[1] see _Authorities_ below.
ATMOSPHERIC RAILWAY. About 1840-1845 great interest was excited by a
method of propelling railway trains through the agency of atmospheric
pressure. Various inventors worked at the realization of this idea. On
the system worked out in England by Jacob Samuda and S. Clegg, a
continuous pipe or main was laid between the rails, and in it a partial
vacuum was maintained by means of air pumps. A piston fitting closely in
it was connected to the leading vehicle of the train by an iron plate
which passed through a longitudinal groove or aperture running the whole
length of the pipe. This aperture was covered by a valve consisting of a
continuous strip of leather, strengthened on each side with iron plates;
one edge was fastened, while the other was free to rise, and was closed
against a composition of beeswax and tallow placed in the groove, the
surface of which was slightly melted by a heater, carried on each train,
in order to secure an air-tight joint. Connected behind the piston was a
frame carrying four wheels which lifted and sustained the continuous
valve for a distance of about 15 ft. Thus the piston having atmospheric
pressure on one side of it and a vacuum equal to 15 or 16 in. of mercury
on the other, was forced along the tube, taking the train with it.
Various advantages were claimed by the advocates of the system,
including cheapness of operation as compared with steam locomotives, and
safety from collision, because the main was divided into sections by
separating valves and only one train could be in each section at a given
time. It was installed on about 2 m. of line between Kingstown and
Dalkey (Ireland) in 1843 and worked till 1855; it was also tried on the
London and Croydon and on the South Devon lines, but was soon abandoned.
The same principle is applied in the system of pneumatic despatch (q.v.)
to the transmission of small parcels in connexion with postal and
telegraph work.
For further particulars see three papers by J. Samuda, P.W. Barlow and
G. Berkeley, with reports of the discussions upon them, in _Proc.
Inst. C.E._, 1844 and 1845.
ATOLL (native name _atollon_ in the Maldive Islands), a horse-shoe or
ring shaped coral reef enclosing a lagoon. The usual shape is that of a
partly submerged dish with a broken edge, forming the ring of islands,
standing upon a conical pedestal. The dish is formed of coral rock and
the shells of various reef-dwelling mollusca, covered, especially at the
seaward edges, with a film of living coral polyps that continually
extend the fringe, and enlarge the diameter of the atoll. The lagoon
tends to deepen when the land is stationary by the death of the coral
animals in the still water, and the patchy disintegration of the "hard"
coral, while waves and storms tear off blocks of rock and pile them up
at the margin, increasing the height of the islands, which become
covered by vegetation. The lagoon entrance in the open part of the
horse-shoe is always to leeward of prevailing winds, since the coral
growth is there slower than where the waves constantly renew the polyps'
food supply. The conical pedestal rising from the depths is frequently a
submarine volcanic cone or island, though any submerged peak may be
crowned by an atoll. For the theory of atoll formation see CORAL-REEFS.