PRESSURE OF TIDAL GLACIERS 211 



2,000 feet to 1,400 feet (c). This is equivalent, so far as 

 the block of ice is concerned, to raising the sea bottom 

 from its position in a until it touches the base of the ice- 

 berg. The ice is not lifted by the sea bottom; it still pro- 

 jects into the air just 200 feet; it is still supported by the 

 water, and though touching the sea bottom does not press 

 on it. Finally, conceive the water drawn down until its 

 depth is but 700 feet (d}. This depth of water is just 

 able to float a berg 800 feet thick. Therefore 800 feet of 

 ice, or one-half the thickness of the block, are now sup- 

 ported by the water, and the remaining 800 feet by the 

 sea bottom. 



Let us now approach the subject in a different way. 

 Begin with a block of ice of the same dimensions as be- 

 fore, resting on a horizontal bed, with which it is every- 

 where in contact (fig. 104, a). The pressure on each square 

 inch of the bed equals the weight of the 

 column of ice above it about 640 

 pounds. Now introduce sea water about 

 the ice until it has a depth of, say, 700 

 feet (b). The water presses horizontally 

 against the vertical faces (as indicated by 

 the arrows), but, as there is no vertical FIG. 104. DIAGRAMS 

 component to a horizontal force, the water ILLUSTRATING NON- 



.,, t . r . ,1 . , , , FLOTATION THEORY 



pressure neither lifts the ice block nor OF TIDAL GLACIERS . 

 pushes it down. The block continues 

 to rest on the bed, exerting still a downward pressure of 

 640 pounds per square inch of base. This line of reason- 

 ing seems quite as plausible as the other, but the result 

 is different. 



A little consideration discovers the cause of the dis- 

 crepancy. In the first analysis it is tacitly assumed that 

 the water exerts its pressure not only on the sides of the 

 ice block but on its base, and this whether the block floats 

 free or touches the sea bed. In the second analysis it is 



