438 METEORIC STONES AND SHOOTING STARS. 



Shooting stars seem to prevail equally in every climate and in every state of 

 the weather. They are occasionally seen at all seasons of the year, but more c 

 frequently in summer or at the end of the autumn. They appear usually to be ] 

 followed by a luminous train of intensely white light. 



A question will immediately arise, whether this be a real continued physical | 

 line of light, or whether it must not rather be ascribed to the same cause which, 

 makes us see a complete circle of light when a lighted stick revolves rapidly in 

 a circle. In that case the circle of light is not real, the effect being an optical 

 illusion. The membrane of the eye which is affected by light has been ascer- 

 tained to preserve the impression made upon it for about one tenth of a second 

 after the cause which produced that impression has ceased to act. We, conse- 

 quently, continue to see a visible object in any position for a tenth of a sec- 

 ond after it has left that position. If, then, a luminous object move over a cer- 

 tain space in one tenth of a second, the eye will see it at the same time in 

 every part of that space, and consequently, that space will appear one contin- 

 uous line of light. 



If, therefore, the luminous train which is visible after a shooting star, extends 

 through a space over which the star 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 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 continued 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. 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. 



Various and discordant have been the explanations offered of these luminous 

 trains. Some have ascribed them to an oily sulphurous vapor 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, explain these 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. 



In the year 1798 an investigation of the heights of shooting stars was un- 

 dertaken by Brandes, at Leipsig, and Benzenberg, at Dasseldorf. Having se- 

 lected a base line (about nine miles in length), they placed themselves at its ex- 

 tremities, on appointed nights, and observed all the shooting stars which ap- 

 peared, tracing their courses through the heavens on a celestial map, and 

 noting the instants of their appearances and extinctions by chronometers pre- ) 

 viously compared. The difference of the paths traced on the heavens afforded 

 data for the determination of the parallaxes, and consequently the heights and 

 the lengths of the orbits. On six evenings, between September and Novem- 

 ber, the whole number of shooting stars seen by both observers was 402 : of 

 these, 22 were identified as having been observed by each in such a manner 

 that the altitude of the meteor above the ground at the instant of extinction 

 could be computed. The least of the altitudes was about 6 English miles. Of 



