80 Experimental Philosophy. [Lecture 7. 



therefore the height of the atmosphere must be 

 much greater than has appeared by the last cal- 

 culation, in which its density was supposed to be 

 every where as great as at the surface of the 

 earth. In order, therefore, to determine the 

 height of the atmosphere more exactly, geometri- 

 cians have endeavoured to determine the density 

 of the air at different distances from the earth. 

 The following sketch will give an idea of the 

 method which some have taken to determine this 

 density. 



If we suppose a pillar of air to reach from the 

 top of the atmosphere down to the earth's sur- 

 face ; and imagine it marked like a standard by 

 inches, from the top to the bottom ; and still 

 further suppose, that each inch of air, if not at 

 all compressed, would weigh one grain. The 

 topmost inch, then, weighs one grain, as it suffers 

 no compressure whatsoever ; the second inch is 

 pressed by the topmost with a weight of one 

 grain, and this added to its own natural weight 

 or density of one grain, now makes its density, 

 which is equivalent to the pressure, two grains. 

 The third inch is pressed down by the weight of 

 the two inches above it, whose weights united 

 make three grains ; and these added to its natu- 

 ral weight, give it a density of four grains. The 

 fourth inch is pressed by the united weight of the 

 three above it, which together make seven grains ; 

 and this added to its natural weight gives it a 

 density of eight grains. The fifth inch, being 



