146 REPORTS OF inve;stigations and projects. 



COMPUTING DIVISION. 



The vv^ork of the computing division, under the direction of Mr. Adams, 

 has progressed very satisfactorily during the year. Miss Ware has devoted 

 all of her time to the measurement of the heliographic positions of flocculi 

 with the heliomicrometer, for the purpose of determining the solar rotation 

 period. The results obtained indicate that the heliomicrometer is admirably 

 adapted for this work, though the proper motions of many flocculi are so 

 great as to suggest that a less precise mode of measurement might serve 

 nearly as well for a study of the mean velocity of the solar rotation. How- 

 ever, it has not yet seemed advisable to modify the system employed, since 

 the discussion of the proper motions themselves, which could not be so well 

 determined with a less accurate measuring instrument, may yield results of 

 value. In case it should become desirable to modify the apparatus so as to 

 permit greater rapidity of operation, it will only be necessary to rule one 

 hemisphere of the globe of the heliomicrometer with meridians and parallels 

 1° apart. The approximate positions of the flocculi can then be estimated 

 (as in the globe-measuring machine used at the Yerkes Observatory) with 

 some gain in rapidity, but with corresponding loss in precision. The selec- 

 tion of flocculi for measurement, as at present carried out, requires consid- 

 erable time, since the greatest care is exercised in the choice of objects which 

 do not undergo material change of form during the interval between the 

 plates. Less care in this regard would probably result in little loss of 

 precision in the determination of the mean rotation period, but, as in the 

 case of less accurate measures, the resulting positions would not be suf- 

 ficiently precise to yield accurate values of the proper motions. The results 

 of the first year's work, which are now under discussion, should leave no 

 doubt as to the most effective means of continuing this investigation. 



As stated in my last report, it is desirable to obtain measurements of the 

 areas covered by the calcium flocculi, to serve as an index to the solar activity. 

 In the first method devised for this purpose the photograph was divided into 

 a series of squares 10° on a side, the total area included extending 40° in 

 latitude and 40° in longitude on each side of the sun's center. The inter- 

 sections of the squares were marked by a dot of ink on the glass side of the 

 plate with the aid of the heliomicrometer. A quantity representing the areas 

 and the intensities of the principal flocculi in each square was measured 

 photometrically. The density of the background was also measured, in 

 order to eliminate large errors which would otherwise result. It was found, 

 however, that the quantity remaining after the background had been elimi- 

 nated was too small to permit its variations, from day to day or from square 

 to square, to be followed with the necessary precision. For this reason a 

 simpler method of procedure was adopted. A piece of plain glass is placed 

 in front of the negative and the principal flocculi in each square are painted 

 out with black paint. Very faint or small flocculi are not included. The 



