For vessels larger than canal barges there was only one method of propulsion and that was steam. For vessels not exceeding a canal barge in size or an omnibus in speed, three other methods of obtaining the necessary power to propel these vessels had been developed. These were: The evaporation of a volatile liquid, such as naphtha; the generation and explosion of gas, and air; and electrical energy stored in secondary batteries. Steam, naphtha, petroleum, and electric motors had their own particular sphere of usefulness. In deciding the kind of motor to be employed in a small vessel, the chief points to be considered were, the speed required, the weight of the motor, the consumption of fuel, and the space occupied by the machinery and fuel; other considerations were the cost of providing the energy, degree of skill required in handling the machinery, effective distance, or the maximum energy endurance. Another and equally important matter was the degree of that indefinite quality known as seaworthiness. Simple steam engines were the most commonly employed means of propulsion in small craft, but compound and even triple-expansion engines with condensers, and forced draught for boilers were used in larger and faster vessels, where the power was up to 150 indicated horsepower, and the speed was 14 to 15 knots. A considerable reduction in the weight of marine steam engines had been achieved, and the use of steam at high pressure had permitted the use of smaller boilers, which in turn had increased bunker capacities. In much the same way in which the gas engine had replaced the low powered steam engine, the petroleum motor had replaced the gas engine. The naphtha engine was in many respects superior to the steam engine and the petroleum engine. The main disadvantage associated with the naphtha engine was that it used a highly volatile and inflammable liquid. Much had also been said in the contemporary press regarding the use of electricity as the motive power for launches.