398 A Short History of Astronomy [CH. xili. 



atmosphere would be sufficient to melt a shell of ice 

 1 20 feet thick covering the whole earth; according to 

 Dr. Langley, the thickness would be about 160 feet.* 



308. With his return to England in 1838 HerschePs 

 career as an observer came to an end ; but the working out 

 of the results of his Cape observations, the arrangement 

 and cataloguing of his own and his father's discoveries, 

 provided occupation for many years. A magnificent volume 

 on the Results of Astronomical Observations made during the 

 years 1834-8 at the Cape of Good Hope appeared in 1847 ; 

 and a catalogue of all known nebulae and clusters, amount- 

 ing to 5,079, was presented to the Royal Society in 1864, 

 while a corresponding catalogue of more than 10,000 double 

 and multiple stars was never finished, though the materials 

 collected for it were published posthumously in 1879. Jhn 

 Herschel's great catalogue of nebulae has since been revised 

 and enlarged by Dr. Dreyer, the result being a list of 7,840 

 nebulae and clusters known up to the end of 1887 ; and 

 a supplementary list of discoveries made in 1888-94 

 published by the same writer contains 1,529 entries, so that 

 the total number now known is between 9,000 and io,oco, 

 of which more than half have been discovered by the two 

 Herschels. 



309. Double stars have been discovered and studied by 

 a number of astronomers besides the Herschels. One of 

 the most indefatigable workers at this subject was the elder 

 Struve ( 279), who was successively director of the two 

 Russian observatories of Dorpat and Pulkowa. He 

 observed altogether some 2,640 double and multiple stars, 

 measuring in each case with care the length and direction 

 of the line joining the two components, and noting other 

 peculiarities, such as contrasts in colour between the 

 members of a pair. He paid attention only to double stars 

 the two components of which were not more than 32" apart, 

 thus rejecting a good many which William Herschel would 

 have noticed ; as the number of known doubles rapidly 

 increased, it was clearly necessary to concentrate attention 

 on those which might with some reasonable degree of 



* Observations made on Mont Blanc under the direction of 

 M. Janssen in 1897 indicate a slightly larger number than Dr. 

 Langley 's. 



