2QO LIGHT SCIENCE FOR LEISURE HOURS. 



Figs. 1 and 2 exhibit a relation midway between these 

 later results and those tabulated above. 



Assuming an average difference of only three-quarters 

 of an inch in the northern and southern zones, between 

 latitudes 40 and 60, let us consider what is the dif- 

 ference of pressure on these two zones of the earth's 

 surface. The area of either zone is 21,974,260*5 square 

 miles, and the pressure on a square mile due to a 

 barometric height of three-quarters of an inch is about 

 670,000 tons, therefore the pressure on the northern 

 zone, between the latitudes named, exceeds the pressure 

 on the southern zone by no less than 14,500,000,000,000 

 tons. Including all latitudes within which there has 

 been ascertained to be a difference of barometric pressure 

 in the two hemispheres, we shall probably be within the 

 mark if we say, that the atmospherical pressure on the 

 northern hemisphere is 20,000,000,000,000 tons greater 

 than the atmospherical pressure on the southern hemi- 

 sphere. 



Such a peculiarity as this may almost deserve to be 

 spoken of in the terms applied by Sir J. Herschel to 

 the distribution of land and water upon our earth, it is 

 ' massive enough to call for mention as an astronomical 

 feature.'' I propose to examine two theories which have 

 been suggested in explanation of this feature of the 

 earth's envelope. These theories are founded on local 

 peculiarities, and the feature considered appears as a 

 dynamical one that is, as a peculiarity resulting from 

 states of motion in the aerial envelope. I shall endea- 

 vour to establish a theory founded on a consideration 



