Photo by Tony Treger, Arbroath
Saturday, 4th October 1810 - the morning on which
the artificers left the Rock for the last time, having completed
the building of the Lighthouse.
That evening Stevenson invited his foremen and captains
of the Service, the clerk of works, Mr David Logan, and
Mr Lachlan Kennedy, engineer’s clerk, and several of their
friends, to one of the local inns, where they spent a pleasant
evening together, at the end of which a toast was given:
“To the Stability of the Bell Rock Light-house” .
. . . And no one would be more pleased to see that
the lighthouse still stands exactly as they had left it,
than the men themselves who had toasted the Bell Rock Lighthouse
that evening in Arbroath almost 200 years ago!
Its design and construction can only be described as one
of the world's great engineering feats of its time. And
yet it is not to Stevenson, or for that matter Rennie, that
the whole credit should go for the construction of this
To both these men, the achievement, yes; but it is John
Smeaton, the builder of the Eddystone of 1759, who deserves
the real credit for the construction technique, for it was
he who pioneered the building of stone-built lighthouses
in modern times. Both Stevenson and Rennie, however, readily
admitted this, and both were continually in praise of this
great engineer. The Eddystone was the blueprint for the
Bell Rock - but "modernised" with a few new features
and improvements which had been developed over the intervening
The design of the lighthouse was in itself a masterpiece.
This can best be illustrated when one looks at just one
area of its construction. The various courses of stones,
built layer upon layer and the method by which they were
secured, was nothing short of mind-boggling!
Stevenson, however, was confronted with problems which
Smeaton didn't have to worry about. The fact that the Eddystone
rock was almost always clear of the water even at high tides;
whereas the Bell Rock was submerged twice daily to a depth
of sometimes 16 feet (4.87 metres), and it was only possible
to work on the rock on an average about 2 hours every low
Because of this Stevenson knew that the lower courses of
the lighthouse which were covered by high tides could not
be left to their own weight and gravity alone to secure
them in position. Trenailling and wedging, as Smeaton had
done, would be the answer.
Another important feature was the angle or curve with which
the base of the tower took off from the horizontal rock
surface. The lighthouse, when finally built, had a broader
base than some of the earlier designs submitted; the theory
being that to minimise the action of the sea against a solid
tower in such an exposed situation the force of the sea
would be better deflected with a shallower rise from the
base than a tower which took off at a steeper angle.
Part of the First Complete
Course and the inset showing clearly the holes for the trenails
and the position of the wedges between the blocks
“. . . by which the stones may be said to hook laterally
into one another, forming a vertical bond of connection;
but it is on the gravity of the materials that the chief
dependence is placed for the stability of the fabric.”
*James Slight (principal mould-maker) and his brother Alexander
were responsible for making the moulds for the stones, following
the designs made by the Clerk of Works, in this very important
aspect of the lighthouse's construction. The moulds were
made from slats of wood (fir timber) and strengthened at
the joints by strips of iron.
Each stone, whether granite or sandstone, was carefully
hewn to this predetermined shape (as per the illustration
below); the whole eventually making up a gigantic circular
jigsaw of interlocking stones.
All that could be done back in the yard ashore was done!
Each course was laid out and carefully checked before shipping
out to the Rock. Stevenson sums it up:
"The whole stones of a course had thus to be laid
with great nicety, corresponding to a number of checks and
marks, previously arranged in the workyard, that the wedges
might fit without trouble at the Rock, and preserve the
respective positions of the superincumbent courses, and
make band throughout the whole fabric."
The implement used to position accurately the stones, mainly
the lower or solid courses. Stevenson, in his own words,
"A Trainer or Rule, framed of timber, applied by
the builders, for ascertaining the exact position of the
stones of the respective courses. At one end is an eye or
socket on which it was fitted to a steady-pin placed exactly
in the centre. This rule was used chiefly for ascertaining
the radiating direction of the stones, from the centre towards
the circumference, being laid agreeably to corresponding
notches and lines marked upon their upper-beds, so as to
preserve band throughout the work, and prevent difficulty
with the closing or finishing stones".
Stevenson says that on one occasion the Trainer had been
left behind in Arbroath by mistake. So much was its importance,
they had to send a boat back to the workshop to collect
it. It was not possible to lay the courses without it! In
modern terms its use could be considered as similar to a
type of compass.
Each course was secured by a series of cubical joggles
(13 in the case of the first complete course) which would
act as steadying pins thus guarding again any horizonal
movement. These would penetrate the lower course at selected
points halfway through the block (ie. 6 inches), as well
as into the superincumbent stone of the course above.
method of trenailing and wedging was carried out exactly
as Smeaton had done 50 years before at the Eddystone. Stevenson
noted that it was probably the most time-consuming of all
the work carried out at the Rock.
Each stone had 2 circular holes (1¾ inches in diameter)
for trenails which would eventually pass through the stone
being laid, and continue 6 inches into the already-laid
stone below. The centre stone had 4 of these holes.
Each trenail measured between 16 and 26 inches in length.
At the lower end a saw-cut was inserted into which a small
wedge was placed. When driven into position, the lower part
of the trenail would split and tighten. It was then cut
flush with the top of the superimcumbent stone and another
wedge was similarly inserted to the upper end of the trenail
and hammered until it split and tightened; thus completing
the stability of the stone onto the course below.
Besides the circular holes for trenailling, grooves, oblong
in shape, were cut into the sides (or waists) of each block.
Pairs of wooden wedges were then driven into the vertical
joints of the stones. These wedges were chiefly intended
for aligniing up the dovetailed stones before grouting.
If Stevenson followed exactly what Smeaton had done, the
grooves would have measured 3 inches by 1 inch.
In the case of the wedges, the thicker end of one of the
wedge was inserted first, followed the the thin end of the
second wedge, and hammered until secure.
Smeaton himself had remarked that by the use of trenails
and wedges "no assignable power, less than would
by main stress pull these trenails into two, could lift
one of these stones from their beds when so fixed, exclusive
of their natural weight, as all agitation was prevented
by the lateral wedges."
The final bond throughout the entire building was maintained
by centering the perpendicular joints of the new course
immediately on the middle of the course below.
The above procedure was carried out, not only for the courses
below high water level, but also throughout the entire solid
part of the building and that section of the between the
door entrance (the 26th course) and the floor of the Provision
room - in all some 38 courses!
The materials used for the first complete course alone
- 123 stones, 13 joggles, 246 oak trenails and 378 pair
The final totals of that part of the house - 1440 stones,
338 joggles, 4065 trenails and 6329 pairs wedges.
From the 39th Course to the top of the House, the trenailing,
wedging, and dovetailing were dispensed with (with the exception
of the end joints and centre stones of the floors). At this
level another method of connecting the courses was adopted,
by means of a raised "zone or belt" (as Stevenson
called it) fashioned on the upper face of the stone (or
more correctly the wall blocks), which would fit into a
similar recessed groove in the stone above. This can be
seen more clearly in the Courses section.
This and the plan of the horizontal floors, were the chief
differences in design between the Eddystone and the
The lower part of the House
showing the change from courses with vertical joggles
to walls-cum-floor/roof using the zonal or belt construction
and horizontal joggles
The problems of construction at these levels were perhaps
not quite so labour-intensive as those of the lower levels,
although the builders at that point had to have more the
attributes of steeplejacks than of men used only to working
on the ground or by the sea! The great size of some of the
stones also worried Stevenson. Any damage or loss to any
of the stones (especially those of the floor of the topmost
room, (weighing over a ton and about 7½ feet in length)
would have caused considerably down-time. Not something
Stevenson was willing to contemplate.
Another variation from Smeaton, who used chains to secure
his arched roofs at Eddystone Lighthouse, was in the construction
of the roofs/floors of the courses of the Bell's upper rooms.
"The floor courses of the Bell Rock lighthouse
lay horizontally upon the walls. They consisted in all of
18 blocks, but only 16 were laid in the first instance,
as the centre stone was necessarily left out to allow the
shaft of the Balance-crane to pass through the several apartments
of the building. In the same manner also, the stone which
formed the interior side of the manhole [entrance to
the next floor], was not laid till after the centre stone,
and the masonry of the walls completed. The number of stones
mentioned above are independent of the 16 joggle pieces,
with which the principal blocks of the floors were connected
[horizontally in this instance, as opposed to vertically
in the lower courses].
Note how the curved roof
is constructed with huge horizontal blocks to achieve the
and how the floor of this room also acts as part of the
"The floors of the Eddystone lighthouse, on the
contrary, were constructed in an arched form, and the haunches
of the arches bound with chains to prevent their pressing
outwards, to the detriment of the walls. In this, Mr Smeaton
followed the construction of the Dome of St Paul's; and
this mode might also have been found necessary at the Eddystone,
due to the want of stones in one length, to form the outward
wall and floor, in the then state of the granite quarries
"At Mylnefield quarry, however, there was no difficulty
in procuring stones of the requisite dimensions; and the
Writer [Stevenson] foresaw many advantages that would
arise from having the stones of the floors to form part
of the outward walls without introducing the system of arching.
In particular, the pressure of the floors upon the walls
would thus be perpendicular; for, as the stones were prepared
in the sides, 'groove-and-feather', after the manner of
the common house floor, they would, by this means, form
so many girths, binding the exterior together."
Another innovation for the Bell Rock that would ensure
greater stability at that part of the house (the 81st course),
considering it had to bear the full weight of the cornice
and the projecting stones of the dome, was the introduction
of a circular flat-bar of Swedish iron, 3 inches deep by
1 inch wide. For the Eddystone arches, Smeaton had used
iron chains instead.
This ring bar (weighing some 400 lb avoirdupois) was fitted
into a groove, 4 inches wide by 3 inches deep, which was
cut in the upper bed of the course. The bar was then heated
to 150 deg Fahrenheit, and sealed into position by molten
To get the entire job done that day, the men worked from
4 in the morning till 8 in the evening - a total of 16 hours.
This technique could be likened to the wright applying
the heated outer metal rim to a wooden spoked wheel or the
bands of metal on a barrel.
important to the Bell Rock works was "Roman Cement",
a type of mortar patented by a Mr Parker of London. Although
not considered appropriate for general use, due to its quick-setting
qualities, it made excellent grouting for the lower courses
and the walls of the entire building where waterproofing
was required. This substance (brownish in colour) is produced
from calcined nodules or argillaceous limestone, found on
the south coast of England. It was also expensive to buy
so it was only used when necessary.
The normal mortar mix was made up of lime from Aberthaw,
Wales; pozzolano or tarras (an earthy substance akin to
lava) from Sicily, Italy, and Andernach, Germany, respectively;
also locally obtained sand.
Throughout the actual building of the lighthouse, Stevenson
continually sung the praises of his men. When the the last
stone was laid, he stated to the gathered artificers and
seamen "that towards those connected with this arduous
work, he would ever retain the most heartfelt regard in
all their interests." Many of the men in Stevenson's
Bell Rock team rose in the service. He kept his word!
* George Anthony Slight, who still lives in Chile and
is a descendant of the man responsible for making the moulds
for the Bell Rock masonry, explained his connection with
Scottish lighthouses and the Bell Rock. He says that his
ancestor James Slight married a daughter
of John Rennie and that his grandfather. George Slight,
went out to Chile in 1886 to design and build the Evangelist
Lighthouse on the west coast of the Magellan Straits and
afterwards stayed on in Chile to build a total of 72 lighthouses.