1874 ] i-Zo [Marsh. 



soTjVuitli of a grain. Hence, the heat acquired by a meteoric mass whose 

 cross section is one square foot, in moving one mile, wovild be one grain 

 raised 7i degrees, or one-fifth of a grain 2,500° in 70 miles. This 

 temperature would undoubtedly be suificient to render meteoric bodies 

 brilliantly luminous," 



The above is a very clear statement of the resistance theory, which is 

 the only one which seems to have met with general acceptance. But 

 when we consider that the heat resulting from the collision of the atmos- 

 pheric molecules with the surface of the meteorite, being developed at 

 that surface, must be to a great extent absorbed by the meteorite ; and 

 that, in the case supposed above, a body more than one foot in diameter 

 had to travel seventy miles, to develop heat competent to raise one-fifth 

 of a grain to a temperature less than that of melting iron, we must con- 

 clude that, at the height of 140 miles, the resistance theory utterly fails 

 to account for any luminosity whatever. 



In order to give some definite form to the discussion of the compara- 

 tive effects of resistance and of latent heat in the production of meteoric 

 luminosity, let us, with Kirkwood, suppose a globular meteor of one 

 square foot section to enter the atmosphere with a velocity of thirty 

 miles per second. 



In traveling one mile, it will sweep a cylindrical space one mile long 

 containing 5280 cubic feet, all the air in which will be compressed to a 

 density at least as great as that of air at the surface of the earth, and be 

 carried forward in front of the meteor. When in approaching the earth 

 denser strata are reached, some portion of the air will of course be 

 merely pushed aside and left behind, the air piled up in a conical mass in 

 front of the meteor, dividing the atmosphere, just as the sharp bow of a 

 vessel divides the water and thus diminishes the resistance ; but at great 

 heights, if the velocity be great, this effect may be neglected. 



Heat will be develoijed at the forward surface of the meteor, firstly — 

 from the resistance of the air, which converts into heat a portion of the ki- 

 netic energy or motive power of the meteor ; its amount, at any given velo- 

 city, depending upon the weiglit of the air met ; secondly — from the release 

 of latent heat, the amount of which depends only on the hulk of the air met. 



The mere mention of the fact that the heating power of "resistance'' 

 depends upon the weight ; and that of "latent heat" upon the bulk of 

 the air encountered, shows the great advantage which the latter has at 

 extreme heights. "Latent heat" is at its maximum at the extreme 

 upper limit of the atmosphere, where there is no appreciable weight of 

 air to absorb the heat developed at the surface of the meteor. Its whole 

 energies are therefore expended on the meteor, the surface layer of which 

 may be so heated as to cause it to burst out in full splendor very soon 

 after entering the atmosphere, and at a height where the weight of air 

 encountered is so infinitesimally small that the effects of "resistance" 

 are not perceptible : but no luminosity can be expected from either source 

 until the heat developed is sufficient to produce incandescence, both in 

 the surface layer of the meteor, and in its atmospheric envelope. 



