ON LIGHT. 283 



mous physical force in perpetual exertion at every point, 

 through all the immensity of space. 



(66.) As this is the conclusion we are landed in (for 

 tl.e evidence for the truth of the undulatory doctrine, or 

 something equivalent to it, accumulating, as we shall see, 

 in all quarters, and in the most unexpected manner re- 

 ceiving confirmation from facts utterly uncontemplated 

 by its originator, obliges us to look on this result as some- 

 thing more than a scientific rhodomontade) we shall 

 endeavour to present it to the conception of our readers 

 in a point of view which may enable them to realize it 

 more distinctly. All who know the nature of a baro- 

 meter are aware that the column of mercury 30 inches 

 in height sustained in its tube, is the equivalent of the 

 pressure of the aerial ocean which covers us, on its sec- 

 tional area ; and is just sufficient to counterbalance the 

 pressure, on an equal area, of an atmosphere five miles 

 in height of air everywhere of the same density as at the 

 surface of the earth. This height (five miles) is what is 

 termed in Barometry, " the height of a homogeneous 

 atmosphere," and affords a measure of what may be 

 called the intrinsic elasticity of the air, of an exceedingly 

 convenient nature ; and which is received as a kind of 

 natural unit, in Meteorology and Pneumatics. Substi- 

 tuting now light for sound, and for air the luminiferous 

 ether, we should have for the corresponding height of 

 our homogeneous atmosphere (gravity being supposed 

 uniform) five and a half billions of miles, or about one- 

 third of the distance to the nearest fixed star! The 

 measure thus afforded of intrinsic elastic power is of the 



