_Signal Mounds_.--It seems quite in keeping with what we have already seen of the sagacity of this wonderful race, that they should erect stations of observation in various suitable regions, so that signals could be given to the multitudes who dwelt in the plain, when they were threatened by an approaching enemy. If a fire were lit on a much burnt mound at the ancient fort near Bournville, it could be seen over a large portion of the valleys, where numerous works are found. No doubt, this was a signal mound, where the appointed watchman, like the watchman of Scripture, could give the alarm of the coming foe, enabling the industrious people to reach the fortress in safety. On a hill 600 feet high, near Chillicothe, Ohio, there is a mound, which in the days of the Mound-builders must have been a signal mound. A light on this can be seen for twenty miles either up or down the valley. The great mound at Miamisburg, Ohio, which is 68 feet high and 852 feet in circumference at its base, served, no doubt, this important department of warfare, as a fire kindled on it could flash light into Butler county, near Elk Creek, where it would again be taken up by the watchman there, and light flashed in the direction of Xenia, and from one signal mound to another until it would reach the great works at Newark. Thus in the course of an hour the whole southern portion of the State of Ohio could be warned of danger and prepare for combat or shelter. Such a system has been used by all nations, both civilized and savage. We need not wonder that the Mound-builders with such sagacity and forethought, should establish such a system of alarm by which the inhabitants could be apprised of invasion. _Indefinite Mounds_.--Of this class there are many. Thousands of such indefinite mounds and squares and circles are to be seen scattered over the various States of the Union. Their structure, composition and contents, give us no clue by which they may be assigned a place. It is believed that many of the strange works that abound in Butler county, Ohio, and which cannot be classified, are among the incomplete works, that is, works left unfinished by the builders.