Account of the Skerryvore lighthouse

by Alan Stevenson

Height.	Radius.	Height.	Radius.	Height.	Radius.	Height.	Radius.
0	21·000	31	14·800	62	11·426	93	9·305
1	20·720	32	14·660	63	11·343	94	9·250
2	20·447	33	14·523	64	11·261	95	9·195
3	20·182	34	14·389	65	11·180	96	9·141
4	19·923	35	14·257	66	11·100	97	9·088
5	19·671	36	14·127	67	11·021	98	9·035
6	19·425	37	14·000	68	10·944	99	8·983
7	19·185	38	13·875	69	10·867	100	8·931
8	18·951	39	13·752	70	10·792	101	8·880
9	18·723	40	13·632	71	10·717	102	8·830
10	18·500	41	13·513	72	10·644	103	8·780
11	18·282	42	13·397	73	10·571	104	8·730
12	18·070	43	13·282	74	10·500	105	8·681
13	17·862	44	13·170	75	10·430	106	8·633
14	17·659	45	13·059	76	10·360	107	8·586
15	17·461	46	12·950	77	10·291	108	8·539
16	17·267	47	12·843	78	10·224	109	8·492
17	17·077	48	12·738	79	10·157	110	8·446
18	16·891	49	12·634	80	10·091	111	8·400
19	16·710	50	12·532	81	10·026	112	8·355
20	16·532	51	12·432	82	9·962	113	8·310
21	16·358	52	12·333	83	9·898	114	8·266
22	16·188	53	12·236	84	9·835	115	8·222
23	16·021	54	12·141	85	9·774	116	8·179
24	15·857	55	12·046	86	9·712	117	8·136
25	15·697	56	11·954	87	9·652	118	8·094
26	15·540	57	11·862	88	9·593	119	8·052
27	15·386	58	11·773	89	9·534	120	8·010
28	15·235	59	11·684	90	9·476	120·25	8·000
29	15·087	60	11·597	91	9·418	...	...
30	14·942	61	11·511	92	9·361	...	...

APPENDIX, No. II.
NOTES ON THE MANUFACTURE OF PARABOLOÏDAL REFLECTORS,
from Information furnished by Mr James Murdoch, of the Northern Lights Service.

Fig. 99.

The reflector-plate consists of virgin-silver and the purest copper (from the ingot), in the proportion of 6 oz. of silver to 16 oz. of copper. The two metals are in pieces, forming a flat parallelopepid of about nine inches of surface. Being first thoroughly scraped and cleared from rust with a file, they are tied together with wire and placed in the furnace, where they are united by means of a flux composed of burnt borax and nitre, mixed to the consistence of cream. Their thickness is sufficient to admit of their being repeatedly passed through the rolling-mill, so as at last to come out a plate twenty-eight inches square. Every time it is passed through the rollers, the plate is annealed in the furnace before being again pressed. It is then cut into a circular disc ready for working; and great care should be taken to keep the metal perfectly clean during the whole of the hammering and polishing processes. The first step towards forming the plate to the curve, is to raise the back or copper side to a slight convexity by beating, with the boxwood mallet ([fig. 99]), rounded at each end, c and d, its inner or silver side upon a large block of beechwood, of a form slightly concave. This beating is begun at the edge of the plate, and gradually reaches the centre. After the disc has been raised to the proper height on the wooden block, the next step is to take it to the horse ([fig. 103], opposite page), where it is beaten with the wooden mallet ([fig. 100]), its concave face being in contact with the bright steel-head a ([fig. 103]), until it has nearly reached the proper height for the reflector, for which the workman has a gauge or mould to guide him; in this course of raising, as it is called, the peened face b a b ([fig. 100]) is first used, and then recourse is had to the opposite or flat face for smoothing it after being raised. In this last course of raising, as well as in the process of smoothing the reflector all over, the workman bestrides the horse.

Fig. 100.

Fig. 101.

During the process of raising with the peened side of the mallet, an external mould FGHF ([fig. 101]), with a needle-point P, at its vertex, is applied, to indicate its proper position with reference to the mould; and allowance is made on the height and diameter of the reflector to meet the expansion of the metal during the hard-hammering which is to follow. After each course of the raising with the wooden mallets, the reflector must be annealed in the following manner:—The reflector is first damped with clean water, and its surface dusted over with a powder, composed of one pint of powdered charcoal to one ounce of saltpetre, which is applied by means of a thin flannel bag. The reflector is then put on a clear charcoal fire, where it is turned round as the powder flies off, which is an indication that the metal is duly heated. Over-heating is very injurious. When removed from the fire, the reflector is plunged into a large tub, containing what is called the pickle, which is a solution of one quart of vitriol in five or six gallons of water. After this it is washed with clean water, and scoured with Calais sand.