Before the commencement of the works, certain ridges, called “fords,” stretched across the bed of the river, at different points between Perth and Newburgh, and obstructed the passage to such a degree that vessels drawing from ten to eleven feet could not, during the highest tides, make their way up to Perth without great difficulty. The depth of water on these fords varied from one foot nine inches to two feet six inches at low, and eleven feet nine inches to fourteen feet at high water of spring tides; so that the regulating navigable depth, under the most favourable circumstances, could not be reckoned at more than eleven feet. The chief disadvantage experienced by vessels in the unimproved state of the river was the risk of their being detained by grounding, or being otherwise obstructed at these defective places, so as to lose the tide at Perth,—a misfortune which, at times when the tides were falling from springs to neaps, often led to the necessity either of lightening the vessel, or of detaining her till the succeeding springs afforded sufficient depth for passing the fords. The great object aimed at, therefore, was to remove every cause of detention, and facilitate the propagation of the tidal wave in the upper part of the river, so that inward-bound vessels might take the first of the flood to enable them to reach Perth in one tide. Nor was it, indeed, less important to remove every obstacle that might prevent outward-bound vessels from reaching Newburgh, and the more open and deep parts of the navigation before low water of the tide with which they left Perth.
The works undertaken by the Harbour Commissioners of Perth for the purpose of remedying the evils alluded to, and which extended over six working seasons, may be briefly described as follows:—
1st, The fords, and many intermediate shallows, were deepened by steam dredging; and the system of harrowing was employed in some of the softer banks in the lower part of the river. Many large detached boulders and “fishing cairns,” which obstructed the passage of vessels, were also removed.
2d, Three subsidiary channels, or offshoots from the main stream, at Sleepless, Darry, and Balhepburn islands, were shut up by embankments formed of the produce of the dredging, so as to confine the whole of the water to the navigable channel, and the banks of the navigable channel were widened to receive the additional quantity of water which they had to discharge.
3d, In some places the banks on either side of the river beyond low water mark, where much contracted, were excavated, in order to equalise the currents, by allowing sufficient space for the free passage of the water; and this was more especially done on the shores opposite Sleepless and Darry islands, where the shutting up of the secondary channels rendered it more necessary.
The benefit to the navigation in consequence of the completion of these works was of a twofold kind; for not only was the depth of water materially increased by actual deepening of the waterway, and the removal of numerous obstructions from the bed of the river, but a clearer and a freer passage was made for the flow of the tide, which begins to rise at Perth much sooner than before; and as the time of high water is unaltered, the advantages of increased depth due to the presence of the tide is proportionally increased throughout the whole range of the navigation; or, in other words, the duration of tidal influence has been prolonged.
The depths at the shallowest places were pretty nearly equalised, being five feet at low and fifteen feet at high water, of ordinary spring tides, instead, as formerly, of one foot nine inches at low and eleven feet at high water. Steamers of small draught of water can now therefore ply at low water, and vessels drawing fourteen feet can now come up to Perth in one tide with ease and safety.
CHAPTER IX.
PRESERVATION OF TIMBER.
1808–1843.
In 1808 Mr. Stevenson was the discoverer of the Limnoria terebrans, that small but sure destroyer of timber structures exposed to the action of the sea, and forwarded specimens of the insect and of the timber it had destroyed to Dr. Leach, the eminent naturalist, of the British Museum, who, in 1811, announced it as a “new and highly interesting species which had been sent to him by his friend Robert Stevenson, Civil Engineer,” and assigned to it the name of Limnoria terebrans (Linnean Trans., vol. xi. p. 37, and Edinburgh Encylopædia, vol. vii. p. 433).