of the rollers to be varied over the roller paths, so as to distribute the wear and prevent deep grooving. With every precaution there is considerable wear, and it is advisable to construct all the rings in halves, an arrangement which renders their renewal possible without taking down the whole apparatus. Brass rollers have been tried with a view to their receiving nearly all the wear, and thus save the steel rings, but they had this disadvantage, that they wore away so rapidly under the heavy pressure that in a few weeks they had large flat edges, requiring much power to grind them round, and producing an irregular motion which soon resulted in their total destruction. Conical rollers have been tried, but they are not much liked, as from the outward thrust there is a difficulty in keeping the rollers up to their work, and there is a large amount of friction against the ring holding them in. It should be remembered, however, that all these trials were made with roller paths of under 2 feet diameter, and with the same diameter of rollers as are now generally used, i. e., about 5 inches. A more favourable result would be obtained from conical rollers with the large diameter of roller paths now used, reducing the outward thrust, and enabling twelve or more rollers to be used to carry the same total weight which formerly was carried by only six. Conical rollers are now being used by the Messrs. Chance. v is a carriage revolving round the central column, upon the friction rollers u u, and receiving its motion from the clockwork, by means of a pinion gearing into the internal gun-metal wheel w screwed to the revolving table v'. The two castings v and v' are bolted to each other. On the top of the casting are bolted eight wrought-iron standards, which carry the whole of the optical apparatus, fitted together in panels as before explained. At the top are guide rollers a, fastened to the upper connecting ring, and working round a turned roller-path, supported by a T iron framing attached to the lantern.

Fig. 29, Plate 3, shows an elevation, and Fig. 26, Plate 2, a horizontal section through the focal plane of a First Order apparatus, commonly called a "Fixed Light varied by short Eclipses," a title which certainly does not state the actual effect, viz., that of a fixed light followed by an eclipse, a flash, and an eclipse, the same phases being continually repeated. Figs. 26 and 29 show an apparatus consisting of four sides, of 45° each, constructed as for a regular eight-sided revolving light; and four complete segments, of 45° each, of a fixed light (as shown in Fig. 21, Plate 2), are fitted in between them, and the whole is mounted on a revolving carriage working on a fixed pedestal, as already explained. If the apparatus revolve once in 8 minutes, there will be 1 minute of fixed light, 26 seconds of eclipse or darkness, 8 seconds of flash, 26 seconds of eclipse, followed by a repetition of the same. In many of these revolving lights it is customary to leave out one of the

lower prism panels, so as to afford access to the lamp, but in some cases a hinged panel is used, and in others sufficient height is given to pass underneath, as shown in Fig. 42, Plate 4. In fixed lights illuminating the whole horizon, access can be had through a man-hole in the service table. Revolving lights are made of eight, twelve, sixteen, and twenty-four sides, to enable the intervals between and the intensities of the flashes to be regulated. In Second Order fixed lights, the panels are made of 60 each, and revolving lights have eight, twelve, and twenty sides.

In Third Order fixed lights, the panels are made of 72° each, and revolving lights have eight, twelve, and sixteen sides.'

The method of framing Second and Third Order lights, both fixed and revolving, is similar to that adopted for First Order lights. Harbour Lights, from their small size, are generally fitted together in one piece. Fig. 30, Plate 3, gives a sectional elevation, and sectional plan through the focal plane of a Fourth Order fixed light illuminating ths of the horizon, the remaining 4th having a silver-plated reflector. The lamp shown is a moderator, placed upon an adjustable stand for regulating the position of the burner. Fig. 31, Plate 3, shows an elevation and sectional plan through the focal plane of a six-sided revolving Fourth Order, mounted on a square pedestal containing the clockwork. The French arrangement of placing the clockwork at the side, and of having a small column to carry the apparatus, is in some cases convenient, in others necessary, on account of the small size of many harbour lanterns.

The construction of panels for the old lights was more complicated than the present method, as there was a regular framework or armature of wrought iron between which the panels were fitted. This framework caused great loss of light, from the joints sometimes having a thickness of 12 inch or 2 inches. The distances from the wrought-iron standards to the indented edges of the panels were filled in with gun-metal, and in the English apparatus no chipping pieces were used, so that the weight of gun-metal and the cost of fitting were much greater than in the modern method. The Author has always been an advocate for a reduction in the thickness of the sides of the panels, to economise light and gun-metal, and to enable all or nearly all the framework to be arranged by machinery in place of by hand. The indented or serrated racks were troublesome to fit, owing to the large extent of edges requiring to be accurately made to correspond. By the method now adopted, and shown in the First Order apparatus, much fitting has been saved, and, by casting the side racks recessed, with only a chipping fillet run round the edges, an economy of metal results,

1 The number of sides may of course vary from those given, and it will be for the Engineer to determine the number suitable for the requirements of the lighthouse about to be erected.

[1868-69. N.S.]

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and increased rigidity is obtained in the panels. Lining plates of gilding metal about 1 inch thick are used to finish off the ends of each panel, and hold in the prisms; at the same time they cover all the recesses, giving the same appearance as if the racks were solid. The intermediate racks cannot be recessed, as there is no covering plate, but the thickness is only about 1 inch, and they weigh less than the former thick indented ones. If desired there is no reason why these latter should not be cast indented as before. The racks are planed on both sides; the outer edges are filed, and the apertures for the prisms (when the castings are accurate) only require cleaning up. The upper and lower connecting rings are turned in one piece, or in segments fitted together. To make a panel, two racks or side frames are butted against the upper and lower segments of rings, and after a couple of wrought-iron screws have been put into each joint, the whole is soldered together, and, in the panels which have an intermediate rack, it is afterwards added in a similar manner. After the panels are made, they are erected upon a table having its upper surface turned and placed perfectly level, with a round centre rod accurately fitted in a socket, so that its centre corresponds with the vertical axis of the apparatus to be erected. By means of gauges, with semicircular ends fitting round this centre bar, and resting on adjustable collars, the panels are fitted in their places, and the holes marked out for the screws to hold the whole together. By the use of templates, and the accurate machinework now applied to all the meeting surfaces, panels can be made interchangeable, which was impossible in the old arrangement of armatures produced by hand. The amount of clearance round each prism is inch, to allow for adjustment in setting the prisms, and for the putty used to secure the glass.

When the fitting is finished, the panels are taken to the erectingshed, where they are erected on their pedestals, or on what is more convenient, a revolving table, specially constructed so that each panel, or part of a panel, can be brought in succession opposite the erecting-post. An arrangement of erecting table is shown in Fig. 7. Plate 1, where a is a ribbed cast-iron bedplate with a turned pathway b on its outer edge for the friction rollers c, and another turned pathway on its vertical central shaft to form a pathway for the guide rollers d. The top table g has a similar rollerpath e, and guide rollers f, on its under side, and the top is turned and pierced with holes to enable the various sizes of apparatus to be secured to it. A wrought-iron plate h covers the central aperture, leading into a pit prepared for the reception of the driving weights of a rotatory machine, or mechanical lamp. As the operation of setting the glass produces much débris, from the plaster of Paris and putty used in fixing the prisms, the erectingtable should be designed to protect the friction and guide rollers

as much as possible from the dust. Apertures are provided in the bedplate to afford access to the guide rollers, and enable the whole of the interior to be cleaned without lifting the upper table: The inclined roller paths do not accumulate dirt and do not require oiling. The prisms are passed into their places, one end covering plate of the panel to be set being removed, and wooden wedges are used to support the glass and enable it to be accurately adjusted in its position by means of internal observation, as explained by Mr. Chance in his Paper. When the prisms are adjusted, plaster of Paris is applied at all the corners to retain them in their correct position, and when it is fairly set the wedges are removed and the remaining spaces filled in with best red lead putty. In the lens panels, as glass butts upon glass with only a thin film of cement intervening, there is no means of adjustment such as exists in the case of the prisms. The only method is to build up each lens panel, beginning at the bottom, and to make each ring correct before another ring is superposed. This latter fact has, up to the present time, prevented any attempt being made to readjust the lens panels in those lights whose upper and lower prisms have been readjusted, as this could only be done at the manufactory. The readjustment of defective lights is a point of importance, for by a small outlay, generally under £100 for a First Order apparatus, a great improvement has been effected and a good result obtained from the upper and lower prism panels, which frequently threw all the light falling on them where it was impossible to be seen. The effect of the lens panels in many lighthouses can be and has been improved by placing all the centres in one level plane with the inner surfaces vertical, and then putting the burner in its most advantageous position in the centre of the whole combination. The labours of the late Royal Commission on Lighthouses have led to the improvement of many defective lights in England and Ireland, and the supply of more perfect apparatus than had before been constructed. In Holland, also, the engineers entrusted with the lighthouse service have had two of their finest lights, at West Kapelle and West Schowen, readjusted under the Author's superintendence, with a beneficial result, which the pilots at once noticed and reported, without knowing anything of the alterations. But it must be borne in mind, that readjustment is only placing in the best possible position prisms incorrectly placed before, and that no actual improvement of the prisms themselves has taken place. It frequently occurs that a prism must be so fixed, as to throw the light too high at its centre and too low at each end, in order that the best result may be obtained from it at the sea horizon. ARRANGEMENT OF THE PANELS AND LANTERN FRAMING. The arrangement of panels generally adopted is that of placing one panel over the other, so that the joints come vertically, over each

other; and it has in its favour simplicity, a minimum loss of light, a minimum cost, and strong convenient-shaped panels. These advantages have been considered of such importance that in France the above method is still adhered to, and all the lanterns are constructed with vertical standards placed in front of the obscuration caused by the sides of the panels. Fig. 32, Plate 3, represents in elevation and plan this arrangement, which has the disadvantage of causing as many points, or rather small arcs, on the sea, as there are standards in the lantern, to be illuminated with a considerably weaker light. In front of each standard of a First Order lantern the light is weakened from 30 to 57 per cent. according to the thickness of standard employed, and there will be sixteen points on the horizon, when it is all illuminated, receiving this weakened light. In harbour lights the obscuration is frequently greater, on account of the small diameter of the flame and the thickness of the standards. The late Mr. Alan Stevenson, aware of this defect, was the first to introduce inclined lens panels, with a view to equalise the distribution of light on the sea, but he was well aware no doubt that there would thus be a diminution of light.

Several lighthouse authorities adopted a lantern with inclined standards, thinking to get over the difficulty connected with vertical standards, but no alteration was made in the construction of the optical apparatus. Fig. 33, Plate 3, represents, in elevation, the effect of an inclined standard projected upon the apparatus, and in plan the actual position is given. The horizontal divergence, resulting from the size of the burner, may be taken at 6°, and the standard is inclined over an angle of 7° in plan, so that when an observer is placed in front of the standard, it nearly stops off the light from him throughout its entire height, commencing on one edge of the flame and finishing on the other, thus obstructing much light which had successfully passed through the apparatus. The defects of this system are apparent, as more light is stopped without there being any greater uniformity, and a more costly and unsightly lantern is obtained. The French Engineers have always

seen and avoided this error.

The lantern of Mr. Douglass (M. Inst. C.E.), Engineer to the Trinity House, is designed to render impossible a correspondence, or optical coincidence, between the framing of the apparatus and that of the lantern. The effect is shown in Fig. 34, Plate 3, where the shaded portions represent the framing of the optical apparatus and the thick black lines the lantern framing. The framing of the apparatus stops 4.5 per cent. of the surface of the apparatus, and the framing of the lantern 6-8 per cent. of the surface of the lantern glazing, and as the two obstructions do not coincide the total loss becomes very serious. These lanterns are expensive, from the amount of workmanship of a costly class, and from the quantity of glass cut to waste.