415 



events, superfluous, since gravity is in that case a sufficient moving 

 force. 



But, it will be argued, if the ice be really acted on by heat and 

 cold as Mr. Moseley supposes, it is a vera causa of motion and can- 

 not be neglected. And here we join issue respecting the physical 

 theory proposed. 



Mr. Moseley's explanation of the descent of the lead on a roof at 

 an angle much below that at which motion could take place by 

 gravity, friction being allowed for (the angle of repose), amounts 

 to this, that every increase of temperature of the mass by the heat 

 of the day expanding it, pushes the lower end downwards more than 

 it pushes the upper end upwards ; whilst the cold of the night re- 

 tracts a little the lower end, but (being favoured by the slope) it 

 pulls down the upper end more than it had been pushed up during 

 the heat of the day, and thus by a species of vermicular motion im- 

 pels the body down the inclined plane. The motion is calculated 

 from a formula including the absolute expansibility of lead, the slope 

 of the roof, the angle of repose, and the diurnal range of tempera- 

 ture. Taking then corresponding data for the Mer de Glace of 

 Chamouni, assuming 30 to be the angle of repose of a glacier upon 

 its bed, taking the expansion of ice to be nearly double that of lead 

 (according to experiments made at St. Petersburgh), and the daily 

 range of temperature of the ice to be the same as that of the air ob- 

 served by De Saussure on the Col du Ge'ant in the month of July, 

 Mr. Moseley calculates the daily descent of the glacier opposite the 

 Montanvert and compares it with my observations. 



Waiving for the moment all other objections, can we possibly attri- 

 bute to the ice of the entire mass of this vast glacier an average daily 

 range of temperature of 4 of Reaumur or 9 of Fahrenheit ? The 

 idea seems to me to be perfectly untenable. 



The expansion and contraction of ice by heat and cold can of 

 course only take place below the freezing-point, or 32. Let it be 

 percolated by water as it may, it cannot rise above that temperature 

 nor expand in the smallest degree. But it is a matter perfectly 

 notorious, that, at least in summer, and throughout the whole extent 

 of the Glacier Proper, and even far into the region of the nv6, the 

 glacier is charged with percolating water derived from superficial 

 fusion. Mr. Moseley admits this, and even attributes the diurnul 



