EROSIOA' PERFORMED BY THE ATMOSPHERE. 325 



tions and as to their capacity for sustaining a load, the current 

 with the greater velocity would be able to remove ten units of 

 sediments, while the slower current would remove one. Multi- 

 plying the fraction expressing the ratios between the cross sec- 

 tions of the two currents d 0") ^7 ^^^ fraction expressing 

 the ratio between their velocities (^ig-), we obtain a fraction 

 which expresses their relative carrying power, if their capacities 

 for sustaining a load were the same. This fraction is TinroVour- 

 If every cubic foot of air in the atmosphere held in suspen- 

 sion as much of sediments as every cubic foot of water in the 

 Mississippi, then the atmosphere would have the power to 

 transport in a given time ten million times the quantity of mate- 

 rial transported in the same time by the Mississippi river. 



With regard to the capacity for holding solid particles in 

 suspension the air is, however, greatly inferior to water. It is 

 evident that the load which can be carried by the air at ordinary 

 and even in high velocities, is a great deal smaller than that 

 which can be carried by water. The capacity in this respect of 

 any current depends on chiefly three factors: (i) the density 

 of the medium, (2) its velocity, and (3) its viscosity. As to 

 the comparative densities of the two fluids, the air is only g^ 

 times as heavy as water. Another circumstance also comes into 

 consideration. When the particles of a material like quartz are 

 suspended in water, they lose about ^|- of their weight in the air, 

 and the force with which they make their way downwards 

 through the water is thus reduced to ^| of what it would be in 

 the air. This still more increases the relative carrying power of 

 water making it 1321 times as great as that of the air (8i3(|-|) = 

 1 321). On account of the greater average velocity of the 

 atmosphere and also by reason of the consequent greater magni- 

 tude of its convection currents, this again has the advantage over 

 water. But exactly to what extent these considerations affect 

 the comparison, data are not at hand to determine. It would 

 appear that the advantage connected with these greater convec- 

 tion currents more than outweighs the disadvantage due to the 

 lesser viscosity of air, when compared with water. At such low 



