Skeptics; A better ogy than greenhouse?

I think John Tyndall summed it up nicely in 1859 when he said, "As a dam built across a river causes a local deepening of the stream, so our atmosphere, thrown as a barrier across the terrestrial rays, produces a local heightening of the temperature at Earth's surface." A dam is a far more accurate ogy for the effect than a greenhouse, if somewhat less intuitive.