September 21st, 1745 - the Battle of Prestonpans. At three in the morning, the the entire Jacobite force walked three abreast along the Riggonhead Defile down towards the sea, crossed a morass, and formed line of battle to the east of Cope. Cope had feared an attack against both his flanks, and had realigned his army on a north-south front, in the position in which they would fight on the next day.
At 6 am Cope's dragoons saw 1,400 Highlanders charging through the early mist, making "wild Highland war cries and with the bloodcurdling skirl of the pipes....". His inexperienced army wheeled to its left by platoons to face the Highlanders, who were charging in from the east following their night march. Cope managed to scramble some cannon up onto his right flank. Because Cope had no gunners, Lt.-Colonel Whiteford (Marines) served the guns with his own hands, and Mr. Griffith (Commissary) the cohorns, and the two officers opened fire as soon as the Highlanders were in range. Undaunted by the light, inaccurate guns, the Highlander army continued its charge; however, the centre became bogged down in marshy terrain, and, as they continued forward, their different speeds of advance caused them to form into a "V". The wings on either side met the inexperienced dragoons on either side of Cope's centre, and the dragoons immediately fled the field. The centre, containing the experienced royal infantry were left facing the centre of the "V" on their front, and the two unopposed wings on either side, leaving them sandwiched. They suffered heavy casualties and gave way. The entire battle lasted just seven or eight minutes.
The Jacobites had around 30 killed and about 70 wounded, and Cope had suffered around 300 dead with 500 wounded. 1,500 prisoners were taken, and many of those who were Highlanders joined the Jacobite army. The wounded and prisoners were given the best care possible at the Prince's insistence. Along the road, Cope's baggage train at Cockenzie, containing £5000 (modern equivalent £600k/US$800k), many muskets and ammunition, was captured with only a single shot fired. Cope tried to rally his men, but could only lead about two hundred stragglers up a side lane (Johnnie Cope's Road) to reorganise in an adjacent field, where they refused further engagement. Cope had no choice but to travel southwards to Lauder and Coldstream and then on to the safety of Berwick-upon-Tweed, 50 miles away. Out of the 2,300 men in the royal army, only 170 troops managed to escape.
This victory did much to bolster the Jacobean cause, and many more recruits were soon gained in Scotland. The Prince spent the night at Pinkie House, while the army occupied Musselburgh.
ETA: The battle was generally called Gladsmuir at the time (sometimes Tranent, especially by the British). A petition of the inhabitants of Prestonpans complains that they are ' deprived of that honour and fame which of right belongs to them,' and demands that the battle should have its title from one of the towns or villages near which it was fought, and not after 'a barren muir'. Since then it has been generally called the Battle of Preston, and more modernly of Prestonpans.
This fine map is a British military one from 1745. It shows the area, where Cope's army had been set out at various times, where they thought the Jacobites were - and where it turns out the Jacobites actually were.