1870.] Astronomy. 519 



the conclusions would still be but slightly affected. Zollncr finds 

 the minimum temperature of the base of the solar atmosphere to be 

 about 27,700° Centigrade, and the corresponding temperature in the 

 region whence the prominences are projected to be about 68,400° 

 Cent. He calculates the pressure in this last-named region at 

 4,070,000 atmospheres, while the pressure outside the stratum en- 

 closing this region he calculates at 184,000 atmospheres. He then 

 shows that whereas these results have been obtained on the suppo- 

 sition that the pressure at the base of the chromosphere is but that 

 corresponding to about 7 inches (180 mm.) of the mercurial baro- 

 meter, the actual pressure of the nitrogen and oxygen atmospheres 

 at this level must be almost infinitely minute. But he remarks that 

 this alone does not suffice to explain the absence of the lines of these 

 elements from the spectrum of the sun, since the fines of volatilized 

 metals are seen. He attributes the visibility of the latter lines to 

 the fact that the vapours of the metallic and alkaline elements have 

 a much greater emissive power, and consequently a much greater 

 absorptive power than those of the permanent gases. 



To proceed further with the discussion of his researches would 

 bring us upon ground altogether removed from astronomy. We 

 may remark, however, that there is one point which seems to us to 

 have been too little considered in these and similar researches. It is 

 assumed that spectroscopic researches enable us to determine the 

 actual pressure at the base of the chromosphere. As a matter of fact, 

 we have no means of knowing whether the estimated pressure belongs 

 to the base of the chromosphere or to a height of ten, a hundred, or 

 even a thousand miles above the solar photosphere. It must be re- 

 membered that 1000 miles at the sun's distance subtends little more 

 than two seconds of arc, the minuteness of which distance will be 

 appreciated by those who have examined double stars two or three 

 seconds apart, even with very powerful telescopes. Assuming that 

 with the magnifying power employed (magnifying power being an 

 essential element in such applications of the spectroscope as we are 

 here considering) an arc of two seconds could be recognized, yet the 

 tenth part of such an arc would be wholly inappreciable. Now the 

 increase of pressure within a distance of 100 miles from the base of 

 the chromosphere is probably more considerable than that occurring 

 throughout all the thousands of miles of chromospheric height above 

 that level. 



The most important astronomical event of the next quarter is un- 

 doubtedly the great eclipse of December 22nd next. In England 

 it will be considerable though not total. The following are the data 

 for the principal places in the British Isles : — 



At Greenwich the eclipse will begin at 11 h. 8 m. a.m., reach its 

 greatest phase at 12 h. 25 m. (when • 814 of the sun's disc will be 

 concealed by the moon), and end at 1 h. 42 m. p.m. 



