THK S?6TS ON THE SUN^S DISC. 289 



cause a pressure 28 times greater than it would on our 

 globe. This great pressure compresses air as much as a tem- 

 perature of 8000 would expand it. 



In a still greater degree than this increased gravity do the 

 qualities peculiar to gases affect the height of the solar atmo- 

 sphere. In consequence of these properties, the density of 

 our atmosphere rapidly diminishes as we ascend, and increases 

 as we descend. Generally speaking, rarefaction increases in 

 a geometrical progression when the heights are in an arith- 

 metical progression. If we ascend or descend 2, 5, or 30 

 miles, we find our atmosphere 10, 100, or a billion times more 

 rarefied or more dense. 



This law, although modified by the unequal temperatures 

 of the different layers of the photosphere, and the unknown 

 chemical nature of the substances of which it is composed, 

 imust also hold good in some measure for the sun. As, how- 

 ever, the mean temperature of the solar atmosphere must 

 considerably exceed that of our atmosphere, the density of the 

 former will not vary so rapidly with the height as the latter 

 'does. If we assume this increase and decrease on the sun to 

 be ten times slower than it is on our earth, it follows that at 

 the heights of 25, 50, and 300 miles, a rarefaction of 10, 

 100, and a billion times respectively would be observed. The 

 solar atmosphere, therefore, does not attain a height of 400 

 geographical miles, or it cannot be as much as ^th of the 

 sun's radius. For if we take the density of the lowest strata 

 of the sun's atmosphere to be 1000 times greater that that of 

 our own near the level of the sea, a density greater than that 

 of water, and necessarily too high, then at a height of 400 

 miles this atmosphere would be 10 billion times less dense 

 than the earth's atmosphere ; that is to say, to human com- 

 prehension it has ceased to exist. 



This discussion shows that the solar atmosphere, in com- 

 parison with the body of the sun, has only an insignificant 

 height ; at the same time it may be remarked that on the 

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