43 



more detailed explanation than any of the other phenomena 

 connected with his subject, he gave a lucid description of the 

 formation and development of the tail, in which, he said, that, as 

 the comet approaches the sun, a violent action commences, the gas 

 bursts forth in jets from the coma toward the sun, and is instantly 

 driven back again, as smoke from a locomotive going at a great 

 speed is driven back on its path. Immediately the jets change 

 their position and direction, and a tail is thus formed, consisting 

 of the vapour thrown off by the combustion of the coma, most 

 likely by some inherent power of the sun or comet, and rendered 

 visible by solar light. In support of this view he read a letter 

 which he had received from Rev. Dr. Robinson, of Armagh. Mr. 

 M'Minn having given an exhaustive explanation of the physical 

 properties of comets, went on to say that Dr. Halley was the first 

 to discover that comets. have stated periods of revolution round 

 the sun — a fact he ascertained from computations made on the 

 appearance of a comet in 1682, from which he found that it 

 revolved round the sun once in seventy-five years. Since his time 

 the periods of revolution of hundreds of these bodies have been 

 found out. The next comet to which he alluded was Encke's. 

 He said that the most remarkable circumstance in connection 

 with this body was the gradual diminution of the length of the 

 period of its revolution, and brought forward several hypotheses to 

 account for the phenomenon, amongst which he mentioned the 

 "resisting medium," the diminution of volume, and also that its 

 retardation might be due to an electric influence. The reader 

 next referred to the appearance of Biela's comet in 1832, and its 

 supposed collision with the earth, and pointed out the reasons 

 why there was ground for such a supposition ; and, in the discus- 

 sion of the matter, stated that there was little danger of such an 

 event occurring, the probability being, according to Arago, that, 

 out of 281,000,000 chances, only one was unfavourable. In 

 conclusion, the reader expressed the opinion that it was impossible 

 to deal with the purpose for which comets were designed, and 

 that these speculations led to the confines of the unknown, in 

 which it was wisdom to confess ignorance. 



