1850.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



IG7 



na\ igations, which, if he might use the expression, formed their 

 only highways of communication with the ocean. 



In introducing this subject, lie would endeavour to explain what 

 ■was implied in the word " tidal," as used in particular with reference 

 to British ports, as he apprehended there was much more importance 

 to he attached to the term than thosewho had not studied the subject 

 were aware of; and he believed he would best explain this by 

 drawing a comparison between Britain and some large tract of Con- 

 tinental country, such, for example, as North America. We there 

 found capacious rivers extending for hundreds, he might say thou- 

 sands, of miles into the interior of the country, and discharging 

 an enormous amount of fresh vrater into sheltered and deeply in- 

 dented bays — these indentations in the line of coast bearing, in 

 fact, some'proportion to the sizes of the rivers which flowed into 

 them ; and such a physical formation afforded facilities of no ordi- 

 nary kind, not only for the establishment of safe harbours on the 

 sea "coast, without the expenditure of capital in their protection, 

 but also for the extension of inland navigation to an almost unlimited 

 degree, by means of the rivers themselves. 



To give a practical idea of this, he stated that when he visited 

 America, twelve years ago, he came to the conclusion, after 

 examining the principal harbours on the sea coast which afforded 

 most perfect shelter and a great amount of accommodation, that 

 the formation of the smallest of our I'ost-Office packet stations in 

 the Irish Channel had consumed a much larger expenditure of 

 capital than the Americans have found it necessary to invest in the 

 formation of harbour accommodation for trading vessels along a line 

 of coast of no less than 4000 miles, extending from the Ciulf of St.- 

 Lawrence to the Mississippi. With reference to the rivers which 

 discharged into these bays, it was impossible in words to convey 

 an adequate idea, or to describe the feelings which the traveller 

 experienced, when, for instance, after crossing the Alleghany 

 Mountains, and completing a fatiguing land journey from the 

 eastern coast of several hundred miles into the interior of the 

 country, he first came in sight of the river t)hio at Pittsburg. 

 There, in the very heart of the continent of North America, he 

 found a large shipping port, containing a lleet cf between thirty or 

 forty steamers, varying from 300 to 700 or bOO tons burthen, 

 moored in the river ; and his astonishment was still nuire increased 

 if he chanced to witness the arrival of one of those steamers, and 

 was told she had come direct from New Orleans in the Gulf of 

 Mexico, and that fifteen days and nights had been occupied in 

 making her inland voyage of no less than 2000 miles among the 

 meanderings of the Mississippi and Ohio ! = 



But Mr. Stevenson stated that with us the case was altogether 

 diffei-ent— the isolated and comparatively contracted limits of our 

 country did not afford area for the collection of such bodies of fresh 

 water. In proof of this, he referred to the comparative areas of 

 the basins and the discharges of the different rivers, viz. : — 



Our streams could therefore, he said, be advantageously navigated 

 only when their waters were deepened by the influx of the tide, 

 and they were consequently closed to all vessels, excepting to 

 those of the smaller classes, during the absence of tidal influence ; 

 and therefore our rivers, when compared to those of our Trans- 

 atlantic or even Continental brethren, could only be regarded as 

 narrow creeks or inlets, kept open by the joint action of the fresh- 

 water stream and the tide; and as the action of the fresh water 

 varied in its extent, and was at best but feeble, that our greatest 

 stronghold in keeping open and deepening our navigations, must 

 be sought for in the action of the tide, which rot only scoured and 

 maintained in a navigable state the sea channels of our rivers and 

 estuaries, but also by its presence increased their depth of water. 

 It was likewise, he said, to be noticed, that the fall or inclination 

 of these large continental rivers has been found to be exceedingly 

 small; — for example, the inclination of the Mississippi had been 

 estimated to average, from its source,* ... per mile Sin. 

 The Amazon ... ... ... ... 5 



The Ganges ... ... ... ... 4 



While the Thames was ... ... ... 21 



The lower part of the Dee ... ... U 



The Lune ... ... ... 23 



The Forth ... ... ... 13 



5 Stevenson's Sketch of Civil Engineering in North America London, 1838. 

 e Traill's Physical Geography. Edinburgh, 1838.— Johnston's Physical Alias. Edin- 

 burgh, l»4a. 



The currents of the larger continental rivers were, therefore 

 more languid and more easily navigated, whereas the currents o^ 

 our rivers were more powerful and less easily overcome. Bii 

 here, again, an important advantage was derived from the tida 

 influence, which ]iroduced an upward current, by which vessel 

 were enabled, without the aid of steam or wind, to reach their 

 port; and he thought that was a view of the subject which could 

 not fail to have struck the most superficial observer, when he saw 

 on any of our navigable rivers or estuaries (such as the Thames 

 or Mersey) a vast fleet of all sizes and from all countries, hurried 

 on by the silent but powerful energy of the flowing tide. ^Vhat 

 an amount of latent power lay there! And how invaluable was 

 that agency to the commerce of this country! If, indeed, the 

 natural power latent in the tides of the Thames and Mersey were 

 suspended, it might truly be said of the stmm power employed on 

 the net work of railways connected with London and Liverpool, 

 that its occupation would be gone. Whatever, therefore, had for 

 its object the improvement or maintenance of tidal navigations 

 was, he submitted, of vast importance to the commerce, and en- 

 titled to the attention of our country.' 



Mr. Stevenson then proceeded to explain that the chief obstruc- 

 tions to the propagation of the tides wei-e the circuitous routes of 

 river.s — the slopes of their beds — the projection of obstacles into 

 their streams; and that the works by which these obstructions 

 could be best overcome consisted in the deepening, straightening, 

 and widening of the channels — the formation of new cuts — the 

 erection of low rubble walls for the guidance of the currents of the 

 first of flood and the last of ebb tide — the shutting up of sub- 

 sidiary channels— and the removal of projecting groins. That the 

 more rigidly that class of works was adhered to, the more gene- 

 rally beneficial would be the effect produced; for not only did they 

 improve the part of the navigation where they were executed, but 

 that their tendency was to increase the back water by which the 

 sea channels were kept open. He then proceeded to illustrate 

 these views by referring to the Tay, Forth, Kilible, Lune, and 

 other rivers, where the duration of the tidal influence had been 

 prolonged from 30 minutes to an hour, and the range of tide in- 

 creased from 9 to 5 feet, while the navigation in all cases had been 

 proportionally improved. Time did not admit of Mr. Stevenson's 

 alluding to many other examples of importance, but tlie Cljdc 

 might be cited as a proof of the length to which such improve- 

 ments had been carried. In 1755, Smeaton proposed to improve 

 that river by erecting a dam across it with locks in the lower part 

 of the river. In 1775, Golburn surveyed the river, and reported 

 that, as far down as Kilpatrick, there were only 2 feet of water in 

 it, but conceived that the river itself might be improved. In 1(S31 

 vessels drawing 13 ft. 6 in. came up to Glasgow, and now large 

 vessels, three or four deep, are to be seen ranged along each side 

 of the harbour. During 1834, 27,000 vessels passed Renfrew ferry, 

 at some periods from 20 to 30 of them in an hour.'* He next 

 stated that, as an engineer could not form a design for such im- 

 provements without accurate data, it was of the highest import- 

 ance to obtain correct information as to the tides, currents, and 

 discharge of rivers, as well as the nature of their beds, and other 

 particulars, and proceeded to explain how these data were ob- 

 tained, and showed the different instruments for ascertaining the 

 velocities of surface and under currents, and for procuring speci- 

 mens of water from different depths; but for details on all these 

 points referred to his treatise on marine surveying.'-' 



He further referred to the plans of the Tay, the Ribble, Mer- 

 sey, Dee, Lune, &c., to show that in each of these rivers there 

 existed a large basin, or estuary, into which the tide flowed, and 

 from which it was discharged twice in twenty-four hours, and stated 

 that it was the flux aiul reflux of the large ^■olume of tidal water 

 from these natural basins which scoured the seaward channels, 

 kept down the tracts of sand-banks by wliich their entrances were 

 encumbered, and maintained a navigable depth of water over their 

 bars. The instances to which he referred were all what were 

 termed bar-rivers, or harbours, in contradistinction to such rivers 

 as the Forth or Clyde, which had not similar obstructions. The 

 fact of the existence of a strong tidal under-current was adduced 

 to prove the effect of the flood-tide as a scouring agent, and it was 

 stated, that while the fresh water, being specifically lighter, floated 

 on the surface, the tidal current flowed in a stronger current be- 

 low. He instanced, in proof of thi.s, various examples, particularly 

 the observations of Professor Traill and Captain Sabine on the Ori- 



7 Stevenson on the Improvement of Tidal Rivers. London, 184i>. 



8 Cleland's Statistical Documents— Transactions of British Association. 18:^6. 



8 Stevenson ■ n the Application of Marine Surveying and Uydiouictr) to tbe pruelive 

 01 Civil Euginveriug. LUiuburgh, U42. 



