How much the man does for us who props our house when it is about to fall, and who, with a skill beyond belief, suspends in the air a block of building which has begun to crack at the foundation; yet we can contract for underpinning at a fixed and cheap rate. The city wall keeps us safe from our enemies, and from sudden inroads of brigands; yet it is well known how much a day a smith would earn for erecting towers and scaffoldings

152 to provide for the public safety.