The machine consisted of a carriage and locomotive behind, supported on four wheels. The boiler was cylindrical, communicating by a steam-pipe with the two horizontal cylinders, one on each side of the engine. When the piston was raised by the action of the steam, a vacuum was produced by the condensation of the steam in a cold-water tank placed underneath the engine, on which the piston was again forced back by the pressure of the atmosphere. The motion was communicated to the wheels by rack-rods connected with the piston-rod, which worked on each side of a drum fixed on the hind axle, the alternate action of which rods upon the tooth and ratchet wheels with which the drum was provided producing the rotary motion. It will thus be observed that Symington's engine was partly atmospheric and partly condensing, the condensation being effected by a separate vessel and air-pump, as patented by Watt; and though the arrangement was ingenious, it is clear that, had it ever been brought into use, the traction by means of such an engine would have been of the very slowest kind. But Symington's engine was not destined to be applied to road locomotion. He was completely diverted from employing it for that purpose by his connection with Mr. Miller, of Dalswinton, then engaged in experimenting on the application of mechanical power to the driving of his double paddle-boat. The power of men was first tried, but the labor was found too severe; and when Mr. Miller went to see Symington's model, and informed the inventor of his difficulty in obtaining a regular and effective power for driving his boat, Symington--his mind naturally full of his own invention--at once suggested his steam-engine for the purpose. The suggestion was adopted, and Mr. Miller authorized him to proceed with the construction of a steam-engine to be fitted into his double pleasure boat on Dalswinton Lock, where it was tried in October, 1788. This was followed by farther experiments, which eventually led to the construction of the _Charlotte Dundas_ in 1801, which may be regarded as the first practical steam-boat ever built.