The lawmakers have tamed and controlled a free flowing river. This use of imagery emphasises that everything in the city including natural forces are enslaved by the city. In the next line, Marks of weakness, marks of woe, there could be a play on words, Mark means both to see or to notice but then again there could be another meaning; like a physical mark upon someone's face like a sign of grief or misery. The use of the word mark I think, is deliberately repeated to sound like the blows of a hammer. Blake uses this imagery to emphasise the pain which industrial London is enforcing on the poor, physically and mentally. The use of mind-forg"d manacles in line 8 is used to describe why the people are so unhappy, this is because they are not free as there lives are being controlled by oppressive or restrictive ideas within their own minds and created by the minds of others. Also by using the manacle the word sounds heavy, just like their plight. Black"ning Church appalls is a vivid and chilling image. The church could be blackened literally because of the soot from London's chimneys, or it could be because the sun is setting and the outline of the church can be seen in the fading light. Blake's use of Black"ning could be symbolic; the church which should be a source of moral warmth and light, is seen as cold and dark. There could be another meaning to the word appalls like a pall over a coffin so it is used to emphasise that the church ignores what it doesn't want to see. Another shocking and surprising image is Runs in blood. This is where the wounded soldier's blood is running down the walls of the rulers for which he has been fighting, so it emphasises the fact that the poor were being blocked out by the government with no means to live, and many to die. The youthful harlots curse is a contradicting image which makes you think how could a harlot be youthful? It shows that even children were subjected to the crimes of London.