1846.] DEDUCTIONS FROM OBSERVATIONS OF VELOCITY. 137 



the variations of velocity could be deduced from them) agree 

 amongst one another, and agree with the phenomena casually 

 noted in the Meteorological Register. They are very trifling 

 in the movement of the Glacier des Bois, which presents a curve 

 of remarkable regularity, giving a minimum about the end of 

 December, and a maximum in July. The coincidence with the 

 curve of temperature is greater throughout than we could have 

 expected, considering the important difference of circumstances 

 which occur in autumn and in spring, when the thermometer 

 stands nearly alike ; the first chill of autumn depriving the 

 glacier of its fluid pressure more effectually than the severer 

 cold of winter which is tempered by its snowy covering, whilst 

 in spring the first relaxation of the bands of frost saturates the 

 icy mass with the impetuous streams of melted snow, as effectu- 

 ally as the intensest heat of summer. In fact, the velocity 

 would probably be greatest in spring, were it not that then the 

 ice has attained its greatest consolidation by the slow but con- 

 tinued effect of the winter's cold penetrating its upper layers, 

 though after all probably to no very great depth. But this is un- 

 doubtedly the reason why the minimum and maximum approach 

 so near to one another in point of time in the torrential glacier 

 of Bossons, and it receives an important illustration from the 

 independent fact of the observed condition of the source of the 

 Arveiron, which (see the Meteorological Register), though very 

 small in February, was still whitish and dirty before a change 

 of weather, showing that the bands of frost were not so strong 

 as to prevent a temporary relaxation of thaw throughout the 

 mass of the glacier even in winter ; and although the mean 

 temperature of the air had been rising ever since the middle of 

 January, and the greatest cold had occurred early in February, 

 we find that at the end of March the source of the Arveiron was 

 still as small as in February, and that owing to the coldness of 

 the spring, it had not even increased very much till the middle of 

 April, when it almost suddenly resumed its summer volume. 

 Now during all this time the velocities of the glaciers underwent 

 but little change, some oscillations backwards and forwards, 



