1841.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



133 



PLYMOUTH BREAKWATER LIGHTHOUSE. 

 A lighthouse is in course of erection upon the western extremity of the 

 Breakwater, the first stone of which was laid by Admiral Warren, on tlie 

 22nd of Feliruary last, it was designed by Messrs. Wallver and lUirges, the 

 engineers of the Trinity Board, in July last, and submitted to tlic Admiralty. 

 Shortly after, their Lordships gave directions for its immediate construction. 

 It is to be erected upon an inverted arch, the foundation of wliich is laid 

 about 1 foot 6 inches below the level of low water spring tides, its centre at 

 top is at the distance of 37 feet 6 inches from the western end or head of the 

 Breakwater, and at the level of low water \0l) feet. The diameter of the 

 head of the Breakwater at the level of low water is 390 feet, and at the level 

 of the top of the Breakwater 75 feet. Tlie Ughthouse is to be of granite 14 

 feet clear diameter, the centre of the light will be 55 feet from the top of the 

 Breakwater. The interior will be divided into floors, forming a store room, 

 a dwelling-room, a bed-room, and a watch-room. The lantern 12 feet wide 

 and 7 feet 6 inches high, is to sliow a Dioptric fixed light of the second order, 

 with mirrors; the south half to show a red light, to (listinguish it from tlie 

 coast lights, and the north side towards the Sound, is to be white. The 

 stones of the lower courses are to be secured with dowels of slate, independent 

 of a vertical and horizontal dovetail, the dowels are 18 inches long and G 

 inches square at the centre, and sunk S inches into the lower course of stone, 

 both ends are dovetailed and secured in their places by plugs in the upper, 

 and by wedges in the lower stone. It is expected that tlie lighthouse will be 

 completed by the end of 1842. 



MERSEY AND IRW ELL NAVIG.\T10N. 



We preseut in the accompanying reports of Mr. Palmer and Jlr. Bateman 

 the groundwork of a long discussion,* which has taken place at the Royal 

 Victoria Gallery, Manchester. In this discussion which lasted for several 

 evenings, Mr. Hawkshaw, Mr. G. W. Buck, Mr. Joseph Radford, Mr. W. 

 Fairbairn, Mr. T. Fairbairn. Mr. Bateman, Mr. Thomas Hopkins, and other 

 engineers took part. The proceedings are of particular interest on account 

 of the important questions concerned in them, and of the public being thus 

 brought to take a part in a professional subject. Of a debate of such length 

 it would be impossible to give even an abstract, but we may mention some 

 of the opinions put forward. Mr. Palmer is in favour of contracting the up- 

 per part of the river estuary, and forming the river as a funnel ; Mr. Bateman 

 is in favour of contracting the upper part, but opposed to interfering with 

 the estuary ; Captain Denham, opposed to contracting the estuary ; Mr. W. 

 Fairbairn, on Mr. Batcman's side ; Mr. Buck, in favour of contracting the 

 upper part, thinks the estuary might be partially contracted ; Mr. Hawkshaw, 

 of opinion that the upper part could not be improved without the neck of 

 the estuary below Liverpool being contracted ; Mr. Radford, Mr. T. Fairbairn, 

 and Mr. Hopkins support Mr. Palmer. Thus in favour of improving the up- 

 per part of the river, the numbers are — 



For, 7 Against, 1 



With regard to the bay at Runcorn Gap, 



For .Mr. Palmer, 4 ; Mr. Bateman, 2 ; against, 2. 



With regard to contracting the Mersey, 



For, 4 Partially, 1 .\gainst, 4. 



Extracts from a " Report on the Improvement of the Riverx Mersey and Irwell 

 between Liverpool and Manchester^ describing the means of adapting them 

 for the nacigation of Sea-going I'essel-w By Henry R. Palmer, F. R. S., 

 Vice-Pres. Inst. C. E." 



At the time when inland navigation by means of artificial canals met with 

 such extraordinary eucouragement, the prevailing opinion was opposed to 

 the use of rivers, chiefly on account of their currents, especially during rainy 

 seasons. Probably this impression may have derived some of its strength, 

 from the well-known bold expression attributed to the late Mr. Brindley, 

 under whose superintendence the Bridgewater Canal was constructed. The 

 advantages which that celebrated work exhibited over the natural line of 

 navigation, at the time the former was constructed, were no doubt obvious, 

 and many other instances might be cited, which would equally point out the 

 superiority of an entirely artificial canal, over an imperfect or ill-regulaled 

 line of river navigation. 



The actual distance, in a straight line, between the quay at Manchester, 

 and the Cofnpany's Dock at Liverpool, is about thirty-three miles ; while 

 the length of the channel, in its natural course, between the same points, is 

 forty-eight miles ; the circuities amounting to no less than fifteen miles. 

 Those circuities have, however, been reduced seven miles, leaving the present 

 length of the line of navigation forty-one miles. 



The width of the river at Manchester is 108 feet, at Warrington 140 feet, 

 at Fidler's Ferry 170 feet, and at Cuerdly Point 650 feet. 



From thence it rapidly widens to 3,500 feet. It is abruptly reduced to 

 1,200 feet at Runcorn Gap, and, within a short distance, is again widened to 

 4,200 feet. 



* It was our original intention to have published the discussion, but it ex- 

 tended to such a great length that we were obliged to abandon our intentions. 

 — Ed. 



The widths continue to vary considerably towards the river's mouth, ex 

 tending in one part to two and a half miles, and again diminishing to 3,300 

 feet at Liverpool. 



The level of the highest tide, uninfluenced by a strong wind, intersects the 

 bed of the river at Woolston, lieing a distance by the course of the channel 

 of about 25J miles above Liverpool, and the bed of the river at Manchester 

 is 49 feet above the level referred to. The first weir in the ascending di- 

 rection is «t Warrington, and the distance from thence to Manchester is 

 divided into 10 pools. 



The navigation of the river between Liverpool and the lock at Warrington 

 is dependant upon the tidal water, and the whole of the remaining distance 

 upon that derived from the uplands. 



At Liverpool the spring tides rise 33 feet 



At Runcorn 16J „ 



At Warrington 8 „ 



The lowest of the neap tides at Liverpool rise 231 feet, and if the wind be 

 strong in the adverse direction they do not extend to Runcorn. The depth 

 of water at Liverpool with a high spring tide is 89 feet, but the bed of the 

 river is rapidly elevafeil, and the depth during the same tide is diminished to 

 33 feet in a distance of 9^ miles. 



.V 33 feet tide at Liverpool occasions a IG^ feet tide at Runcorn; thus 

 sliowing the bed of the river at Runcorn to be about 16^ feet above the 

 level of low water mark at sea, assuming the line of high water mark to be 

 level between the two places. This, however, is not strictly the fact, and 

 will he hereafter the subject of explanation. 



The river is subject to considerable land floods, wliich descend with great 

 impetuosity, and overflow tlie banks, laying under water extensive areas of 

 marshes. .\ land flood implies an accumulation of the water of drainage de- 

 rived from a more than ordinary quantity of rain. The river channel being 

 jiroportioned only to an average quantity, the surface of the stream is neces- 

 sarily raised. But the accumulations that are so injurious, and which are 

 complained of, are not to be .-ittributed to any natural deficiency in the ca- 

 pacity of the channel, but to the permanent barriers or weirs that have been 

 erected, which diminish the water space nearly three fourths, without any 

 compensation having been provided. 



The evil consequences of such circumstances are of far greater magnitude 

 than has been supposed. It is well known that the water in its descent over 

 the lands, washes down such loose soil as it is capable of removing ; the 

 same being conducted into the channel of the river, it is carried out to sea, 

 if the moving power continue to be suflicient throughout the whole distance. 

 The natural slope of the Mersey above the tideway is such as would occasion 

 a considerable velocity of the water, but by dividing it into a series of pools, 

 the velocity is, as it were, concentrated at the weirs, and the motion betweea 

 them is much inferior to that which is required for removing the soil brought 

 down by the rains. The cleansing of the channel is therefore exclusively 

 dependant upon extraordinary quantities of rain from whence an increased 

 velocity is obtained. 



But if the weirs were altogether removed, it is obvious the river above the 

 tideway would cease to be navigable ; weirs of some kind are indispensable 

 where the slope of a river is great, but it is equally clear, that they should 

 be so constructed as to prevent the least hindrance to the motion of the 

 floods. 



Seeing that the fixed weirs contribute so largely and injuriously to impede 

 the motion of the water, and tlierefore to elevate its height during floods, we 

 find that a large proportion of it is made to pass over surfaces which are in 

 no way benefitted, but which are damaged by it ; while its use as a scouring 

 power is altogether lost. While these effects cannot, perhaps, be entirely 

 prevented, they may be greatly diminished, by so constructing the weirs that 

 the impediment they cause shall have relation to the quantity of water in 

 the river. If the weirs were properly made self-adjustable, according to cir- 

 cumstances, the bed of the river would be acted upon during longer periods, 

 and therefore more eft'ectually cleansed. 



Frotn the parallelism of the upper division, its bed is comparatively regular. 

 The lower division is, however, of a contrary character ; the extent of surface 

 covered by the tides is such as to permit an effect upon their motion caused 

 by the winds. The sands of which the bed is composed are therefore sub- 

 ject to a change of place, and hence the positions of the shoals are everMable 

 to variation. 



From this circumstance the channel or line of deepest water varies also, 

 and becomes divided in various places ; so that instead of one permanent 

 course, having a depth which is due to the naturiil force of the descending 

 waters, several channels are formed, of which neither can be of the depth, 

 that in a single channel, would be maintained. 



There can be no doubt that the condition of a river is best for the purposes 

 of navigation, when the deepest part is limited to one permanent and regular 

 track. This can be effectually obtained only by causing the flowing and ebb- 

 ing waters to act in the same lines ; such a condition may not be practicable 

 where the scale of the river is of so great a magnitude, that the motion and 

 action of the water is influenced by winds. 



The principle, however, should be kept in view, and should be approached 

 as nearly as the means extend. A regularity in the outhne or borders of 

 the river is essential for the production of the effect required ; and while the 

 opposite banks of the Mersey remain as they now are, totally inconsistent 

 with each other, we cannot hope for the improvement so much needed, and 

 which is obviously within the power of art greatly to assist. 



