An Accidental Circumstance

I just finished The Great Bridge, David McCullough's 1972 opus about the building of the Brooklyn Bridge. One quote struck me as particularly relevant. When asked to compare the Brooklyn Bridge to the Great Pyramid of Cheops, young Roebling wrote:

To build his pyramid Cheops packed some pounds of rice into the stomachs of innumerable Egyptians and Israelites. We today would pack some pounds of coal inside steam boilers to do the same thing, and this might be cited as an instance of the superiority of modern civilization over ancient brute force. But when referred to the sun, our true standard of reference, the comparison is naught, because to produce these few pounds of coal required a thousand times more solar energy than to produce the few pounds of rice. We are simply taking advantage of an accidental circumstance.

It is fascinating that, in 1883, an engineer would have such a firm grasp on the energy dependence of construction. Roebling lived to 1926 and I'd love to know what he thought of the nascent petroleum based economy. Perhaps he would have some useful advice for us now that this accidental circumstance seems to be drawing to a close.