Every portion of the Earth's substance in its daily rotation; describes acurve which is in the main a resultant of that resistance which checks itsnearer approach to the centre of gravity, that momentum which would carryit off at a tangent, and those forces of gravitation and cohesion which keepit from being so carried off. When with this axial motion is contemplatedthe orbital motion, the course of each part is seen to be a much more involvedone. And we find it to have a still greater complication on taking into accountthat lunar attraction which mainly produces the tides and the precessionof 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 unceasingmovements in the Earth's atmosphere; descend to the slow alterations in progresson its surface; and then to the still slower ones going on beneath.
Masses of air absorbing heat from surfaces warmed by the Sun, expand,and ascend: the resistance being less than the resistance to lateral movement.
Adjacent atmospheric masses, moving in the directions of the diminished resistance,displace the expanded air. When, again, by the ascent of heated air fromgreat tracts like the torrid zone, there is produced at the upper surfaceof the atmosphere a protuberance -- when the air forming this protuberanceoverflows laterally towards the poles; it does so because, while the tractiveforce of the Earth is nearly the same, the lateral resistance is diminished.
And throughout the course of each current thus generated, as well as throughoutthe course of each counter-current flowing into the space vacated, the directionis always the resultant of the Earth's tractive force and the resistanceoffered by the surrounding masses of air: modified only by conflict withother currents similarly generated, and by collision with prominences onthe Earth's crust. The movements of water, in both its gaseous and liquidstates, furnish further examples. Evaporation is the escape of particlesof water in the direction of least resistance; and as the resistance (whichis due to gaseous pressure) diminishes, the evaporation increases. On theother hand condensation, which takes place when any portion of atmosphericvapour has its temperature much lowered, may be interpreted as a diminutionof the mutual pressure among the condensing particles, while the pressureof surrounding particles remains the same; and so is a motion taking placein the direction of lessened resistance. In the course followed by the resultingraindrops, we have one of the simplest instances of the joint effect of thetwo antagonist forces. The Earth's attraction, and the resistance of atmosphericcurrents ever varying in direction and intensity, give as their resultants,lines which incline to the horizon in countless different degrees and undergoperpetual variations. In the course the rain-drops take while trickling overthe surface, in every rill, in every larger stream, and in every river, wesee them descending as straight as the antagonism of surrounding objectspermits. So far from a cascade furnishing an exception, it furnishes butanother illustration. For though all solid obstacles to a vertical fall ofthe water are removed, yet the water's horizontal momentum is an obstacle;and the parabola in which the stream leaps from the projecting ledge is generatedby the combined gravitation and momentum.
The Earth's solid crust undergoes changes which supply another group ofillustrations. The denudation of lands and the depositing of the removedsediment in new strata at the bottoms of seas and lakes, is a process throughoutwhich motion is obviously determined in the same way as is that of the watereffecting the transport. Again, though we have no direct inductive proofthat the forces classed as igneous expend themselves along lines of leastresistance, yet what little we know of them is in harmony with the beliefthat they do so. Earthquakes continually revisit the same localities, andspecial tracts undergo for long periods together successive elevations orsubsidences: facts which imply that already-fractured portions of the Earth'scrust are those most prone to yield under the pressure caused by furthercontractions. The distribution of volcanoes along certain lines, as wellas the frequent recurrence of eruptions from the same vents, are facts oflike meaning. §78. That organic growth takes place in the direction of least resistance,is a proposition set forth and illustrated by Mr. James Hinton, in the Medico-ChirurgicalReview for October, 1858. After detailing a few of the early observationswhich led him to this generalization, he formu1ates it thus: --"Organic form is the result of motion."
"Motion takes the direction of least resistance.""Therefore organic form is the result of motion in the direction ofleast resistance."
