Titus Livius (Livy), The History of Rome, Book 33
Evan T. Sage, Ph.D. Professor of Latin and Head of the Department of Classics in the University of Pittsburgh, Ed.

7. Philip wished to hurry and so ordered an advance, undeterred by the low-hanging clouds after the rain; [2] but so dense a fog obscured the day that the standard-bearers could not see the road nor the soldiers the standards, and the column, straggling along in obedience to the various cries, was as disorderly as if wandering about at night. [3] They crossed the hills which are called Cynoscephalae¹ and encamped after leaving there a strong guard of infantry and cavalry. [4] Although the Roman had stayed in the [p. 293]same camp near Thetideum, nevertheless he sent out² ten troops of cavalry and one thousand infantry to discover where the enemy was, with orders to guard against ambushes, which the darkness would hide, even in open country. [5] When they came to the guarded hills, both forces remained passive, as if struck with a mutual fear; then they sent messengers back to the camps to their commanders, as soon as their panic from this unexpected contact had subsided, and did not longer postpone the fight. The battle began at first with skirmishes of a few scouts in advance, then assumed larger proportions as reinforcements came to the aid of the defeated. [6] In this battle, when the Romans were not holding their own, but kept sending message after message to their commander that they were hard pressed, five hundred cavalry and two thousand infantry, [7] mostly Aetolians, under two military tribunes, were speedily sent and restored the unfavorable battle, and as fortune changed the Macedonians, finding themselves in difficulties, begged through messengers for aid from the king. [8] But since he had expected anything but a pitched battle that day, on account of the general darkness from the fog, having sent most of his troops of every sort out to forage, he hesitated for a time, not knowing what to do; [9] then, as messengers kept urging him, and the fog had now uncovered the ridges of the mountains and he could see the Macedonians crowded together on the highest of a number of hills, defending themselves more with the advantage of position than with arms, thinking that he must at any [10] rate stake everything, lest he suffer the loss of some of his men, left unsupported, he sent Athenagoras, commander of the mercenaries, [11] [p. 295]with all the auxiliaries except the Thracians and with³ the Macedonian and Thessalian cavalry. [12] On their arrival the Romans were driven from the ridge and checked their retreat only when they reached more level ground in the valley. The Aetolian cavalry was the greatest safeguard to prevent their utter rout. [13] At that time their cavalry was by far the best in Greece; in infantry they were inferior to their neighbours.