240 DISCUSSIONS IN CLIMATOLOGY. 



be otherwise than of great thickness. Were not the 

 sheet enormously thick the quantity of ice annually 

 discharged would not equal that being formed, and 

 consequently the ice would of necessity increase in 

 thickness year by year, till the rate of discharge 

 became equal to that of growth. We have just seen 

 that it would require a thickness of not less than 1400 

 feet at the very edge of the cap to make the two rates 

 equal, even although the ice was moving outwards with 

 a velocity of a quarter of a mile per annum, a rate of 

 motion greater than that of an Alpine glacier; and, on 

 the other hand, to produce such a rate of motion as 

 this„ a thickness in the interior enormously greater 

 than 1400 feet is required. If, from an increase in the 

 snowfall, or from a decrease in the quantity of snow 

 and ice melted, or from both combined, the annual 

 amount of ice requiring to be discharged were doubled, 

 the velocity remaining the same, the thickness of the 

 sheet would ultimately become doubled also. Or, if the 

 thickness of the sheet remained the same the velocity 

 would be doubled. The actual result in such a case, 

 however, would be that a restoration of equilibrium 

 between supply and discharge would take place, by an 

 increase both of thickness and velocity. As the quarter 

 of a mile per annum of velocity would only be sufficient 

 to discharge one-half the amount of ice being formed, 

 the sheet would increase in thickness year by year. 

 But this increase in thickness would produce an 

 increase of velocity, and the increase both in thickness 

 and velocity of motion would continue till the quantity 

 of ice discharged would be equal to the 12 inches over 

 the whole area, instead of the 6 inches as before. 

 Equilibrium being now established, no further increase 

 would take place either in the thickness of the sheet 

 or in the velocity of its motion. If, on the contrary, 



