About three miles west of Alloa, the navigable River Devon falls into the Forth; but between it and the town above-mentioned, a stratum of rock occupies the bed of the river, which constitutes a kind of bar, over which vessels of more than seventy tons seldom venture. This place is a little above the largest island in the river, and is designated the Thrask Shallows. From the Devon, the Forth gradually opens into an estuary, which, opposite the mouth of the Carron, is two miles in width; and a few miles further down, between Borrowstounness and Culross, it is full three miles; but again contracts to little more than one mile a short distance below Queen's Ferry. At Leith, which is seven miles and a half below the last-mentioned place, it is nearly six miles in width; and between Preston Pans and Kirkaldy, it is above twelve miles.
The length of this magnificent river and estuary, from Stirling to the Isle of May, where it may be said to enter the German Ocean, is about seventy English miles, viz. from Stirling to opposite the River Carron and Forth and Clyde Canal, twenty-four miles; thence to Leith twenty miles; and to the Isle of May it is twenty-six miles. Within the limits above described are five ports, viz. Leith, Alloa, Anstruther, Grangemouth and Preston Pans.
The tide flows up this river to Craigforth Mill, a short distance beyond Stirling; and at Cambus Quay, at the mouth of the Devon, (though above fifty miles from the sea,) it is frequently known to rise 20 feet at spring tides. Several attempts have been made to improve the navigation beyond Alloa; and in particular by Messrs. Watt and Morrison, in 1767. These gentlemen proposed to extend the navigation from Stirling to the lime and slate quarries at Aberfoil, and, by four cuts, to shorten the course from Stirling to Alloa seven miles. Mr. Smeaton's opinion was taken on these proposed improvements, and also as to the removal or avoiding the Thrask Shallows; and though all was proved quite practicable, they have been suffered, either from the want of spirit in the parties most interested, or from one cause or other, to remain in statu quo, with all their imperfections.
The River Forth is a free navigation; the only tolls paid on it being for the use, and towards the support of several ferries, for which an act was obtained in the 32nd of George III. cap. 93,
