IxXXvi PROCEEDLN^GS OF THE GEOLO&lCAL SOCIETY, [vol. liXvii, 



latched and cannot yield, in this case the resistant pressure of the 

 door is felt and appreciated, yet the same takes place when it is 

 free to move, though the opposing pressure is luiiited to that 

 necessary to overcome the friction of the hinsres and the inertia of 

 the door. Once this limit is reached the door begins to moye, and 

 if the pressure exerted by the pei-son is greater than that needed to 

 move the door, at the rate which he wishes to impart, the result 

 may be that he falls forward until he meets the greater resistance 

 of the floor. 



A similar fallacy is commonly to be found in tlie interpretation 

 of experiments on the small scale, intended to illustrate the 

 foldings and faultings of rocks. These generally take the foi-m 

 of a box-shaped receptacle, tilled with sand, clay, or other material, 

 the bottom and three sides being solid with eacli other, while the 

 fourth side can be advanced by means of a screw or other 

 mechanical contrivance. Here we appear, at first sight, to have 

 a case of a solid immovable obstacle and a pressure, combined 

 with movement, exerted from one side in the direction of the fixed 

 obstriiction ; but Avhen the circumstances are more closely examined, 

 Ave see that whatever pressure is exerted by the movable side on the 

 contents of the receptacle must be met by an equal and opposite 

 pressure on the nut of the screw, the fulcrum of the lever, or 

 generally on the fixed point from which the purchase is obtained. 

 Looking still farther into the matter, we find that this fixed point 

 must be connected, directly or indirectly, with the body of tlie 

 receptacle, and so we see that tlie sides are only nominally fixed 

 or movable, and that the one is drawn in exactly the same degree 

 as the other is pushed ; consequently, the pressure on the contents is 

 not from one side towards the opposite one, but in equal amount and 

 opposite directions from each towards the other. When, however, 

 we transfer our consideration from the movements of the sides of 

 the receptacle to the resistance offered by the contents, the con- 

 ditions become much less simple : in the one ease, we have three 

 sides and the bottom all locked together and moving in unison, 

 so that there is no frictional resistance to the forward movement 

 of the contents as a whole : in the other, any displacement of the 

 contents would be resisted by friction against two sides and the 

 bottom of the box, this resistance being apart from that opposed 

 by the contents to defoi-mation. resulting from change in the 

 dimensions of the receptacle. Hence it results that, while the 

 pressure must in every ease be equal and opposite, the resistance 



