MOUNT WILSON SOLAR OBSERVATORY. 287 



SUMMARY. 



The chief results of the year may be summarized as follows: 



(1) A tentative working hypothesis of sun-spots, proposed in the 

 annual report for 1912, has been tested by solar observations and 

 laboratory experiments. 



(2) As suggested by the hypothesis, the free end of a colunmar vor- 

 tex in water turns up to meet the surface, thus affording a possible 

 explanation of the formation of the second member in a bipolar spot- 

 group. 



(3) The tentative conclusion stated in the last annual report, that 

 the vortices of bipolar spot-groups in high and low latitudes rotate in 

 opposite directions, has been confirmed.^ 



(4) It follows from the observed directions of rotation that bipolar 

 groups, if semicircular vortices, should move toward the poles from 

 high latitudes and towards the equator from low latitudes. 



(5) The sun-spot measures of Carrington, Dyson, and Maunder 

 show small motions in latitude of the right sign, but their magnitudes 

 appear to be too low. 



(6) Kelvin's approximate formula for the velocity of vortex rings 

 has been confirmed in water and is now being tested in air at various 

 pressures in the hope of explaining this apparent discrepancy. 



(7) Secondary vortices, produced in smoke above single vortices in 

 water, show a circulation similar to that found by Evershed and St. 

 John in the solar atmosphere above sun-spots. 



(8) Right-handed and left-handed smoke vortices, set up by right- 

 handed and left-handed vortices in water, show characteristic asym- 

 metrical stream Unes in close agreement with the structure of he 

 hydrogen fioccuh above certain bipolar spot-groups. 



(9) In other cases these fields of force in the solar atmosphere 

 resemble more closely the lines of force about a bar magnet. 



(10) It therefore remains for further work to decide as to the pre- 

 ponderance of the hydrodynamic or the electromagnetic forces which 

 jointly define the structure of the hydrogen flocculi. 



(11) Hydrogen flocculi photographed simultaneously with the red 

 and ^dolet edges of Ha show remarkable differences in structure. 



(12) The characteristic structure and motions of prominences flow- 

 ing into sun-spots can be closely imitated in vertical sections of the 

 smoke vortices. 



(13) Prominences photographed simultaneously in calcium and 

 hydrogen light show interesting differences in structure. 



(14) The application of differential methods of observation has ren- 

 dered possible a marked increase in the accuracj^ of measurements with 

 the^spectrograph and spectroheUograph. ^ 



^See footnote on p. 258. 



