152 Observations on the Tails of Halley^s Comet. 



of the one now seen ; and that this which is now seen would, if it 

 were nearer to the earth, appear to constitute a kind of tail to the 

 other. Yet, notwithstanding the indistinctness arising from distance, 

 the head of the comet I should judge to be brighter than it was two 

 weeks since. 



Concluding remarks. — Length of the tail by direct and indirect 

 vision. — It appears from the preceding observations, that on those 

 days in October in which the length of the tail was observed in 

 these two modes, its mean length by direct vision, was 5° ; by indi- 

 rect vision, ITj degrees ; i. e. as one to three and a half* In a ma- 

 jority of instances, the length by indirect vision was between two 

 and four times that by direct vision. 



Now the tails of some comets are said to have exhibited instanta- 

 neous variations in length, like the coruscations of an aurora borea- 

 lis ; and this circumstance has led some astronomers to consider them 

 of an electrical nature. But has not this phenomenon been rather 

 physiological than physical ? Has it not been chiefly occasioned by 

 a change in the position of the eye ? The other hypothesis presents 

 insuperable difficulties, when we consider the distances of comets, 

 and the immense velocity with which electricity must move, to pro- 

 duce any sensible and sudden variation of length, at such distances 

 from the observer. It will be seen from the preceding observations, 

 that immense and instantaneous variations of length were observa- 

 ble by us at almost any time when the tail was visible. They were 

 observed hundreds of times ; so that a tail eight or ten millions of 

 miles in length, would frequently become in a second of time twenty 

 or thirty millions. 



Mean lengths, 1 : 3.5, i. e. 5° 17.5° 



