Like an army, a herd of buffaloes put out vedettes to give the alarm in case anything beyond the ordinary occurred. These sentinels were always to be seen in groups of four, five, or even six, at some distance from the main body. When they perceived something approaching that the herd should beware of or get away from, they started on a run directly for the centre of the great mass of their peacefully grazing congeners. Meanwhile, the young bulls were on duty as sentinels on the edge of the main herd watching the vedettes; the moment the latter made for the centre, the former raised their heads, and in the peculiar manner of their species gazed all around and sniffed the air as if they could smell both the direction and source of the impending danger. Should there be something which their instinct told them to guard against, the leader took his position in front, the cows and calves crowded in the centre, while the rest of the males gathered on the flanks and in the rear, indicating a gallantry that might be emulated at times by the genus homo. Generally buffalo went to their drinking-places but once a day, and that late in the afternoon. Then they ambled along, following each other in single file, which accounts for the many trails on the plains, always ending at some stream or lake. They frequently travelled twenty or thirty miles for water, so the trails leading to it were often worn to the depth of a foot or more. That curious depression so frequently seen on the great plains, called a buffalo-wallow, is caused in this wise: The huge animals paw and lick the salty, alkaline earth, and when once the sod is broken the loose dirt drifts away under the constant action of the wind. Then, year after year, through more pawing, licking, rolling, and wallowing by the animals, the wind wafts more of the soil away, and soon there is a considerable hole in the prairie. Many an old trapper and hunter's life has been saved by following a buffalo-trail when he was suffering from thirst. The buffalo-wallows retain usually a great quantity of water, and they have often saved the lives of whole companies of cavalry, both men and horses.