COMETS. 133 



senting the intensity of the repulsion fell into 

 three classes. This was the first hint of a 

 classification of cometary tails. Meanwhile he 

 carefully studied the tails of comets both from 

 direct observation and from drawings. 



In 1877 he wrote: "I suspect that comets 

 are divisible into groups, for each of which the 

 repulsive force is perhaps the same." Subsequent 

 investigations led Bredikhine to divide the tails 

 of comets into three types. The first type con- 

 sists of long, straight tails, pointed directly away 

 from the Sun, represented by the tails of the 

 great comets of 1811, 1843, and 1861. In the 

 second type, represented by Donati's and Coggia's 

 comets, the tails, although pointed away from 

 the Sun, appear considerably curved. In the 

 third type the tails are, to quote Miss Clerke, 

 " short, strongly-bent, brushlike emanations, and 

 in bright comets seem to be only found in com- 

 bination with tails of the higher classes." 



In 1879 Bredikhine fully developed his com- 

 etary theory. Assuming the reality of the re- 

 pulsive force, he concluded that to produce tails 

 of the first type, the repulsion requires to be 

 twelve times greater than the solar attraction ; 

 the production of tails of the second type 

 necessitates a repulsive force about equal to 

 gravity ; while the force producing third - type 



