Al-Madinah was protected in almost every direction, either by ramparts, fortlets, or gardens. At the north only would it have been possible for the enemy to arrange a formidable assault. A learned Persian, Salman-i-Farisi, recently converted, explained to the Prophet a system of efficacious protection. It was by means of a ditch, and Salman had seen it practised in his own country. Mohammad was so struck by the Persian's arguments that this ditch was ordered to be dug immediately. All the Believers, confiding in their chief's farsightedness, set ardently to work. Nevertheless, they were in a state of extreme distress. An icy north wind, such as blows frequently in winter on these table-lands of the desert where there is intense radiation, benumbed their shivering bodies. Roads where the work of revictualling was carried on were blocked by the enemy; provisions were lacking. The pangs of hunger would have paralysed their strength if it had not been kept up and rekindled by faith, for all they had to eat were a few grains of barley cooked in rancid, nauseous mutton-fat. Meanwhile, the shovelfuls of earth accumulated, thrown up with great spirit by the workers, and the ditch had reached a good depth, when suddenly the pickaxes struck against a rock which they were powerless to uproot. Mohammad filled his mouth with water and spat it out on the stone, at the same time as he implored the help of the Almighty. The diggers again applied themselves to their task and the vigour of their arms, increased tenfold by the certainty of success which the Prophet's action had instilled into their hearts, met with no further obstacles. It seemed to them that the hard stone had become as friable as the sand; the rock splitting into countless fragments under the attack of their tools.