He set up the model and operated it, noticed how the alternate heating and cooling of its cylinder wasted power, and concluded, after some weeks of experiment, that, in order to make the engine practicable, the cylinder must be kept hot, "always as hot as the steam which entered it." Yet in order to condense the steam there must be a cooling of the vessel.

The problem was to reconcile these two conditions.
At length the pregnant idea occurred to him--the idea of the separate condenser.

It came to him on a Sunday afternoon in 1765, as he walked across Glasgow Green.

If the steam were condensed in a vessel separate from the cylinder, it would be quite possible to keep the condensing vessel cool and the cylinder hot at the same time.

Next morning Watt began to put his scheme to the test and found it practicable.