72 



G. F. BECKER — STRUCTURE OF THE SIERRA NEVADA. 



and with coal tars I found that the weight did sink in the manner expected, 

 so that the masses assumed a position about as shown in figure 11, the surface 

 of the load remaining sensibly horizontal and the unloaded portion of the 

 viscous mass being compressed laterally. It is also possible to measure the 

 horizontal component of the force by attaching to the load a cord passing 

 over a pulley and adding just enough weight to the vertical end of the cord 

 to keep the load from diverging from simple vertical motion. With a tar 

 which yielded to the pressure of the finger about as easily as the softest 



Figure 11— Result of Experiment. 



India rubber, I found by this means that the horizontal traction was 7i per 

 cent, of the load. The experiment was much too rough to make this per- 

 centage of any value except as showing that the horizontal traction, even in 

 a very soft mass, is a considerable fraction of the weight. 



My tar was a very soft one, but it diflfered from a perfect fluid in the same 

 sense as other viscous substances do, though to a smaller degree than most 

 of them. The experiment might have been made with hard aspnalt or 

 sealing wax if it had seemed worth while, but qualitatively the results 

 must have been the same, while, had pressure enough been applied to pro- 

 duce deformation at the same rate, these substances must have given a larger 

 angle of deflection from the vertical and a larger horizontal traction. 



Direction of Upheaval. — In my experiments I found that if after a consid- 

 erable deformation had been produced all the weights w^ere removed except- 

 ing a very thin and light one, this would return to substantially the same 

 position which it occupied at the beginning of the experiment, however often 

 the experiment might be repeated. This was entirely expected, for just as 

 it was shown that on a viscous earth the mass at B in figure 9 must be im- 

 pelled by a force directed downwards and with an inclination towards A, 

 so it can be proved that the force acting on the bottom of the pit dug at A 

 must be upwards and away from B. 



Geological Application. — The geological application of these inferences 

 and experiments is obvious. That the earth tends to an isostatic condition, 

 or to a condition of hydrostatic equilibrium, is shown by pendulum experi- 



