Observations on the Tails of HaUey^s Comet. 153 



Again, in estimating the rapidity with which a comet's tail is de- 

 veloped, we are liable to err, from a neglect of meteorological, as 

 well as physiological influences. Estimates have been made in re- 

 lation to the number of millions of miles to which the matter of a 

 comet's tail has been projected, during the interval between the ob- 

 servations on two consecutive nights. By overlooking or underesti- 

 mating the immense influence, which slight changes in the transpa- 

 rency of the atmosphere must have on the apparent length of such 

 faintly luminous bodies, we might draw from several of the prece- 

 ding observations, astonishing conclusions with regard to the velocity 

 with which this luminous matter is projected. We might thus infer 

 the projection of luminous matter to the extent of fifteen or twenty 

 millions of miles in a single day ; for the tail of Halley's comet, ac- 

 cording to some of the preceding observations, must have appeared 

 to receive such an augmentation of length in a single day.* The 

 greatest observed length was real, and indeed from the gradual man- 

 ner in which the brightness diminished, it must have been less than 

 the actual length. It is not improbable, from the length of the tail 

 at different altitudes, that, in the absence of terrestrial atmosphere, 

 it would have presented a length of some hundreds of millions of 

 miles. Even the part actually seen on the 16th, if it had been di- 

 rected toward the earth at the time of the comet's nearest approach, 

 must have nearly reached us, and by gravitation, (if it were ponder- 

 able,) mingled with our atmosphere. What effects may sometimes 

 be produced in the planetary atmospheres in this way, is not known. 



By what has been said in relation to sources of exaggeration, re- 

 specting this developement in length, it is not intended to deny that 

 it does take place with astonishing rapidity, as comets approach the 

 sun. This has been often observed. 



Direction of the principal tail. — By referring to those of the pre- 

 ceding observations, made when the direction of the tail was seen, 

 and its place among the stars recorded, it will be found that it was 

 never directly opposite the sun, but always inclined towards the re- 

 gion from which the comet was departing. This was always the 

 case when the position was observed and recorded, as it was on ten 



* The angular lengths above given would, with the comet's place in its orbit, 

 enable one to calculate the absolute lengths ; but what has been said above in rela- 

 tion to this last point, has been founded only on hasty and approximate estimates. 



Vol. XXXL— No. 1. 20 



