Then, too, some very solid and substantial barrier was required to be constructed between the excavated basins made and the bank of the river, to prevent the water of the river from bursting in upon the workmen while they were digging. In such a case as this they make what is called a coffer dam, which is a sort of dam, or dike, made by driving piles close together into the ground, in two rows, at a little distance apart, and then filling up the space between them with earth and gravel. By this means the water of the river can be kept out until the digging of the basins is completed. The first set of docks that was made was called the West India Docks. They were made about the year 1800. Very soon afterwards several others were commenced; and now there are five. The following table gives the names of them, with the number of acres enclosed within the walls of each:-- NAMES. ACRES. West India Docks, 295 East India Docks, 32 St. Catharine's Docks, 24 London Docks, 90 Commercial Docks, 49 If you wish to form a definite idea of the size of these docks, you must fix your mind upon some pretty large field near where you live, if you live in the country, and ask your father, or some other man that knows, how many acres there are in it. Then you can compare the field with some one or other of the docks according to the number of acres assigned to it in the above table. If you live in the city, you must ask the number of acres in some public square. Boston Common contains forty-eight acres. St. Catharine's Docks contain only twenty-four acres; and yet more than a thousand houses were pulled down to clear away a place for them, and about eleven thousand persons were compelled to remove. Most of the docks are now entirely surrounded by the streets and houses of the city; so that there is nothing to indicate your approach to them except that you sometimes get glimpses of the masts of the ships rising above the buildings at the end of a street. The docks themselves, and all the platforms and warehouses that pertain to them, are surrounded by a very thick and high wall; so that there is no way of getting in except by passing through great gateways which are made for the purpose on the different sides. These gateways are closed at night. Mr. George and Rollo, when the time arrived for visiting the docks, held a consultation together in respect to the mode of going to them from their lodgings at the West End. Of course the docks, being below the city, were in exactly the opposite direction from where they lived--Northumberland Court. The distance was three or four miles. "We can go by water," said Mr. George, "on the river, or we can take a cab."