228 David Forbes — On Meteorites. 



matter thrown off from the meteor, as is seen on a small scale 

 with the magnesium light, is a problem as yet unsolved. The colour 

 of the light is usually white next the zenith, changing to bluish, or 

 sometimes reddish over the horizon, the nucleus being oftener red or 

 deep orange, whilst the tail is bluish or more rarely reddish. The 

 meteors of Nov. 7, 1799, and Oct. 23, 1801, emitted a greenish 

 light, thought by some to indicate the presence of copper. Although 

 the majority of meteors seen in Europe show primitive colovirs, it 

 appears from the Chinese records that lilac and other compound 

 colours are more frequent in that country. 



The luminosity of meteors is not regarded as d,ue to their being 

 actually burning bodies, but is usually attributed to their being sur- 

 rounded by an atmosphere rendered luminous through the enormous 

 pressure due to the rate at which they fly through the heavens 

 (a velocity of twenty miles per second being equal to a pressure of 

 100.000 atmospheres) ; an explanation not altogether satisfactory 

 however, seeing that whilst they are observed to be less luminous 

 after entering our atmosphere, they are very brilUant at altitudes 

 beyond it where the air must be so rarefied as to be nearly a vacuum 

 not containing ^-^-^J___ of oxygen ; a difficulty which made Poisson 

 suggest that the luminosity of meteors might be due to electrical 

 phenomena. Various calculations make the velocity af meteors to 

 be from sixteen to thirty-two miles per second. It is evidently 

 greater than that of the earth's rotation, since meteors are often seen 

 to, as it were, catch up and outstrip the earth. The enormous resist- 

 ance which bodies moving at such high speeds must encounter from 

 the air, when they once enter our atmosphere, sufficiently explains 

 why they do not do greater damage, or why, as at Hessle in Sweden, 

 the two pound stone which fell on the ice only scored it up some 

 three or four inches, and rebounded, instead of breaking through it. 

 Several meteorites, and especially the one last referred to, contain 

 considerable carbonaceous or combustible matter, and this may pos- 

 sibly explain why, in certain instances, meteorites have been seen to 

 fall or burst into fragments, and apparently burn away without 

 leaving any or but very little solid matter behind. 



Meteorites are known to have fallen in all climates, seasons of the 

 year, and hours of the day and night, but any generalization on 

 these points in the present incomplete state of our knowledge 

 would be premature ; it being only comparatively of late years that 

 attention has been directed to their registration, which in itself ex- 

 plains why only about 100 falls were recorded between the years 

 1000 and 1800, a period of eight centuries, whilst no less than 

 191 have already been observed in the first sixty years of the present 

 century, and it is more than probable that a far larger number fall 

 into the sea, at night, and in out-of-the-way places, than those which 

 come under scientific observation. Schreibers, however, taking the 

 number of instances recorded in the quarter of a century between 

 1790 and 1815, and the relation of the surface area on which they 

 fell to that of the entire earth, showed that about 700 meteorites fell 

 per annum, which is equivalent to about one in each year on a tract 



