A. MATHEMATICS AND ASTRONOMY. 35 



as to its suitableness for a permanent astronomical observa- 

 tory. He seems to have devoted himself mainly to his fa- 

 vorite branch, solar spectroscopy, in which he was eminently 

 successful. He gives a list of no less than 273 lines which 

 he has determined satisfactorily, hardly a tenth part of which 

 were ever seen by any other observer. He conceives that 

 the dark lines always seen in the spectrum have their origin 

 at the base of the chromosphere, and that, with proper in- 

 strumental power and favorable atmospheric conditions, they 

 might all be seen reversed to bright lines at any time. The 

 variations of brightness were very considerable and sudden 

 when the chromosphere was much disturbed. Sometimes 

 one set of lines would be particularly bright, and at other 

 times another. In addition to the elements formerly known 

 to exist in the solar atmosphere, the following seem to be 

 pretty positively indicated namely, sulphur, cerium, and 

 strontium, while zinc, erbium and ytrium, lanthanum and 

 didymium, are indicated with a less degree of probability. 



EEPOKT ON EXCKE'S COMET. 



The Washington Observatory has lately published a re- 

 port, by Professors Hall and Harkness, of observations on 

 Encke's comet during its recent return. It was first seen at 

 Washington on the 11th of October last, and continued to 

 be observed on favorable nights until the 7th of December. 

 The observations on the movements and relations of the 

 comet are detailed by Professor Hall, while the spectroscop- 

 ic investigations were conducted by Professor Harkness. 

 The results of the latter are summed up in the following 

 propositions : 



1. Encke's comet gives a carbon spectrum. 



2. From November 18 to December 2 the wave length of 

 the brightest part of the second band of the comet's spec- 

 trum was continually increasing. 



3. No polarization was detected in the light of the comet. 



4. The mass of Encke's comet is certainly not less than 

 that of an asteroid. 



5. The density of the supposed resisting medium in space, 

 as computed from the reserved retardation of Encke's comet, 

 is such that it would support a column of mercury some- 

 where between ttttt an( l tWit of an inch high. 



