in Geological Climatology. 261 
TThen the high grounds of Scotland and Scandinavia, with 
those of the northern parts of America, "became covered with 
snow and ice, and the eccentricity went on increasing, a 
diminution of the Gulf-stream and a host of other physical 
agencies, all tending towards a glacial condition of things, 
would be brought into operation. This would ultimately and 
inevitably lead to a general state of glaciation, without the 
aid of any of those additional geographical changes of land 
and water which some have supposed. 
The Mutual Reaction of the Physical Agents. — Those who 
think that the agencies to which I refer would not by them- 
selves bring about a glacial condition appear to overlook a 
most important and remarkable circumstance regarding their 
mode of operation, to which I have frequently alluded in 
' Climate and Time ' (pp. 74-77) and other places. The cir- 
cumstance is this : — The physical agencies in question not 
only all lead to one result, viz. an accumulation of snow and 
ice, but their efficiency in bringing about this result is actu- 
ally strengthened by their mutual reaction on one another. 
In physics the effect reacts on the cause. In electricity and 
magnetism, for example, cause and effect in almost every case 
mutually act and react upon each other ; but the reaction of 
the effect tends to weaken the cause. Those physical agents 
to which I have referred, no doubt, in their mutual actions 
and reactions obey the same law; but in reference to one 
particular result, viz. the accumulation and conservation of 
snow, those mutual reactions strengthen one another. This is 
not reasoning 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 direc- 
tion is strengthened by their mutual reactions. This curious 
circumstance throws a flood of light on the causes which 
tended to bring about the glacial epoch. 
To begin with, we have a high state of eccentricity. This 
leads to long and cold winters. The cold leads to snow ; and 
although heat is given out in the formation of the snow, yet 
the final result is that the snow intensifies the cold : it cools 
the air and leads to still more snow. The cold and snow 
bring a third agent into play — -fogs, which act still in the 
same direction. The fogs intercept the sun's rays; this inter- 
ception of the rays diminishes the melting-power of the sun, 
and so increases the accumulation. As the snow and ice con- 
tinue to accumulate, more and more of the rays are cut off; 
and, on the other hand, as the rays continue to be cut off, the 
