340 Professor Forbes's Fourteenth Letter on Glaciers. 



Yet, lest we might attribute the irregularities of the torrential gla- 

 cier to causes quite local and uncertain, we find them reflected more 

 or less distinctly in the movements of the neighbouring one. Thus 

 the anomalous retardation in the end of March and beginning of 

 April appears in three stations out of four, as does that in the first 

 half of June, showing clearly that it is not an error of observation. 

 It appears that the thaw of the winter's snow during the month of 

 May, saturating th(. pores of the glacier with water, produced (as 

 we know that a thaw always does) a sudden and violent march, es- 

 pecially of the more susceptible or torrential glacier. So completely 

 had this sudden move forced on the glacier of Bossons, encumbered 

 by the spring avalanches, and loaded with all the fragments and 

 snow masses which had remained temporarily suspended during 

 the winter months, that the lower part of the glacier (as we read in 

 the memoranda to the register) advanced and widened greatly, to an 

 extent which it had not done for many years past, and seemed to 

 change its whole character ; and in February a similar temporary 

 increase of volume had taken place ; " on ne s'y reconnait presque 

 plus," writes Balmat ; thus accounting for the particular accession 

 of speed which appears in that month. In both cases, after the 

 rapid march in February and in May, a reaction takes place ; the 

 material is deficient, the excessive pressure has been removed by the 

 previous overflow, and a lull occurs in March and in June. 



VII. These irregularities, such as they are, even should we fail 

 in entirely explaining them, are, at least, not to be attributed en- 

 tirely to errors of observation, since different observations (which it 

 is to be recollected were sent to England in so rough a state, that 

 they required to be reduced and computed before the variations of 

 velocity could be deduced from them) agree amongst one another, and 

 agree with the phenomena casually noted in the Meteorological Re- 

 gister. They are very trifling in the movement of the Glacier des 

 Bois, which presents a curve of remarkable regularity, giving a mi- 

 nimum 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 diff'erence of cir- 

 cumstances which occur in autumn and in spring when the thermo- 

 meter 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 effectually 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 con- 

 solidation by the slow but continued effect of the winter's cold pene- 

 trating its upper layers, though after all probably to no very great 



