10 SCIENCE IN SHORT CHAPTERS. 



still unable to detect any indication of any limit to its ex- 

 pansibility. 



The most remarkable part of Dr. Wollaston's paper is 

 the reductio ad absurdum by which he seeks to finally demon- 

 strate the finite extent_of our atmosphere. He maintains, 

 as I do, tluit if the elasticity of our atmosphere is unlimited, 

 its extension must be commensurate with the universe, that 

 every orb in space will, by gravitation, gather around itself 

 an atmosphere proportionate to its gravitating power, and 

 that, by taking the known quantity of the earth's atmos- 

 phere as our unit, we may calculate the amount of atmos- 

 phere possessed by any heavenly body of which the mass is 

 known. On this basis Dr. Wollaston calculates the atmos- 

 phere of the sun, and concludes that its extent will be so 

 great as to visibly affect the apparent motions of Mercury 

 and Venus, when their declination makes its nearest ap- 

 proach to that of the sun. No such disturbance being 

 actually observable, he concludes that such an atmosphere 

 as he has calculated cannot exist. In like manner he cal- 

 culates the atmosphere of Jupiter, and finds it to be so 

 great, that its refraction would be sufficient " to render the 

 fourth satellite visible to us when behind the centre of the 

 planet, and consequently to make it appear on both (or all) 

 sides at the same time." 



On examining these calculations, I have discovered the 

 very curious error above referred to. As this is a matter 

 of figures that cannot be abridged, I must refer the reader 

 to the original calculations. I will here merely state that 

 Wollaston's method of calculating the solar gravitation at- 

 mosphere and that of Jupiter and the moon leads to the 

 monstrous conclusion that, in ascending from the surface 

 of the given orb, we always have the same limited amount 

 of atmospheric matter above as that with which we start- 

 ed, although we are continually leaving a portion of it 

 below. 



Wollaston's mistake is based on the assumption that, 

 under the circumstances supposed, the atmospheric pres- 

 sure and density, at any given distance from the centre of 

 the given orb, will vary inversely with the square of that 

 distance. As the area of the base upon which such pressure 



