238 THE DIRECTION OF MOTION. 



of all the forces engaged; and how this line becomes 

 more complicated in proportion as the forces are multi- 

 plied. If instead of the motions of the planets and satel- 

 lites as wholes, we consider the motions of their parts, we 

 meet with comparatively complex illustrations. Every por- 

 tion of the Earth's substance in its daily rotation, describes a 

 curve which is in the main a resultant of that resistance 

 which checks its nearer approach to the centre of gravity, 

 that momentum which would carry it off at a tangent, and 

 those forces of gravitation and cohesion which keep it from 

 being so carried off. If this axial motion be compounded 

 with the orbital motion, the course of each part is seen to be 

 a much more involved one. And we find it to have a still 

 greater complication on taking into account that lunar at- 

 traction which mainly produces the tides and the precession 

 of the equinoxes. 



§ 77. We come next to terrestrial changes: present ones 

 as observed, and past ones as inferred by geologists. Let us 

 set out with the hourly-occurring alterations in the Earth's 

 atmosphere ; descend to the slower alterations in progress on 

 its surface; and then to the still slower ones going on be- 

 neath. 



Masses of air, absorbing heat from surfaces warmed by 

 the sun, expand, and so lessen the weight of the atmospheric 

 columns of which they are parts. Hence they offer to ad- 

 jacent atmospheric columns, diminished lateral resistance; 

 and these, moving in the directions of the diminished resist- 

 ance, displace the expanded air; while this, pursuing an up- 

 ward course, displays a motion along that line in which there 

 is least pressure. When again, by the ascent of such heated 

 masses from extended areas like the torrid zone, there is pro- 

 duced at the upper surface of the atmosphere, a protuber- 

 ance beyond the limits of equilibrium — when the air form- 

 ing this protuberance begins to overflow laterally towards 

 the poles; it does so because, while the tractive force of the 



