In this development shipping lines are the vital factors. Individual ships are merely pawns on a world-wide chessboard. A single ship can do nothing in the complex structure of modern commerce. Lines must maintain regular service. They must maintain home and foreign offices. They must know where cargoes are to be had and where they are to go. They must have armies of agents and brokers constantly in touch with them. Their ships must be able to voyage and return, voyage and return again, always filled, never idle, never at a loss for cargoes, else their costly structures will crumble, their finances wane, and they will find themselves faced with bankruptcy, disruption, reorganization or destruction. Because of world economics shipping lines find it possible to develop or find themselves broken. Because the margin between success and failure is usually a narrow one shipping lines find it essential to seize upon every development that increases efficiency and decreases cost. Simple steam engines became compound, because shipping lines had to operate their ships with a smaller outlay for fuel in order to compete with sail. Iron gave way to steel, because greater strength was thus secured with less weight. The turbine has made its way against the reciprocating engine because of its increased efficiency and its consequent saving in expense. Oil is being more and more widely burned instead of coal, because its efficiency makes it cheaper through the use of fewer men, through increased steaming ability and less weight, as well as its cleanliness (on passenger ships) and the reduction in time used in coaling.