J. Croli— Geological Climatology. 269 
strengthened by their mutual reaction on one another, In 
physics the effect reacts on the cause. In electricity and mag- 
mutual reactions strengthen one another. This is not reason- 
ing in a circle, as Mr, Searles Wood supposes; for the reaction 
of an effect may on the whole weaken the cause, and yet in 
regard to a particular result it may strengthen it. In the case 
under consideration the agents not only act in one direction, 
but their efficiency in acting in that one direction is strength- 
ened by their mutual reactions, This curious circumstance 
throws a flood of light on the causes which tended to bring 
of the rays diminishes the melting power of the sun, and so 
Increases the accumulation. As the snow and ice continue to 
tfade winds on the cold hemisphere and weakens those on the 
warm. The effect of this is to impel the warm water of the 
tropics more to the warm hemisphere than to the cold, Sup- 
pose the northern hemisphere to be the cold one, then as the 
Stow and ice begin gradually to accumulate, the ocean currents 
