Marsh.] ±^0 [March 6, 



had before been developed by the motion of the meteor itself. The 

 intensity of the light must of course, diminish with the loss of rela- 

 tive velocity, and altogether cease whenever the cloud and the air are 

 relatively at rest, or nearly so.* 



The motion of a meteoiic cloud, relatively to the air, m^y result either 

 from its own momentum, from atmospheric currents, or from the diur- 

 nal rotation of the atmosphere, in which the meteor, of course, had not 

 participated ; or from any or all of these causes combined, so that it 

 must in almost all instances be very considerable. 



The light of the aurora may perhaps in like manner be due to latent 

 heat; for although rarefied air is a very good conductor, it probably offers 

 resistance to the passage of electric currents sufficient to produce a 

 momentary condensation quite competent to illuminate their paths. 



It is evident that if the upper air be in the condition suggested, the 

 track of ev&ry mechanical impulse, traversing it with considerable velocity, 

 must become luminous. 



This igneous condition of rarefied air necessarily implies a definite limit 

 to the atmosphere of each member of the solar system : otherwise, meteors 

 — being constantly subjected to the action of latent heat — would b'e lumi- 

 nous, not merely when within one or two hundred miles of the earth, 

 but at all distances. 



The depth of highly rarefied air which a meteor can traverse before 

 becoming luminous, must of course, depend upon its velocity, tempera- 

 ture, and conducting power ; but the height at which their luminosity is 

 seen to commence must afi'ord some clue to the determination of the 

 height to which the atmosphere extends. 



The great comet of 1843, when in perihelion, Feb. 27, passed within 

 sixty thousand miles of the surface of the sun, at a velocity of about 850 

 miles per second, and the next day was seen " as a brilliant body within 

 less than two degrees of the sun."f 



It was not seen again until about seven o'clock on the evening of March 

 7, when although the tail was a very conspicuous object, the brilliancy of 

 the nucleus did not exceed that of a star of the third magnitude. 



This change, so much greater than could reasonably be expected to re- 

 sult from increased distance from the sun, occasioned great surprise, and 

 has not been satisfactorily accounted for. 



Is it not possible that its splendor was temporarily increased by the 

 latent heat developed during its passage through the solar atmosphere ? 



The great day-light meteor of Nov. 15, 1859, was seen at 9 o'clock in 

 the morning, in full sunshine, by persons who wei'e not within two 

 hundred and fifty miles of any portion of its path, appearing so very 

 bright that they thought it close at hand. Comparing the probable size 



*Prof. Newton remarks (Silliman's Journal, vol. 47, p. 407,) ""What kind of matter It 

 is which remains visible in the cold upper air for three-fourths of an hour until, by 

 gradual dissipation, the light fades out, I leave for others to say." 



tKirkwood'8 Oomets and Meteors, Phila.,1873, p. 17. 



