Account of the Skerryvore lighthouse

by Alan Stevenson

The pieces must be placed end to end on the horizontal plane or table of the lathe at AA ([fig. 110]), and must rest on the exterior arris A of the reflecting side, on which arris there is ground a narrow plane whose width is proportionate to the projection of the outer edge beyond the inner edge of the zone, foreshortened by the bevel or inclination of the reflecting side, when resting (as in [fig. 110]) on the circular iron belt, which is screwed to the table of the lathe provided for its reception. This narrow plane at the arris A should be sufficient to give the prism a solid and regular bearing on the circular iron belt. In this figure ([fig. 110]) a b is the vertical axis of the lathe, n the point from which the co-ordinates for O, the grinding centre for the exterior concave refracting surface AC, are measured, and e AC the arc swept by the grinding surface. Conversely, n′ is the origin of the co-ordinates for the grinding centre O of the interior convex refracting surface BC, and e CB the arc swept by its grinding surface. Some skill is required in fixing the prism on the belt, for, on the one hand, there is an obstacle to correct workmanship from the dragging motion of the platform, and, on the other, by the unequal subdivision of the weight of the glass, which should be nearly balanced. This narrow plane being perfectly adjusted for all the segments so as to bed them quite level, the circular iron belt on which the ring should be ground, is placed on its platform, in the manner represented in the [figure]. It should be as truly levelled as possible, otherwise all the subsequent operations will be deranged by it. This iron belt is heated by means of heating pans; and the degree of heat may be practically judged of by the ebullition of drops of water let fall on it. The segments of glass are also at the same time placed in a stove heated with steam, and are generally raised to about 120° centigrade. The difference between the time required for the two operations of heating the iron belt and the glass segments is employed in laying or bedding a quantity of cement on the reflecting side of the segment, so as to fill up the angular space between the glass and the iron belt, and also to serve as a seat for the segment in the manner shewn in [fig. 110]. This operation is performed on a plane surface, in order that the lower part of the mastic may be precisely on a level with the narrow plane already ground on the outer arris of the reflecting side. After being sure that the heat is equally spread over every part of the circular iron belt, the segment is arranged on it; and the workman must, at this juncture, exert all his skill in placing the parts of the segment in a position nearly concentric with the belt, or in a truly circular form, making due allowance, however, for the inequalities existing at various parts of the rough material, and at the same time taking care that there should be an interval of at least two millimètres (or about ¹⁄₁₂th inch) between the ends of each of the two adjacent segments. Without this interval the heat evolved during the polishing would either dilate the glass so much as to cause the ends of the segments to fly into splinters, or make it needful to remove the zone before this should take place, the inevitable consequence of which would be the fracture of the pieces. Those intervals between the segments are filled with statuary’s plaster, which must be carefully washed and brushed at each change of the emery employed in grinding.

The exterior diameter of the circular iron belt must be precisely equal to that of the ring, because, if larger, the free movement of the rubbers to and fro on the concave refracting surface AC ([fig. 110]) could not take place.

By what is already said, it will be obvious that the grinding process is begun at the refracting sides AC and BC, and a few words will shew that this could hardly be otherwise. If a commencement be made on the reflecting side, which appears at first sight more natural, the consequence is obvious. Having provided for an excess of material in every direction, the segment must consequently be larger than it will be when finished; and the surfaces therefore cannot be true and perfect, except they be ground throughout their entire segmental section, from their centres of curvature, in reference to some given apex of the generating triangle. Now, if the reflecting side were finished first, it might continue to possess this excess of size after being finished, and would, therefore, afford no accurate starting point for the grinding of the other surfaces; it would also present no surface or narrow plane for resting firmly on the iron belt, but would then depend merely on its own finished plane, which, being curved and considerably inclined, would not give a solid bearing for the glass. The other mode of commencing with the two refracting sides, on the contrary, gives a solid bearing on the narrow plane already ground on the reflecting side at A; and after these surfaces have been ground, and the segments inverted (as shewn in [fig. 111]), the outer edge of this narrow plane at the arris A, which has been fully defined by the intersection of the finished surface AC just ground, and also the apex at C, which has been determined by the intersection of AC and BC, combine to fix an accurate starting point for the rubber, in grinding the reflecting surface AB.

Fig. 111.

Dressing off the rough part of the Ring.

The ring is generally reduced from the rough state by means of fixed rubbers, the adjustment of which is more easily regulated than that of the moveable beam or radius of the arc, which is used to give the exact curvature of the surface. Those fixed rubbers are 150 millimètres (nearly 6 inches) wide, by 200 millimètres (nearly 8 inches) long, and are of cast-iron. Three such rubbers are placed at equidistant points of the circle. Two cutters of sheet-iron attached to arms placed vertically (as are also those which carry the rubbers), and moving in grooves radiating towards the centre of the lathe, so as to admit of adjustment to suit the varying radii of the zones, serve gradually to abrade the outer and inner arrises of the segments, so as to prevent the splintering to which, from becoming too sharp, those arrises, without this precaution, would be liable. Those rubbers are, besides, fixed by stems to frames, in the form of quadrants of the circle, which allow of a change in the direction of the planes, as occasion may require.

Instead of the siliceous sand formerly used, the powder of pounded freestone is employed, as it is found to wear the tools less, and to form a better preparation for the subsequent grinding operations. It is easy to conceive that the action of the fixed rubbers necessarily produces ruts or inequalities in the circular direction. The operation of rough dressing, therefore, is not finished until, when those first rubbers are removed, the surfaces of the segments have been subject, for the required time, to the action of moveable rubbers, attached to arms working as radii of curvature, in a plane at right angles to the horizontal movement of the lathe, which carries the zones.

The Emery Grinding.

The form of the segment should be nearly perfect, after the rough grinding is finished. The lathe and the zone are then subjected to an extremely careful washing. Every place where the stone-powder might adhere is dusted. The radius of curvature is verified afresh, agreeably to the co-ordinates ([Appendix, No. IV.]); and emery is used instead of powdered stone; beginning with that called No. 1, which is drawn after suspension in water for one minute. Brushes are used for spreading the emery on the surface of the glass. The quantity ought always to be sufficient to prevent the direct contact of the cast-iron rubber with the glass. Splintering or scratching, which cannot be easily effaced, may result from the neglect of this precaution.