Design and Construction of the Skerryvore Lighthouse

When it comes to lighthouse construction in exposed locations, a fundamental question arises: should stability depend more on strength or weight^[1]? The text favors weight, noting that 'in preferring weight to strength, we more closely follow the course pointed out by the analogy of nature'^[1]. It argues that inertia, inherent in a weighty structure, offers a more constant and reliable resistance compared to the potentially compromised strength of lighter materials fixed together^[1]. The constant tremor from waves can loosen fixtures, reducing the effectiveness of relying solely on the tensile strength of building materials^[1].

The ideal form for a lighthouse combines a low center of gravity with minimal wave resistance. The text suggests a conical shape achieves a low center of gravity, but acknowledges practical difficulties^[1]. A cone's base can create an angular space where waves may break violently^[1]. The text mentions a non-professional friend suggesting a wedge shape for lighthouses, but that the direction of winds and waves is too variable to guarantee consistent effectiveness^[1]. It asserts there seems little reason for any doubt as to the circular section being practically the most suitable for a Tower exposed in every direction to the force of the waves^[1].

The design of the Skerryvore Lighthouse intentionally diverged from the forms of the Eddystone and Bell Rock Lighthouses^[1]. One key difference was to give the tower the Skerryvore such dimensions as would not be widely discordant with these general proportions^[1]. This primarily involved making the sides of the tower less concave to the sea^[1]. The text explains a preference for a less curved profile writing, 'the more nearly we approach to the perpendicular, the more fully do the stones at the base receive the effect of the pressure of the superincumbent mass'^[1]. This vertical pressure is seen as a means of retaining stones and creating a strong bond, contrasting with elaborate dovetailing.

The text indicates that at the Skerryvore site, geological considerations influenced construction decisions. The rock was a syenitic gneiss, consisting of quartz, felspar, hornblende, and mica^[1]. The text states that this rock was 'exceedingly difficult and tedious to excavate'^[1]. A dyke of basalt also transected the gneiss^[1]. The presence of this basalt and other geological features influenced the location and construction of the tower, necessitating adjustments to avoid undermining its foundations^[1].

The text provides a comparative analysis of the Skerryvore, Eddystone, and Bell Rock Lighthouses: 'In both the Bell Rock and the Eddystone, the thickness of the walls had been reduced to the lowest limits of safety towards the top'^[1]. The author sought to avoid the tremors that can result from a heavy cornice and thinner walls^[1]. By thickening the walls at the top, the Skerryvore design made a 'near approach to the conic frustum'^[1]. This, the author believed, would better ensure 'that the stones at the base receive the effect of the pressure of the superincumbent mass'^[1].

A significant departure from previous designs was in how the stones were united. The text notes, 'In both these Towers the stones were dovetailed throughout the buildings...with the view of preventing the sea from washing away the courses which might be left exposed to the winter storms...'^[1]. For the Skerryvore, the author 'entirely dispensed with dovetailing and joggles between the courses' in the lower parts, using common diamond joggles and wooden treenails for temporary fixtures^[1]. The decision to de-emphasize dovetailing and joggling reflected a belief that the weight of the structure and the mortar's adhesion would be sufficient to maintain stability^[1].