NOTES. 457 



NOTE F. CHAPTER XIII. 



The investigations pursued in this and the previous chapter, explain the cause 

 of the oversetting of the large icebergs which sometimes float within the limits of 

 the temperate zone. These enormous blocks of frozen fresh water assume various 

 forms i some are columnar, others approach the parallelepiped in their outline, 

 others again resemble mis-shapen cylinders; but all evidently different in form 

 below the plane of floatation to what they exhibit in their extant volume. The 

 action of the atmosphere as the summer advances, slowly thaws the upper surface ; 

 the under side likewise melts at first, but becomes soon protected by a pool of fresh 

 water of the same temperature, consisting of the dissolved portion of the ice which 

 is upheld by the superior density of the surrounding medium. The principal waste 

 of the icy mass taking place along its immersed sides, the current of melted water 

 continually rises upwards, and leaves a new surface to the attack of a warmer 

 current. Whenever therefore, the breadth of the vast column becomes so reduced 

 that it approaches to three fourths of its altitude, the icy parallelepiped will 

 overset, and present a new position of equilibrium. Thus, if the whole height 

 of the mass were 1000 feet, 890 feet would be submerged in the ocean, and 110 feet 

 would be extant, towering amidst the waves. In this case, the elevation of the 

 centre of gravity beyond that of buoyancy would be 35 feet, which is the limit of 

 the metacentre after the base of the column has been reduced to a breadth of 

 766 feet. 



An iceberg of a cylindrical form 1000 feet high, would sink 889 feet in the 

 ocean ; but when the diameter of its base was reduced to nearly the same dimen- 

 sions, say 885, it would overset and take a new position. The instability of the 

 cylinder takes place earlier than that of the parallelepiped, or when the width below 

 becomes eight ninths, instead of three fourths of the whole height. Since then the 

 extant portion wastes more slowly than the immersed portion, the greater the 

 extension of the summit, the more it will hasten the change of position by over- 

 whelming the icy mass. 



And if the block be wasted and rounded below into the shape of a parabolic 

 conoid, it will suffer a total inversion the moment its base is reduced to Its depth 

 in the ratio of about 11 to 20, and its lowest point will become the summit of the 

 extant mass. This form of a body of ice would therefore suffer a greater previous 

 waste ; but its balance is in the end more effectually destroyed. In every case, 

 stability becomes precarious after the breadth of the block is inferior to its depth. 



NOTE G. CHAPTER XIV. 



Upon the pressure, cohesion, and capillary attraction of fluids that are heavy, 

 depends their transmission through fissures of the earth and between its strata, 

 which are pervious to the percolation of water. We can penetrate but a small 

 distance, say 500 fathoms, in digging for coal ; a less depth suffices for some ores, 

 and water is found at all depths, from a few feet to three hundred, as in the neigh- 

 bourhood of London. In the great coal area of Britain, extending lengthwise 260 

 miles, and in breadth about 150 miles, in a diagonal line from Hull to Bristol, in 

 England, and from the river Tay to the Clyde, in Scotland, we find a great variety 



