{139}It was not until the passing of the Mersey and Irwell Navigation Act, in 1720, that the work of rendering these rivers navigable between Warrington and Manchester was begun, and another twenty years elapsed before it was completed. The result of this "conveniency of water carriage" when it was, at last, obtained, was to reduce the cost of transport of goods and merchandise from forty shillings a ton by road to ten shillings a ton by river. The goods traffic between Liverpool and Manchester at this time amounted to about 4000 tons a year; but it had, prior to the provision of water transport, naturally been restricted to the quantity that could be carried by the packhorses, carts and waggons of those days. Hence the river navigation gave the advantage of a transport not only cheaper in price but greater in capacity. It will be seen later on, however, that the Mersey and Irwell navigation subsequently developed disadvantages for which a remedy was sought in the construction of the Duke of Bridgewater's canal. An Act, passed in 1720, for making the river Weaver navigable from Winsford Bridge, beyond Northwich, to Frodsham Bridge, near the junction of the Weaver with the Mersey (a distance of about twenty miles), was not only of further material advantage to the port of Liverpool but a first step in an important development of the salt mines of Cheshire. These mines have been described as "incomparably the richest of the salt mines and brine pits of England"; but at the date in question their working was greatly hampered by transport costs and difficulties in the matter both of fuel and of the distribution of the salt, when made. Fuel was required for heating the furnaces and the pans in which the brine was evaporated into salt; and in the earliest days of the industry the salt-makers used for this purpose faggots of wood brought from the forests on the borders of Cheshire and Staffordshire. As long as these supplies were available, the principal seat of the salt trade was at Nantwich, in the higher part of the Weaver, and near to the forests where the wood was obtained. But the forests got depleted in course of time, and the industry then moved to other works lower down the river which could be operated with coal brought from the Lancashire coal-field. This coal, however, had to be carried, by cart or packhorse, a distance of twelve {140}or fourteen miles; and inasmuch as two tons of coal were required for every three tons of fine salt made, the cost of transport of raw materials was a serious item.