Caesar and the Rhine 56BC

Julius Caesar, man of his times, undertook and completed many wondrous accomplishments during his tenure in consul in Rome. In June 56 BC, he took his legions into Gaul to further subjugate the locals. But, his aggression resulted in the slaughter of hundreds of thousands of local. In a show of political propaganda and military strength, he built a bridge to allow his army to cross the Rhine river and for the first time, extend Rome's influence directly into the territory of the Germanic tribes.

Engineering a bridge has never been trivial and the Romans had little historical precedence to build upon. Nevertheless, they were master engineers and had the necessary material at hand. With his 40000 men and the surrounding forest, Caesar created a solid platform for his force to safely cross the Rhine river and undertake a few weeks pillage.

Caesar recounts this amazing endeavour within his diaries. Within, he graphically describes using huge stones to ram logs into the river bottom and then laying long timbers to establish a support and platform.

This undertaking utilized energy through a number of ways. The men had to be feed. The trees contained large quantities of energy to grow. And the effort to chop, clean, move and install the wood added to the total.

Caesar had 40000 men work for 10 days thus needing 1.3e13 Joules.

Assume Caesar used fir trees. Multiplying the energy content of each with the likely number of trees results in an energy content of 8.8e13 Joules.

Felling and clearing the trees would use considerable effort but is difficult to measure. However, Caesar spoke of a stone pile-driver pushing the logs into the river bottom. Lifting and dropping the stone the likely number of times results in an energy expenditure of 2e7 Joules.

The rough energy total for building the bridge is 1e14 Joules. This is greater than the energy needed to loft the space shuttle into orbit. It is approximately the same as the energy released by the atomic bomb dropped on Hiroshima.

Sadly, given that the bridge was for propaganda purposes rather than practical purposes, Caesar destroyed it on his return.