscope to the problem, which we owe to 
Professor Dunér. His conclusions are based on the displacements shown by the 
Fraunhofer lines at the solar limb, where the gases producing these absorptions are 
carried by the rotation either towards us or from us. His results are, it is true, not 
directly comparable to those derived from the movements of the spots, because both refer 
most probably to different levels, and therefore perhaps to different conditions of motion ; 
but the great advantage of the spectroscopic method seems to me to lie in the fact that 
we always measure at the same level, wherever this level may be—a point on which we 
are by no means certain in the case of the spots. Besides, we are independent of the 
uncontrollable vicissitudes of proper motions, and we are able to extend the investiga- 
tions from the equator to the immediate vicinity of the poles. The results obtained by 
Professor Duns&R may be summarised by the statement that the retardation of the 
angular velocity discovered by CaRRINGTON and SpoErRER was found to be also shared 
by the photospheric layer emitting the Fraunhofer lines, and that the amount of this 
TRANS. ROY. SOC. EDIN., VOL. XLI. PART I. (NO. 5). 15 
