ATMOSPHERIC AGENCIES. 39 



and borne onward by the rivers to the ocean. We often 

 see the effect of heavy rainfalls on exposed soils and sur- 

 faces in our own islands how they batter, loosen, and 

 carry away ; but our rainfall, amounting annually to some 

 30 or 40 inches, is trifling compared with the rainfall of 

 tropical and sub-tropical countries, ranging from 200 to 400 

 inches, and this concentrated for the most part within one 

 period of the year. It is not uncommon to hear travellers 

 speak of the soils being converted into mud, and of the 

 rivers running mud rather than water, and this solely 

 through the battering and dissolving influence of the 

 periodical rains. Again, frost in all the higher latitudes 

 and altitudes is annually performing a similar function. 

 The moisture that inserts itself into the pores and inter- 

 stices of all rock-substances is converted into ice during 

 frost ; ice occupies more space than the water of which it 

 consists, or, in other words, water expands during freezing ; 

 the particles of rock-matter are distended or forced asunder; 

 and when thaw comes, their cohesion being loosened, they 

 are washed away by the rains and carried down by the 

 streams and rivers. Every winter we see the disintegrating 

 effects of frost on the ploughed soils, road-cuttings, and sea- 

 cliffs of our own islands ; and this effect is manifested a 

 hundredfold in all the colder latitudes and" in all the higher 

 mountains, whether within tropical, temperate, or arctic 

 regions. The destructive power of frost is stupendous, 

 whether silently crumbling away the cliffs and precipices ; 

 discharging the avalanche and landslip down the mountain- 

 slope ; slowly grinding its way as the glacier through the 

 Alpine glen ; or transporting and dropping, as the iceberg 

 does, its burden of rock-debris over the floor of the ocean. 

 As with the rains and frosts, so to a certain extent with 

 the winds or aerial currents of the atmosphere. Wherever 

 there is rock-matter sufficiently light and loose, thence the 



