It was the custom in those days, as it is now when armies are marching, to send forward small bodies of men in every direction to explore the roads, remove obstacles, and discover sources of danger. These men are called, in modern times, _scouts_; in Alexander's day, and in the Greek language, they were called _prodromi_, which means forerunners. It is the duty of these pioneers to send messengers back continually to the main body of the army, informing the officers of every thing important which comes under their observation.