[PLASKETT-DELURY] THE SOLAR ROTATION , 6 
the lines are of much better quality for measurement than at X 5600. 
When the probable errors (in kilometres) are reduced to linear measure 
they become more than twice as great at À 5600 as at À 4250. The 
probable errors for a single line obtained at Mt. Wilson are 
p.e. = = 0.015 km. per sec. (1906-7). 
Die —— = 00 a mea loos) 
The Ottawa value as above stated is = 0.015. It must not be forgotten, 
however, that the Mt. Wilson values are from one or two plates, the 
Ottawa from the mean of all the plates; On the Mt. Wilson plates 
the lines giving, systematically, velocities differing from the mean were 
excluded, on the Ottawa plates these and all lines were included; and 
lastly that the Mt. Wilson linear dispersion was in 1906-7, 10 per cent. 
and in 1908 30 per cent. greater than the Ottawa. Hence it is evident that 
the probable error of measurement is about the same at the two places. 
Although the probable error of a plate determined from the agreement 
among the plates is not given, it is readily computed, and for the 
equator (1908) is = 0.011 km. per sec. as compared with * 0.018 here. 
This is considerably smaller, but yet about 5 times that obtained from 
agreement among the lines. 
28. It is evident from the ratios of the probable errors that a great 
many more lines than necessary for the actual determination of the 
rotation have been measured, and that it would be preferable to measure 
four or five times as many plates with only one fourth or fifth the 
number of lines, and that even then the probable error obtained from 
comparison of the plates would be twice that deduced from the internal 
agreement of the lines. However, in this investigation a larger number 
of lines was measured for the purpose of determining whether different 
elements and different lines of the same element give different velocities 
of rotation. 
SYSTEMATIC DIFFERENCES OF VELOCITY FOR DIFFERENT ELEMENTS. 
29. Considerable attention has been devoted to this phase of the 
investigation which is of importance not only because of its interest 
in the theory of the sun, but also because it was one of the questions 
proposed by the Rotation Committee, and because Adams has found 
some small systematic differences for different elements and his results 
should be confirmed. 
As previously mentioned in the A 5600 region the lines were 
chosen particularly with this point in view and include as large a 
number of elements as is possible among the limited number avail- 
