Reference: These lines have been taken from the lesson ‘On an African Rives’ written by David Livingston. 

[ N.B. : The above reference will be used for all explanations of this lesson. ] 

Context: In this lesson the writer describes his personal experiences of wildlife in South Africa. He sailed to a hill station with his family and a fellow explorer named Oswell in April, 1851. 

Explanation : In these lines the writer tells us that hippos generally live together in a herd and herd is never dangerous to man. But if a canoe passes through their midst, when they are sleeping they become dangerous and may attack the canoe. So, the travellers are advised that they should always direct the boat near the bank during day time and in the middle of the river in the night. Generally hippos run away to see a man. But if a hippo is turned out of its herd it becomes violent like an elephant. The writer saw a boat which was broken into pieces by any one of the hippos by striking its hind foot against it again and again.