LECTURE VII. 



EXPERIMENTAL PHILOSOPHY. 



THE PHENOMENA OF THE ATMOSPHERE. 



THE word phenomenon, the plural of which 

 stands at the head of this lecture, and which we 

 shall frequently have occasion to use, means 

 simply an appearance. It is derived from the 

 Greek verb PHAINOMAT, which signifies to ap- 

 pear; but it is generally used to imply any 

 striking or remarkable appearance. The atmo- 

 sphere was before explained t'o mean that mass of 

 air which surrounds the earth. Various con- 

 iectures have been made with respect to the 

 neigm 01 me CILUIOO^U.^ . nn( ^ as we know to a 

 certainty the relative weight of a column of ti*e 

 atmosphere by the height to which its pressure 

 will raise water or mercury in an empty tube, so 

 different calculations have been founded on these 

 data, to ascertain its extent as well as its density 

 at different heights. If the air of our atmosphere 

 were indeed every where of an uniform density, 

 the problem would be very easily solved. We 

 should, in that case, have nothing more to do, 

 than to find out the proportion between the 

 height of a short pillar of air, and a small pillar 

 of water of equal weight ; and having compared 



