It is not difficult to ascertain how this marvellous development of population and trade has taken place. The situation of Liverpool, with its practically open though well protected roadstead, has, of course, had much to do with the change. But this natural advantage has its drawbacks, and these were sufficiently serious to have prevented progress beyond a certain point had not there been public-spirited and large-minded men to direct the enterprise of the community. To attract navigation, the channels of the river had to be defined, and they had to be kept clear. They had to be buoyed and provided with beacons on both sides. Notable among the guiding influences are the New Brighton Lighthouse (known also as the Perch Rock Lighthouse) at the mouth of the river on the Cheshire side, and the Formby and numerous other lights on the other side along the stretch of the Crosby channel until safe passage out to sea is secured. But something more was needed. The tidal rise and fall of the water-level meant a variation of some 30 feet at spring tides, which made the loading and unloading of vessels difficult, and at times dangerous; besides, the vessels soon became too numerous for ordinary quay accommodation. It was necessary to provide special basins, and the first step in this direction was taken as far back as 1699, when the Pool was deepened and improved. This was but an insignificant beginning to what has now grown to such vast dimensions, but it solved a serious problem for the trade of Liverpool of that day; and in about ten years afterwards the Pool was made into a dock some four acres in extent, giving accommodation for 100 small vessels, Liverpool securing its reward in Parliamentary permission "to impose a duty for twenty-one years upon the tonnage of all ships trading to or from the port for making a wet dock." This earliest of the docks no longer exists; but others were soon afterwards constructed in its vicinity, though parallel with the river, and some of these are still in use. The expansion of the dock system eventually necessitated the formation of a Dock Estate and the acquisition of property along the whole city front. The docks now stretch along the line of the Mersey for a distance of from six to seven miles, and comprise some 25 miles of quay space and 380 acres of water space. In addition, there are nine miles of quay space and 164 acres of water space in the dock accommodation provided across the river at Birkenhead. This is irrespective of graving dock arrangements. The area of the Dock Estate exceeds 1,600 acres, inclusive of provision for extension. [Illustration: SWING-BRIDGE OVER THE ENTRANCE TO STANLEY DOCK, LIVERPOOL (_p. 266_).]