Chap. VIII.
3C1
entirely through it, and entering 8 inches into the lower stone. On the upper bed ofeach course is a projecting fillet, which enters a groove formed to receive it in the uppercourse, and by this means no water can be admitted. A gallery is formed above by pro-jecting the courses inside and out; this supports the lantern, which is of east-iron.
The light is stationary, red, dioptric, of the first order, without mirrors. The burnerhas four concentric wicks, the largest of which is 3J inches in diameter, and consumes apint of oil per hour. The mortar used in the construction consisted of three of sand, oneof lime, and one of pozzolana. A foot-bridge unites this lighthouse with the shore. Oil arock at a short distance is a beacon formed of a cone of masonry, 20 feet in diameter at thebase, and 37 feet high : on the top is a staff and globe, which rise 13 feet above the apex;the globe, 4 feet in diameter, formed of copper bands, is 36 feet above high water mark.
Lighthouse lamps, as now’ supplied with oil, are of the Argand or Fresnel construction,and great attention is requisite to the ventilation of the chambers; where a quantity of oilis burnt in a short space of time a great quantity of carbonic acid is produced, w’hichrenders the atmosphere unwholesome. A pound of oil in combustion produces 1*06 poundsof water, and 2*86 pounds of carbonic acid; this increase of weight being due to theabsorption of oxygen from the atmosphere, one part of hydrogen taking eight parts byweight of oxygen to form w r ater. An Argand gas-burner in four hours, when placed inthe window of a shop, produces 2.1 pints of water. Lighthouses are now usually fitted withone large central lamp, the outer wick of which is 3 or 4 inches in diameter, or with manysingle Argand burners, each having a parabolic reflector. Professor Faraday has contriveda method to keep the air in the chamber pure, and to ventilate the lamps by flues whichmaintain fresh air within the lantern ; this is performed by means of a chimney or coppertube, 4 inches in diameter, not in one length, but in three or four ; the lower end of eachportion, for about 1^ inch, is opeued out into a conical form, measuring inches in dia-meter at the lowest part. When these pipes are put together to form the chimney, theupper end of the bottom piece is inserted about ^ inch into the cone of the next pieceabove, and fixed there by three ties or pins, leaving at the same time plenty of air-way.
After the ventilating chimney is thus completed, it is placed in such a manner, that thelamp chimney enters about £ inch into the lower cone, and the top of the ventilatingchimney enters into the cowl or head of the lantern. The ventilating flue, thus ingeniouslycontrived, is found to carry off all the products of combustion into the cowl; none passesinto the conical apertures from the flue into the air of the lantern, hut a portion of theair passes from the lantern by these apertures into the flue, and thus ventilates it.
A sudden gust of wind striking the cowl does not in any degree affect the steadiness ofthe flame, the ventilating flue carrying up every thing, and bringing nothing down. Inlighthouses, where many separate lamps and reflectors are made use of, a system ofgathering pipes is resorted to; these are brought together behind the reflectors, and enterone large pipe, which passes off to the cowl at top. A seven-eighth inch pipe is foundsufficient for a single lamp, and this is made to pass downwards through the aperture in thereflector over the lamp, and dips an inch into the lamp glasses, when the draught upwardsis such, that not only do all the products of combustion enter the tube, hut air alsopasses down between the top edge of the lamp-glass and the tube, and is finally carried offwith the smoke. The tube should not dip more than ^ inch into the lamp-glass, or thewhole of the burnt air would not escape.
Holyhead, or the Caer Cybi of the Britons is situated on a small island at the north-westextremity of Anglesea; here are the remains of many Roman constructions, as well asBritish . This port, being the nearest to Dublin, has occasioned it at all times to be muchfrequented ; latterly it has undergone considerable improvement, and a new lighthouse beenerected. The piers in front of the port are upwards of 1300 yards in length, and thebase 90 yards in breadth. The slope towards the sea is made 5 to 1, and the breadth attop is 8 feet. At 8 feet below the top a way is formed, 48 feet in breadth, which answersthe purpose of a quay. At the end of the pier the quay is 47 feet above the level of lowwater mark, and 6 feet above high water. All the masonry is executed with graniteobtained in the vicinity, and towards the head of the pier is formed a jetty or spur whichadvances 60 feet, and has a width of 164 feet.
It appears from Tacitus (in Vita Agricola!), that the Romans had a settlement here, andcarried on a considerable trade with the people of Ireland . No place could be moreadvantageously situated for such a purpose, as it projects far into the Mare Yergivum ofPtolemy , and lies in the neighbourhood of the Roman stations on the western shores ofFlavia Ca'sariensis: Mr. Pennant has described a Pharos at the summit of the mountaincalled Pen Caer Cybi, 10 feet in diameter, and at no great distance a regularly faced wall,10 feet in height and 6 feet in thickness, which surrounds three sides of a parallelogram,the fourth being open to the harbour. At each angle was a circular tower. By some thesefortifications are supposed to he the work of the celebrated chieftain Caswallon Law-hir,for the purpose of repelling the l’icts, who infested these coasts after the final departure ofthe Romans.