KNOWLEDGE OF THE SUN — ST. JOHN 183 



means an outflow of the gases from tho spot tangential to the sun's 

 surface. An extensive series of observations at the Mount Wilson 

 Observatory gave the data for a more complete analysis of the 

 motions. 



The results are graphically given in the accompanying diagram 

 (fig. 1), which shows a vertical section of the surroundings of a 

 spot, the axis of the vortex lying in the ])lane of the paper. Im- 

 mediately above the shaded umbra, the gases in which the low-level 

 lines of iron and those of the heav}' elements originate are flowing 

 outward with velocities proportional to the lengths of the heavy 

 lines. At higher levels the velocity of outflow decreases and be- 

 comes zero for the strongest and highest-level lines of iron, the 

 level of velocity -inversion lying at about 1,200 km. Above this 

 level the motion is inward, the velocity increasing with height. The 

 field-forming vortex is below the level of the umbra and in it the 

 gases are rising as in a terrestrial tornado and spreading outward 

 above the photosphere. The high-level cliromosi)here layers tend 

 to settle, owing to the decreased pressure of radiation over the spot 

 caused by the cooling of the expanding gases, and form a secondary 

 vortex, in which the flow, as shown by Doppler displacement, is 

 inward. The magnetic lines of force are indicated in the usual way 

 jis if diverging from the pole of a magnet. As seen from the earth 

 in the red light of hydrogen the high-lying cyclone appears as in 

 Plate 4, Figure 2, which pictures a horizontal cross section of it. 



ORIGIN OV FRAUNHOFER LINES 



Spectrum lines are due to transitions between two energ}?^ states of 

 an atom, the frejpiency of the emitted or absorbed radiation being 

 exactly proportional to the difference in energy between the two 

 states. Every absorption line is associated with the transition from 

 one energy state to another of higher energy content and can only be 

 produced when the atom is already excited to the lower energy state 

 of the transition to which the line is due, measured by the excita- 

 tion jxjtential expressed in volts. The absorbed energy, hv, corre- 

 sponding to the line is diffusely scattered or reradiated without change 

 of wave length. This alone would give completely black lines, but 

 the effect of collisions of the "second kind" is to supply radiation 

 to the middle of the line so that none are completely black; this 

 residual intensity is what makes tiie spectroheliogiam possible. 



LEVELS 



Lines in the solar spectrum give informatitm of the conditions ob- 

 taining in the region of their origin. Like infornuition concerning 

 the conditions is given by lines originating in the same layer, but 



