of Molecules at very loiv pressures. 1051 



All those whose directions are included in angles DOC and 

 AOB will either be returned through BC directly, or will be 

 directed on to the opposite wall, and thence returned through 



Fig. 2. 



BC. Thus more than half the total number of molecules 

 entering through such a point as this will be returned 

 through BC. 



Case 2 : Any point under AB or i)C. 



A perpendicular through will cut AB (or DC similarly, if 

 U be under DC). Let this point of intersection be called P. 

 Since POB is itself a right angle, and angle DOC not small, 

 therefore angle POD is considerably less than a right angle. 

 Molecules entering in directions included in angle DUO are 



Those entering in directions 



returned directly through BC. 



included in angle POB are also returned through BC, either 

 directly or after impact with DC. Only molecules enterino- 

 in directions included in angle POD * succeed in reachino- 

 the top. Thus more than half of the total number of 

 molecules entering through such a point as this will be 

 returned through BC. 



Thus of all possible points of entry through the line, BC, 



* And only some of these, namely those which enter in directions 

 included in angle DOX, where X is the point on BA such that angle 

 OXB is equal to angle I3XA. 



