The principles of attack and repulse by means of mines dug beneath the walls of fortresses had reached a high state of development by the end of the sixth century B.C. At the siege of Barca in Libya, about 510 B.C., the Persians excavated underground tunnels that reached to the walls. Among the Barcaeans there was a skilled worker in brass who took a brazen shield and, carrying it round within the wall, applied it here and there at places where he thought the workings might be. Where there were no mines the shield was silent, but at places near mining operations the shield made a vibrating sound. By countermining at these points the Barcaeans broke into the enemy’s works and slew the men they found there.