144 ON THE REVOLUTIONS 



in twenty-three. But without inquiring wliether or not this 

 declivity would be sufficient for the purpose, it is of conse- 

 quence to attend to another circumstance pointed out in these 

 measurements. The line above which snow lies perpetually, 

 during all summer, is there noted, and lies by the scale at 

 7500 feet perpendicular below the summit of Mont Blanc. It 

 is easy, then, to calculate to what horizontal distance from the 

 centre of the ridge, this limit of perpetual snow would extend, 

 and we thus find it to be 32 miles *. But Saussuke has found 

 the junction of the granite with the surrounding strata at the 

 Btict, and by the same scale, I find the distance of this moun- 

 tain from JNIont Blanc is about 10 miles. It is obvious, then, 

 that in the supposed situation of the Alps, in which the gra- 

 nitic 



• Let AD be Jura, and BC Mont-Blanc. Let D be the place on Jura where 

 the blocks lie ; the line DC will be the outline of the supposed surface. Let 

 AB be drawn horizontally at the level of the surface of the lake, and DE at a 

 level 2000 feet higher, meeting BC in E. We have then AB rr DE = 54 miles 

 = 2S5120 feet ; and CE = CB— EB = 14432 — 2000 = 12432 feet. The mea- 

 sure of this declivity along CD may therefore be easily obtained- Thus, 

 CE: ED: : 12432 feet: 285120: : 1 :22.9, nearly one in twenty-three. Mr 

 Playfair states it at one in thirty ; but he has reckoned to the summit of Jura ; 

 whereas the blocks under consideration lie at some distance below that summit. 



Let CF be equal to 7500 feet, and let FG be drawn horizontally; then 

 G will be the lowest point of perpetual snow upon the supposed surface. 

 The horizontal distance of G from the centre wiU thus be obtained, 

 CE : CF : : ED : FG, or 12432 : 7500 : : 54 miles : 32^ miles. Let H be the ex. 

 treme point of the granitic mass at the Buet, and let HK be drawn vertically^ 

 the point K will denote the limit of the granite upon the supposed surface. 



