270 Progress in Science. (April, 
greatest depth of water impounded is 60 feet, and the average depth 22 feet. 
The area of the réServoir is 409 acres, and it is capable of holding 2400 million 
gallons of water, equal to two hundred days’ supply for the city of Dublin and 
suburbs. Two mains, 48 inches and 33 inches in diameter respectively, are 
carried through the bank, in a tunnel excavated out of the rock, and arched 
over. The 48-inch main, which terminates in the bye-wash, is a provision 
solely for the purpose of being able rapidly to lower the water in the reservoir, 
if necessary. The 33-inch main conveys the water to a circular receiving-— 
basin, situated at, the outer toe of the embankment, and from this basin the 
water is distributed by side canals on to seven filter-beds. After being fil- 
tered, the water is colle¢ted into two pure-water tanks, from whence it is 
carried for about 700 yards in an iron pipe, 42 inches in diameter, with a fall — 
of 6 feet per mile, until it reaches the tunnel, into which it is laid for 120 yards, | 
The tunnel is 4367 yards long, and conveys the water from the natural valley 
of the Vartry, under a range of hills separating that valley from the distriés 
sloping towards the sea to the east. This tunnel is from 5 feet to 6 feet high 
and 4 feet wide, with a gradient of 4 feet in a mile. At Callow Hill, the 
northern or Dublin end of the tunnel, there is a circular receiving-tank, © 
go feet in diameter, from which a main, 33 inches in diameter, conveys the 
water to the distributing reservoir at Stillorgan a distance of 17} miles. 
Three tanks relieve the pressure at different points, viz., at Kilmurray, Kil-— 
croney, and Rathmichael. The water is distributed through the streets of the = 
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y 
city by mains varying from 27 inches to 18 inches in diameter, which form a 
zone round the central parts of the city, from which the service-mains — 
diverge. Screw-valves at the intersection of the streets enable the waterto 
be turned off or on, either to repair the mains or to concentrate the pressure 
in case of fire. Hydrants are placed in every street at intervals of 100 yards, 
and the system is so perfect that since the introduction of the Vartry water 
no steam or hand fire-engine has been used to extinguish fires. The total 
cost of the works has been £610,000, or at the rate of about £1 16s. 6d. per t 
head of the population. , 
Great Basses Lighthouse.—In 1855 instructions were given for the prepara-_ 
tion of a design for the erection of a beacon on the Great Basses Rocks, off — 
the coast of Ceylon. The design submitted was for a cylindrical cast-iron 
tower, secured within an enlarged basement of masonry, which basement was 
to be enclosed within an outer casing of cast-iron, and both tower and casing 
were to be sunk into the rock. After three years’ operations, and the ex- 
penditure of £40,000, only a few landings had been effected on the rock, and 
the authorities therefore suspended further proceedings. In June, 1867, the 
whole question was referred to the Trinity House authorities, who recom- 
mended for approval a design prepared by their Engineer, Mr. J. N. Douglas, 
for a granite structure in which the base of the former structure was proposed 
to be utilised. The lighthouse, which has now been constructed, consists of 
a cylindrical base, 30 feet in height and 32 feet in diameter, on which is placed 
a tower, 67 feet 5 inches in height, 23 feet in diameter at the base, and 17 fee 
in diameter at the springing of the curve of the cavetto. The thickness of 
the wall is, at the base of the tower, 5 feet, and at the top 2 feet. The 
accommodation within consists of six circular rooms, each 13 feet in diameter; 
and there is also a room, 12 feet in diameter, in the basement, for coals and 
water, and a rain-water tank below, 7 feet 6 inches in diameter. The tower 
contains 12,288 cubic feet of granite, and the cylindrical base 25,077 cubic feet, 
making a total of 37,365 cubic feet, weighing about 2768 tons. Above the 
tower is a lantern and dioptric revolving apparatus of the first order. 
cylindrical 14-feet lantern of the Trinity House has been adopted. The 
dioptric apparatus has eight panels of refractors, with upper and lower prisms 
for emitting flashes of red light at intervals of 45 seconds. A 5-cwt. bell, for” 
a signal during foggy weather, is fixed on the lantern gallery. The cost of the 
building was £64,300, and the light was first exhibited on the roth of March, 
1873, and has since been regularly continued every night from sunset to” 
sunrise. 
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