58 ELEMENTS OF LABORATORY WORK 



The practical requirements for observations with the lever 

 are as follows: A wooden rod about 4 square centimetres 

 section and about H metre long ; a wooden edge placed so as 

 to act as fulcrum ; a pulley fixed so that the rod may be 

 balanced if necessary by means of a cord passing over it. At 

 the ends of the cord are fixed two hooks, one to receive the 

 rod at its centre, and the other to hold a body just able to 

 counterpoise the rod. In this way we get rid of the difficulty 

 of calculating for the mass of the rod, and may pay attention 

 solely to various masses which may be hung from the rod at 

 various positions, while at the same time the position of the 

 fulcrum may be altered. 



42. The Pressure of Liquids. When a liquid is at rest its 

 free surface is horizontal, except where it is in near contact 

 with a solid ; and this is true however much the surface may 

 be modified by the shape of the containing vessel. In other 

 words, all portions of the free surface of any homogeneous 

 liquid at rest are in the same horizontal plane. But when we 

 speak of a free surface we mean that portion of the liquid 

 which is in contact with the atmosphere. 



The first and second of the figures below show a liquid 

 with two equal free surfaces (A and B), each in contact with the 

 atmosphere, and in the same plane. The third shows that 

 they are still in the same plane when they are unequal. In 

 the fourth case, the closed space B c must contain air, or other 

 gas, in the same condition as the air outside the tube, or, as we 

 have seen, the two surfaces A and B would not be in the same 

 plane. In the fifth case the air has been driven out of the 

 tube, while mercury has been poured in, and the space c D is 

 a vacuum. We must conclude, in this case, that the column 

 of mercury B C has the same effect on the imaginary surface 

 B as the atmosphere would have had, if the surface had been 

 exposed to it as in No. 1. In other words, the atmosphere at 

 the surface A, and the column of mercury above B, are in 

 equilibrium. 



Further investigation of these facts must be postponed 

 until more is learnt of the structure of matter. We shall 

 require help from the theory that matter is discontinuous 



