It was never a foregone conclusion that the Roman Empire should have made any significant use of steam power. The basic principles of the steam engine were certainly known by the mid-first century A.D., as seen in the ‘wind-ball’ ( aiölipile ) described by Hero of Alexandria in his treatise on Pneumatica . Hero's device, in which a copper sphere was made to rotate by jets of stream when the reservoir of water underneath was heated to boiling point, clearly demonstrated that steam could serve as a source of propulsion. It was, admittedly, a very inefficient design: in modern reconstructions, either too much steam escaped through the joints or the joints had to be made so tight that friction became a serious problem. Such deficiencies were by no means insurmountable, and all the other elements necessary for the construction of a working steam engine – pistons, cylinders, an effective valve mechanism – can be found in Hero's writings or in those of his contemporaries.