Sentence:  Let the orders now be that each <WSD>demon</WSD> is to stop all molecules from crossing his area in either direction except 100 coming from "A", arbitrarily chosen to be let pass into "B", and a greater number, having among them less energy but equal momentum, to cross from "B" to "A". 
Meaning of the word marked with <WSD> tags:  An hypothetical entity with special abilities postulated for the sake of a thought experiment in philosophy or physics.