134 A CENTURY'S PROGRESS IN ASTRONOMY. 



tails has only one-fourth the power of gravita- 

 tion. It was concluded that the tails are formed 

 by particles of matter repelled from the comet 

 by the repulsive force of the Sun, and in tails 

 of the first type the velocity with which these 

 particles leave the body of the comet is four or 

 five miles a second. Bredikhine reached the 

 conclusion that the Sun's repulsive force is in- 

 variable, and that the different types of tails 

 are formed by the same force acting on different 

 elements. The numbers 12, 1, and J, are in- 

 versely proportional to the atomic weights of 

 hydrogen, hydrocarbon gas, and iron vapour. 

 Here, then, was the key to the mystery. Bre- 

 dikhine pointed out that in all probability the 

 first-type tails are formed of hydrogen, the second 

 of hydrocarbon, and the third of iron, with a 

 mixture of sodium and other elements. 



Within a few years of the publication of Bre- 

 dikhine's theory, five bright comets made their 

 appearance, and there was abundant chance of 

 testing the theory spectroscopically. In 1882 

 Well's comet was particularly studied at Green- 

 wich by Maunder, who discerned a sodium-line 

 in its spectrum. The magnificent comet which 

 appeared in 1882 was spectroscopically studied 

 at Dunecht in Aberdeen shire by Ralph Gopeland 

 (1837-1905), Astronomer-Royal of Scotland, who 



