34 INDUCTION. 



is still a progressive effect, produced however, not by the mere 

 continuance of a cause, but by its continuance and its progres- 

 siveness combined. 



A familiar example is afforded by the increase of the tem- 

 perature as summer advances, that is, as the sun draws nearer 

 to a vertical position, and remains a greater number of hours 

 above the horizon. This instance exemplifies in a very inter- 

 esting manner the twofold operation on the effect, arising 

 from the continuance of the cause, and from its progressive 

 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 diurnal rotation than the counteract- 

 ing cause, the earth's radiation, can carry off, the mere continu- 

 ance of the cause would progressively increase the effect, even 

 if the sun came no nearer and the days grew no longer ; but in 

 addition to this, a change takes place in the accidents of the 

 cause (its series of diurnal positions), tending to increase the 

 quantity of the effect. When the summer 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 temperature continues to increase. 



Again, the motion of a planet is a progressive effect, pro- 

 duced 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, which alternately increases and diminishes as the planet 

 draws nearer to or goes further from its perihelion, and which 

 acts at every point in a different direction ; 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 perihelion, 

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

 recedes from its perihelion : and this force as well as the other 

 acts at each point in a different direction, because at every 

 point the action of the central force, by deflecting the planet 

 from its previous direction, alters the line in which it tends to 

 continue moving. The motion at each instant is determined 



