Jamieson, Fausset and Brown on 1 Samuel 14:14-15

14-15. that first slaughter, which Jonathan and his armour-bearer made, was about twenty men, within as it were an half acre of land, which a yoke of oxen might plow--This was a very ancient mode of measurement, and it still subsists in the East. The men who saw them scrambling up the rock had been surprised and killed, and the spectacle of twenty corpses would suggest to others that they were attacked by a numerous force. The success of the adventure was aided by a panic that struck the enemy, produced both by the sudden surprise and the shock of an earthquake. The feat was begun and achieved by the faith of Jonathan, and the issue was of God.