Intelligence and Miscellaneous Articles. 303 



Johnson, upon this subject, and very recently a work by Mr. Pogson 

 on the valuations of magnitudes has appeared. From this we learn 

 that a star of the fifth magnitude is about two hundred and fifty 

 times more brilliant than a star of the eleventh magnitude. Thus 

 the illumination of the atmosphere by the moon is much more intense 

 than the illumination of the cometary substance by the sun itself, 

 since it would be necessary to render the comet three thousand six 

 hundred times more luminous to enable it to extinguish a star of the 

 eleventh magnitude, whilst the luminosity of the atmosphere illumi- 

 nated only by the moon is sufficient to render invisible stars which 

 are two hundred and fifty times more brilliant. 



The disproportion becomes still more striking when we consider, 

 that, according to the measurements of Wollaston, to which Sir John 

 Herschel says he sees no objections to be made, the illumination of 

 the full moon is a little less than the eight hundred thousandth part 

 of the full illumination of the sun. 



To complete the data of our definite calculation, we shall call 

 to mind, that, according to the density of the air in the lower strata 

 of the atmosphere and its total weight as indicated by the barometric 

 column, the whole stratum of air which constitutes the atmosphere 

 is equivalent to a stratum of about 8 kilometres in thickness, and 

 possessing the density of the air at the surface of the earth. 



We have alseady found that it would be necessary to render the 

 comet 3600 times more luminous for it to extinguish the lustre of a 

 star of the eleventh magnitude. To render a star of the fifth magni- 

 tude invisible, it would require to be made 3600 x 250 times more bril- 

 liant than it is. In other words, if the atmosphere were 3600 X 250 

 times less compact than it is, it would be equivalent to the comet. 



As 3600 X 250 make 900,000, the nine hundred thousandth part 

 of the atmosphere wovdd suffice to produce the same eflfect of illu- 

 mination as the comet ; but as the latter is in the full light of the 

 sun, whilst the atmosphere is only illuminated by the moon when it 

 extinguishes stars of the fifth magnitude, this circumstance gives the 

 atmosphere a further advantage in the proportion of 800,000 to 1 ; 

 which under ordinary circumstances gives the atmosphere a supe- 

 riority equal to 900,000 x 800,000, or 720 billions. 



But this is not all : the thickness of the cometary substance being 

 500,000 kilometres, whilst that of the atmosphere is only 8 kilo- 

 metres, we must increase the above relation in the proportion of 

 500,000 to 8, which brings it to forty-five millions of billions, thus — 



45,000,000,000,000,000. 



Thus, according to these data, the density of the substance of a comet 

 could not be calculated at so high a quantity as that of the atmo- 

 sphere diminished by the enormous divisor, forty-five millions of 

 billions. The shock of a substance so rarefied would be nothing at 

 all, and not the least particle of it could penetrate even into the 

 most rarefied parts of our atmos])here. 



According to experiments of my own, gases lose their property of 

 elasticity long before they are reduced to such a low density. I do 



