0 a wave moving in the same direction of travel as the vibrations (= fast movements backwards and forwards) of the particles of the substance through which it is travelling --
1 a wave in which the substance the wave travels through moves in the same direction of the wave itself. Sound waves are longitudinal waves. --
We can complete it step by step however long the road is and it can't be completed however short the road is if you don't even mark your footprint.
Vi kan slutföra det steg för steg hur lång vägen än är och den kan inte slutföras hur kort vägen än är om du inte ens markerar ditt fotavtryck.