302 Royal Society : — 



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 everj' 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 bodj'' 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 temj)erature of the ice to be the same as that of the air ob- 

 served by De Saussure on the Col du G^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 pejfectly untenable. 



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

 course only take place belotv 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 neve, the 

 glacier is charged with percolating water derived fi'om superficial 

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

 oscillation of temperature vi^hich he assumes, to the action of water, 

 as in the following passage: "Glaciers are, on an increased scale, 

 sheets of ice placed upon the slopes of mountains, and subjected to 

 atmospheric variations of temperature throughout their masses by 

 variations in the quantity and the temperature of the water, which 

 flowing from the surface everywhere percolates them" (p. 339). 

 This action therefore clearly brings the temperature of the ice up 

 to 32° during the day. But how is the cold of the night to operate 

 in reducing the temperature of a mass of ice certainly from 300 to 

 600 or more feet in thickness through the enormous average depres- 

 sion of 9^ degrees ? The water so efiicient by its percolation in 

 raising the temperature (if necessary) to 32°, being frozen, is now 



