opposite end of the beam was lowered and its rod also. But through the arrangement of the two cog-wheels the connecting rod caused the fly-wheel to revolve, and at twice the rate at which it would have gone round had Watt’s original rod and crank idea been employed, for the “planet” cog-wheel goes round in a circle but does not revolve on its own axis. Some of his engines of this type were so arranged that the speed of the fly-wheel shaft was not so much greater than in the case where a crank was employed. Thus, in this important adaptation of the vertical to the rotary movement, we get the nucleus of the future steamboat engine, which was to turn the paddle-wheels round. But Watt did not stop there. We have seen that whilst it was the steam which pushed the piston and its rod upwards, it was yet the pressure of the air and the weight of the parts which caused the piston and rod to descend. Now, as we have seen, Watt had already resolved to cover in the top of the cylinder in order to keep out the air from cooling the latter. It was, then, but a natural transition to utilise the steam not merely for pushing the piston upwards, but also for sending the same down after its ascent had been made. We thus get what is the well-known double-action of the modern reciprocating engine, in which steam is employed from either side of the piston alternatively, so that each stroke becomes a working stroke and the power of the engine is doubled. It was Watt who, as early as the year 1782, discovered the advantages which were possessed by the expanditure of steam, but as this does not enter into practical application just yet, we can postpone the subject to a later chapter. We need only emphasise the fact that the fly-wheel which is so familiar to all of us was the invention of Watt, and it is perhaps scarcely necessary to explain that the reason for the existence of this wheel is in order that it may, at the beginning of the stroke, when the engine is at its strongest, store up the surplus energy and give it back towards the end of the stroke. It thus maintains an equal motion throughout the whole stroke given forth by the piston and its rod.