302 LIGHT SCIENCE FOR LEISURE HOURS. 



other words, our column of air owes one-thirty-sixth 

 part of its height to the presence of aqueous vapour. 

 If we suppose this thirty-sixth part to flow off not 

 from the upper regions only, but in such a manner that 

 one complete thirty-sixth part of the volume of the 

 column should pass off then, instead of standing at a 

 height of thirty inches, the barometer would stand at a 

 height of 29 inches, less by only one-third of an inch 

 than the height of 29J inches due to the dry air alone. 

 Now we cannot, in accordance with Maury s theory, 

 legitimately add the five-sixths of an inch of barometric 

 pressure to the height of the barometer under a neigh 

 bouring column. For we have no evidence to show 

 that the air assumed to be expelled from the southern 

 temperate zone is heaped over the southern tropical 

 zone ; on the contrary, we have a barometer in the 

 latter zone not quite so high even as the barometer in 

 the corresponding northern zone. Therefore if air is 

 expelled in the manner supposed by Maury, it must be 

 distributed over a very much greater portion of the 

 globe s surface than it had been expelled from. Hence, 

 returning to our imaginary column of air, but a small 

 fraction of the five-sixths of an inch due to overflow 

 must be added to the barometer under a neighbouring 

 air-column. The latter barometer originally at 29 J 

 may be fairly assumed to rise at most to about 29-J 

 inches. We have, then, a difference of 29f 29^- 

 inches, or two-thirds of an inch ; so that despite all the 

 opposing considerations we have neglected, we still 

 have a difference less by one-third than that for which 



