SPECTROSCOPIC OBSERVATIONS OF THE ROTATION OF THE SUN. 101 
The probable value of a difference “ obs. — comp.” is in all the groups approximately 
the same, viz., +0°010 km., whereas the probable errors of an observed single value of v 
are respectively +0°013, 0°013, 0°018. The formula therefore represents the obser- 
vations in all the groups with sufficient accuracy. The constancy of ¢ is a somewhat 
significant feature. If it should be confirmed by future observations, it would almost 
certainly point to the conclusion that the formula is not purely empirical, but has some 
physical meaning. A decision on this question would be highly important, for if the 
rotation of the surface layers be actually governed by a law which has its analytical 
expression in the formula here adopted, we may conclude that swrface phenomena are 
the cause of these peculiarities. I shall not dwell, however, on this at present, but 
shall now draw attention to the result of these computations with regard to the 
constants a and b of our equation. Here we find a satisfactory agreement between the 
values for 1887-9 and 1901-2. These are also the two groups which have exactly 
the same position in the sun-spot cycle, both embracing a time of minimum activity. 
On the other hand, as already pointed out, the year 1903 was characterised by 
vigorous displays of spots and solar eruptions. Simultaneously with this activity we 
notice an enormous change in the values of the constants of our equation. The 
“retardation” of the higher latitudes appears now to be reduced to less than half its 
former amount. ‘This is a novel and, I am sure, unexpected result. So far, it is true, 
it can only be said to represent a coincidence in time, but I trust the discussion has 
made it sufficiently clear that the discrepancies observed, since they can in no way be 
ascribed to observational errors, must, in all probability, be interpreted as a new and 
very peculiar feature of the still mysterious mechanism of the sun. Care has been 
taken throughout not only to investigate all possible errors which may arise from 
deficiencies of the instrument, and to eliminate such errors from the reduced values 
of the observations, but the measurements were also arranged in such a way that 
the possible remainders of such errors which could not be elicited in the reductions 
should have a minimum effect on the results. 
The possible bearing of an investigation of this kind on important questions of solar 
physics makes it very desirable that the observations should not only be continued at 
this Observatory, but that other observatories should also take part in them. Next to 
the question of possible alterations in the amount of solar heat, an answer to which may 
now be expected from the ingenious researches of Professor Lanctry and his staff, the 
problem of solar rotation should command the greatest attention from the part of 
solar physicists, for no other seems to me so well adapted to give us information on the 
mechanism of the solar forces. That the periodic play of these forces should in some 
manner be conducive to changes of the distribution of the moments of rotation is a 
logical conclusion on which I trust astronomers are unanimous. It is well to remark in 
this connection that the idea has already received theoretical consideration in an in- 
teresting paper by Mr Empen. 
The construction of the apparatus required for these observations is well within the 
