Autobiography of Sir John Rennie, F.R.S. 4

Autobiography of Sir John Rennie, F.R.S. 4

artificial harbours for packets, for the purpose of establishing the best

and most direct communication between the south-west coast of Scotland

and the opposite coast of Ireland. In addition to making the surveys of

the different ports above mentioned, it was absolutely necessary to make

a correct chart of the channel within the above limits, comprehending

a coast-line of about 30 miles on each side, including the Copeland

Islands. It was also necessary to determine the exact distances between

the different ports, together with the soundings, currents, rises of

tide, prevailing winds, and all the other attendant hydraulic phenomena.

This was a very extensive survey, and required great skill, judgment, and

experience, and Mr. Giles was fully competent to undertake it.

 

As the shore, particularly on the Scotch side of the channel, was very

precipitous, rugged, and mountainous, it was impossible to measure with

anything like accuracy a base line from which a series of triangular

observations could be made so as to connect the two coasts together;

Mr. Griles was therefore obliged to resort to the Bay of Luce, situated

about 10 miles to the east of Port Patrick; it had this disadvantage,

however, that in consequence of the intervening mountains neither the

Scotch nor Irish coast could be seen. But there was no alternative, for

no other convenient place could be found to measure a base line. The

head of the Bay of Luce, however, at low water consists of an extensive

district of flat sand 6 miles long, admirably adapted for the purpose of

measuring a correct base line. This plan was accordingly adopted, and

a base was measured 6 miles in length, first by the common chain, then

by another chain consisting of steel links each 5 feet long, and lastly

by well-seasoned deal rods each 10 feet long; these measurements were

repeated with great accuracy several times, and a mean was then struck

by which the variation was reduced to a fraction of an inch. It should

also be observed that the measurements were taken when the atmosphere was

about the same temperature, so that the final measurement was reduced

to as near accuracy as practicable. Having established the base, strong

vertical staffs with flags at their summits were then accurately fixed

upon the summits of the neighbouring mountains which overlooked the Irish

Channel, and from which on a clear day the opposite coast of Ireland

could be distinctly seen. From each end of the base the above-mentioned

angles were taken with one of Troughton’s best 7-inch theodolites,

between the different mountain stations, and the distance between the

extreme points of these stations was accurately calculated, so that it

gave a base line of about 30 miles along the Scotch coast, from whence

correct sights were taken on flagstaffs fixed on the high land above

Bangor, Donaghadee, and Ballantrae, including the Copeland Islands on the

opposite coast of Ireland. These observations being taken on different

levels, were subsequently reduced to the same plane, so that the exact

distance was obtained between the different stations on each side of

the channel, and a correct chart was made. In addition to the general

survey, detailed hydraulic charts were made of the places on each coast

which were best adapted for making harbours of the size required, viz.

Port Nessock, Ardwell, and Port Patrick, on the Scotch side, and Bangor,

Ballyhone, Donaghadee, and Ballantrae, on the Irish side of the channel;

and as there was a great variety of interests concerned, each proprietor

being desirous of having the permanent ports established on his own

property, it was finally decided by the Government, at the recommendation

of my father, that the whole of the surveys should be submitted to the

arbitration of the Trinity Board, to select one port on each side of

the channel which was best adapted for establishing a communication by

packets between the two countries for letters and passengers in the most

expeditious, convenient, and effective manner. The Trinity Board, after

having given the subject their most mature consideration, ultimately

decided on Port Patrick on the Scotch, and Donaghadee on the Irish

side. This decision was approved of by the Government, and Mr. Rennie

was desired by them to prepare designs for harbours in both of the

above places; he did so, and the Government ordered them to be carried

into effect under his direction: for a more particular description the

reader is referred to my work on British and Foreign Harbours. During

the progress of these surveys I learned a great deal in this important

department of civil engineering, and personally surveyed the bay of Port

Nessock and the Copeland Islands, which gave me an excellent lesson, as,

on account of the rugged nature of the coasts, they were attended with

considerable difficulties.

 

As already observed, in the following year, 1815, I was placed under

Mr. Giles’ direction during the whole period of the survey of the river

Tyne, which had been entrusted to him by my father also. In the same

year I was under Mr. Giles during the surveys for the Barnard Castle,

and Stockton and Darlington railways and canal; and I subsequently made

a hydraulic survey of the port of Blythe for my father, and for which

he afterwards made a design for its improvement. I had been previously

employed under Mr. Giles in the surveys for the eastern extension of the

Kennet and Avon Canal. During a part of this time our head quarters were

at Windsor; and one Sunday afternoon I recollect very well attending

the promenade on the terrace at Windsor Castle when His Majesty King

George the Third, accompanied by the Queen and Royal Family, made their

appearance with their attendants and joined the promenade, and were

received in the most loyal and affectionate yet respectful manner. I also

about the same time assisted Mr. Giles in the survey of the Thames in

the vicinity of Woolwich Dockyard, the accumulation of the mud in front

of which was so great at that time that it threatened to render that

dockyard useless. The evil to a certain extent has since been remedied by

removing a number of the projections which interfered with the currents

of the ebb and flood tide, according to the plan laid down by the late

Mr. Rennie, although, as he clearly pointed out, it would be impossible

to improve the river to such an extent in front of Woolwich and Deptford

dockyards as to render them fit for the construction and accommodation

of large vessels of war; and therefore he recommended that they should

be abandoned and sold, and that a proper establishment should be made

at Northfleet capable of accommodating at all times of tide any number

of the largest vessels of war at that time in the navy, or that might

be built hereafter. This dockyard was intended to be so arranged that

all classes of vessels of war could be built there; and it would contain

establishments for manufacturing cordage, sails, anchors, guns, smith

work of every kind, together with depôts of provisions, and stores of

all sorts, all of which were to be arranged in such a manner that each

operation of building, repairing, storing, and fitting out for sea should

be completed in the order required; so that a vessel, after having been

built, might be transferred at once into a dry dock to be coppered,

then take in her masts, ballast, rigging, sails, stores, provisions,

guns, boats, and seamen, and sail at once complete to her station from

the centre dock-gates; in the same manner, when returning from her

station, she might be repaired and fitted out again ready for sea. The

saving effected by such an establishment would have been immense, and

the service would have been performed in a far more expeditious and

effective manner. Mr. Pitt was then Prime Minister, and his master mind

at once acknowledged the advantages of such an establishment; it was

accordingly approved of by the Government, the land was bought, and it

was ordered to be carried into effect, but the unfortunate death of

that great minister and the change of Government effectually put a stop

to all further proceedings in this direction. Since that time the old

dockyards of Woolwich and Deptford, Sheerness and Chatham, have been

improved and enlarged, at an expenditure as great as would have completed

Mr. Rennie’s establishment at Northfleet, without half the efficiency or

accommodation. The old useless dockyards of Woolwich and Deptford, which

the late Lord Melville, when First Lord of the Admiralty, condemned and

ordered to be sold, have been retained, and the increase of unnecessary

expense has been enormous, whilst the evils complained of have not been

remedied till quite lately.

 

Having by this time a considerable knowledge of surveying, practical

and theoretical, in all its departments either on land or water, so

that I could undertake either, my father deemed that I had learned

sufficient, and directed me to return to my practical duties in the

construction of the Southwark Bridge. As this was considered a work of

great importance and difficulty I felt highly honoured by my father’s

confidence, and devoted my energies to it with the greatest anxiety and

with a determination to do everything in my power to make it successful.

The difficulty of obtaining the large blocks of granite and other stone

had been successfully overcome, and they arrived with great punctuality,

and the masonry of the abutments and piers was successfully carried into

effect as previously described. Then came the important question of

erecting the cast-iron arches; each arch consisted of eight main ribs,

and each rib consisted of thirteen pieces, the lower or main part of

which formed the chord or arch upon which the whole of the superstructure

was to be supported. These thirteen pieces were solid, 2½ inches thick

in the mass and 3 inches thick at the bottom, and 2½ thick at the top,

and formed so many radiating blocks, like arch-stones. At the end of

every block there was a transverse frame extending from one side to

the other through the whole width of the bridge; against each side of

these frames the main ribs abutted and were nicely fitted to them, and

in order to prevent them from moving laterally there were projecting

dovetailed cheeks cast on the frame, and between these cheeks and the

ends of the main ribs solid cast-iron wedges the whole depth of the main

rib were fitted, then drawn home against the ends of the rib; by this

means the ribs were kept firmly within their places, and as an additional

precaution strong diagonal braces, having a strong feathering rib on

each side, were inserted diagonally between the ribs from one end of

the arch to the other, and secured to the ribs by projecting dovetailed

cheeks on them, and wedges and bolts, so that these cast-iron arches were

constructed in the same manner as a stone arch, being almost as it were

a solid mass depending upon the equilibrium of the different pieces for

its stability. The depth of the main rib of the centre arch at the crown

is 6 feet and 8 feet at the piers, whilst the depth of the ribs of the

side arches at the crown is 6 feet and 8 feet at the abutments. As these

ribs with their attendant transverse frames and diagonal braces formed

the main part of the arches upon which the whole of the superstructure

depended, it was necessary that they should be extremely well put

together and properly united to the piers and abutments. Contrary to

the usual mode of constructing stone arches, they were commenced at the

centre instead of the sides, a strong wooden centring supported upon

tiers of piles having been previously constructed between the piers

and abutments to support them whilst being put in their places. Each

piece of each rib was carefully placed upon the centre, resting upon

nicely-adjusted strong wooden double wedges, and connected together

as they proceeded by the transverse frames and diagonal braces before

mentioned. By this means the whole of the arches were constructed at

the same time from the centre to the skewbacks or bearing parts of the

piers and abutments; but in order to connect them properly with them it

was necessary to devise a particular arrangement. For this purpose a

transverse frame, similar to those already described for connecting the

rib pieces together, was accurately imbedded and fitted to the skewbacks