I was leafing through a novel in which a group of frogmen attached an engine to the back of an inflatable raft. Or at least it was called an engine at first; later it was called a 'motor'.

Which makes me wonder, what's the difference between and engine and a motor?

Historically an engine was simply the name for a machine and is still used in that sense in such examples as a "portable engine" or a "steam engine".

A motor is the provider of motive power, not on its own an engine in the previous sense.

Richard, if I understand you, you're saying that that historically an engine and a motor were distinct in that the engine was the entire machine, while the motor was its source of power.

I doubt not that that is historically correct, but is it true today?

For example, an automobile is a machine, but you wouldn't call it an engine, would you? Rather, if you spoke of a car's "engine", you'd mean just the part taht provides the power (excluding, for example, the passenger compartment, the transmission, etc.) And wouldn't "motor" would be an alternate name for the same thing?

Rockets can be motors; computer programs can be engines.

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I doubt not that that is historically correct, but is it true today?

I suspect that in modern useage it has changed. An engine is now a part of the whole and as such has taken on the meaning once reserved for "motor". Not, though, in the case of traditional vehicles like steam engines where the engine is the whole.

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Rockets can be motors

Can they? I have heard of a "rocket motor" but that's a motor for a rocket, surely.

The Space Shuttle's solid rocket boosters are also called motors.