One of the great sights on the river is the Tower Bridge. This is not the newest bridge, but it was built later than most of the others. It has two great towers rising one on each side, to the sky, and the bridge lies across low down between these towers. But when a big ship comes and wants to get up the river under the bridge, what is to be done? The bridge is not high enough! Well, what does happen is this, and I hope that every one of you will see it one day, for it is one of the grandest things in all London: a man rings a bell, and the cabs and carriages and carts and people who are on the bridge rush quickly across to the other side, and when the bridge is quite empty then the man in the tower touches some machinery, and slowly the great bridge, which is like a road, remember, rises up into the air in two pieces, just as you might lift your hands while the elbows rested on your knees without moving, and the beautiful ship passes underneath, and the bridge goes back again quite gently into its place. This bridge has been called the Gate of London, and it is a very good name, for it looks like a giant gate over the river. Close to it is the Tower, of which you must often have read in your history books--the grim Tower where so many people who were not wicked at all were imprisoned, and where some of them were beheaded because, in the time when they lived, there were no laws such as there are now safeguarding people's lives. The Tower will have a chapter to itself later on.