1054 Mr. A. Fairbourne on Restricted Movements 



Actual^ the result must lie intermediate between the two 

 extremes, (1) perfect reflexion, and (2) absolute inde- 

 pendence of the angles of approach and departure. The 

 first of these extremes has been considered. In the second 

 it is desirable to consider a vessel whose height is greater 

 than in those previously drawn, and whose angle, BZC, is 

 small {fig. 5). 



Fig. 5. 

 A D 



B 



Let the vessel be imagined filled with a gas whose mean 

 free path is relatively so large that intermolecular collisions 

 during the short journeys in the vessel are occurring with 

 only a negligibly small fraction of the total number of 

 molecules which pass through in any representative period 

 of time ; and, at the first instant of consideration, let the 

 molecules in the vessel be moving in all directions equally. 

 If the walls were ideally smooth and the usual laws of 

 reflexion were in operation, then any impact of a molecule, 

 whatever its direction, with either wall (on the inside of the 

 vessel) would necessarily increase the downward motion of 

 that molecule, since all impulses caused by the smooth walls 

 must be normal to their surfaces. Further, in this case, 

 since the distance from wall to wall is less in the neighbour- 

 hood of AD than in that of BC, and since molecules pass 

 from side to side without intermolecular interruption, there- 

 fore laterally moving molecules near the top of the vessel 

 would receive more impacts from the walls in unit time than 



