PROGRESSIVE EFFECTS. 303 



it is no longer by the addition of equal quantities in equal times ; the 

 quantities added are unequal, and eventhe quality nyiy now be differ- 

 ent. If the change in the state of the pennanent cause be progressive, 

 the effect will go through a double series of changes, arising partly 

 from the accumulated action of the cause, and partly trom the changes 

 in its action. The effect is still a progressive effect, produced, how- 

 ever, not by the mere continuance of a cause, but by its continuance 

 and its progressiveness combined. 



A familiar example is afforded by the increase of the temperature 

 as summer advances, that is, as the sun draws nearer to a vertical 

 posititju, and remains a greater number of hours above the horizon. 

 This instance exemplifies in a very interesting manner the twofold 

 operation on the ettect, arising from the continuance of the cause and 

 from its progi'essive change. When once the sun . has come near 

 enough to the zenith, and remains above the horizon long enough, to 

 give more warmth during one dim'nal rotation than the counteracting 

 cause, the earth's radiation, can caiTy off, the mere continuance of] the 

 cause would progressively inci'ease the effect, even if the sun came no 

 nearer and the days gi'evv no longer ; but in addition to tliis, a change 

 takes place in the accidents of the cause (its-series of diurnal posi- 

 tions), tending to increase the quantity of the effect. When the suni- 

 mer solstice has passed, the progressive change in the cause begins to 

 take place the reverse way ; but, for some time, the accumulating 

 effect of the mere continuance of the cause exceeds the effect, of the 

 changes in it, and the tempei-ature continues to increase. ■ 



Again, tlife motions of a planet are a progressive effect, produced 

 by causes at once permanent and progressive. The orbit of a planet 

 is determined (omitting perturbations) by two causes: first, the action 

 of the central body, a- permanent cause, wliich alternately increases 

 and diminishes as the planet draws nearer to or goes further from its 

 perihelion, and wliich acts moreover at every point in a- difllerent direc- 

 tion; and, secondly, the tendency of the planet to continue moving in 

 the direction and with the velocity which it has already acquired. 

 This force also grows greater as the planet draws nearer to its perihe- 

 lion, because as it does so its velocity increases; and less, as it recedes 

 from its jierihelion : and this force as well as the other acts at each point 

 in a different directioi^, because at every point the action of the central 

 force, by deflecting the platiet from its previous direction, alters the 

 line in which it tends to continue moving. The motion at 'each instant 

 is determined by the amount and dirc.-ction of the motion and the 

 amount arid direction of the sun's action at. the previous instant: arid 

 if we- speak of the entire revolution of. tho planet as one pheilomenon 

 (which,, as it is periodicidand similar to itself, we often find it conve- 

 nient to do), that ptienomenon is the pi-ogre^sive effect of two perma- 

 nent and progressive causes, the central force andi the acquired motion. 

 Those causes happening to be progressive in tlie particular way which 

 is called periodical, the effect necessarily is so too ; because, the quan- 

 tities to be added together returning in a regular order, the same sums 

 inust also regularly return. 



This example is, well worthy of consideration also in another respect. 

 Although the causes themselves are permanent, and hiflepcndcnt of all 

 conditions known to us, the changes' which take place in the quantities 

 and relations of the 'causes are actually caused by tKe periodical changes 



