Dr. Forbes on the Descent of Glaciers. 303 



powerless. Cold can be conveyed downwards, or to speak more 

 correctly, heat can be transmitted upwards through the ice only by 

 the slow process of conduction, and this on the supposition that the 

 depression of supei'ficial temperature is all that the theory might 

 require. But how stands the fact } Mr. Moseley quotes from De 

 Saussure the following daily ranges of the temperature of the air in 

 the month of July at the Col du Geant and at Chamouni, between 

 which points the glacier lies. 



At the Col du Geant 4°"257 Reaumur. 

 At Chamouni 10°-092 Reaumur. 



And he assumes " the same mean daily variation of temperature to 

 obtain throughout the 'ength" [and depth ?] " of the Glacier du 

 Geant which De Saussure observed in July at the Col du Geant." 

 But between what limits does the temperature of the air oscillate ? 

 We find, by referring to the third volume of De Saussure's Travels, 

 that the mean temperature of the coldest hour* (4 a.m.) during his 

 stay at the Col du Geant was 0°-457 Reaumur, or 33°- 03 Fahrenheit, 

 and of the warmest (2 p.m.)4°-714 Reaumur, or42°-61 Fahrenheitf. 

 So that even upon that exposed ridge, between 2000 and 3000 feet 

 above where the glacier can be properly said to commence, the 

 air does not, on an average of the month of July, reach the freezing- 

 point at any hour of the night. Consequently the range of tempera- 

 ture attributed to the glacier is between limits absolutely incapable of 

 effecting the expansion of the ice in the smallest degree. This would 

 of course be still more applicable if we take the mean of the tempe- 

 ratures at Chamouni and the Col du Geant to present the general 

 atmospheric conditions to which the glacier is exposed. 



It is in summer that the glacier moves fastest : it is with my 

 observations of motion in July that Mr. Moseley compares the results 

 of his theory : and therefore it is of no avail to say that there are 

 periods of the year when congelation penetrates at night some inches, 

 or even it may be some feet into the ice, and when therefore the 

 sensible heat of the glacier may be considered to vary, though, if 

 regai'd be had to its vast thickness, it must be on an average and in 

 the most extreme circumstances to an absolutely inappreciable 

 degree. 



Lastly, Mr. Moseley, whilst condemning in the following passage 

 the theory of glacier motion by the dilatation of water in the inter- 

 stices of the ice, clearly passes sentence on his own, which could not 

 come into action until tlie other had already produced its effects : 

 " The theory of Charpentier, which attributes the descent of the gla- 

 cier to the daily congelation of the water which percolates it, and 

 the expansion of its mass consequent thereon, whilst it assigns a 

 cause which, so far as it operates, cannot, as I have shown, but 

 cause a glacier to descend, appears to me to assign one inadequate 

 to the result ; for the congelation of the water which percolates the 



* The obgervatioDS were made every two hours day and night. 



t The corresponding extremes at Chamouni are 53°"25 and /S'-QG Fahr. 



