THE TWO COMETS OF THE YEAR 1868. 159 



intensity of the sun's action, to be the most heated por- 

 tion of a comet's volume. 



None of the comets which have been examined have 

 had a tail, so that we are unable as yet to form any 

 certain opinion respecting the nature of this portion of 

 a comet's volume. It seems almost certain, however, 

 that the tail shines by reflected light, because in every 

 known instance the tail has appeared as an extension 

 from the outer part of the coma, and may therefore be 

 expected to resemble that portion of the comet in its 

 general characteristics. 



One of the comets which has been examined with 

 the spectroscope, though it has not a visible tail, has 

 been shown to have an appendage of a very remarkable 

 character, respecting which, also, we have been able to 

 learn several interesting particulars. 



In the year 1866 a telescopic comet was discovered 

 by M. Tempel. This was the first comet examined by 

 Dr. Huggins. Its orbit was carefully calculated by the 

 German astronomer Oppolzer, and found to pass very 

 near the orbit of our own earth. Soon after this, Pro- 

 fessor Adams calculated the orbit of the November 

 shooting stars ; and to the surprise of the astronomical 

 world it was found that these minute bodies travel 

 along the very path in space which had been already 

 assigned to Tempel's comet. We need not here discuss 

 the circumstances of this discovery. Let it suffice to 

 state that all astronomers who are competent to form 

 an opinion on the subject are agreed that the Novem- 

 ber shooting-stars are certainly due to the existence of 



