LUMINOUS TRAIN OF THESE STARS. 



it for about one-tenth of a second after the cause which pro- 

 duced that impression has ceased to act. We, consequently, 

 continue to see a visible object in any position for a tenth of a 

 second after it has left that position. If a luminous object 

 move over a certain space in one-tenth of a second, the eye will, 

 therefore, see it at the same time in every part of that space, and 

 consequently that space will appear one continuous line of light. 



If, then, the luminous train which is visible after a shoot- 

 ing star, extends through a space over which the star has 

 moved in one-tenth of a second, it is then possible that such 

 luminous train may be illusory, being a mere optical effect of 

 the rapid motion of the star. But if it be longer than this, or if 

 it be visible in any one place for more than the tenth of a second 

 after the star has moved from that place, then it cannot be 

 explained on this principle, and must be admitted to be an actual 

 train of light. Now it is stated by observers of these meteors 

 that the trains are sometimes seen for several minutes. In the 

 case of actual fire-balls, Dr. Olbers observed trains which con- 

 tinued visible for six or seven minutes, and Brandes in one 

 instance estimated that fifteen minutes elapsed between the 

 extinction of the fire-ball and the disappearance of the luminous 

 train. Admiral Krusenstern, in a voyage round the world, 

 saw the train of a fire-ball, which continued to shine for the 

 space of an hour after the ball itself had disappeared, during 

 which interval the train appeared almost stationary. 



In general, the trains have the same hollow, cylindrical 

 appearance as the tails of comets, their inner part appearing to be 

 void of luminous matter, and a further resemblance to comets is 

 exhibited in the curved form, which they sometimes assume. 



4. Various and discordant have been the explanations offered of 

 these luminous trains. Some have ascribed them to an oily 

 sulphurous vapour existing in the atmosphere, which, being 

 disposed in thin layers, and becoming inflamed, would exhibit 

 the appearance of a brilliant spark passing rapidly from point to 

 point. Beccaria and Vassali considered them to be lines of 

 electrical sparks ; an hypothesis, however, which has been 

 abandoned. Lavoisier, Volta, and others explained theae meteors, 

 by supposing that hydrogen gas, accumulated, by its lightness, 

 in the higher regions of the atmosphere, was inflamed. But the 

 general law of gases, which gives them a tendency to mingle, 

 notwithstanding the effect of their specific gravities, puts aside 

 this hypothesis. 



5. In the year 1798, an investigation of the heights of 

 shooting-stars was undertaken by Brandes, at Leipsig, and 

 Benzenberg, at Dusseldorf. Having selected a base line (about 



L 2 147 



