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Rh carbonic acid amounting to from 15 or 20 to 60, 70 and even 89 parts in 10,000. But since the adoption of electricity as the motive power the atmosphere of the tunnels has much improved, and two samples taken from the cars in 1905 gave 11-27 and 14-07 parts of carbonic acid in 10,000.

When deep level “tube” railways were first constructed in London, it was supposed that adequate ventilation would be obtained through the lift-shafts and staircases at the stations, with the aid of the scouring action of” the trains which, being of nearly the same cross-section as the tunnel, would, it was supposed, drive the air in front of them out by the openings at the stations they were approaching, while drawing fresh air in behind them at the stations they had left. This expectation, however, was disappointed, and it was found necessary to employ mechanical means. On the Central London railway, which runs from the Bank of England to Shepherd's Bush, distance of 6 m., the Ventilating plant installed in 1902 consists of a 300 h.p. electrically driven fan, which is placed at Shepherd's Bush and draws in fresh air from the Bank end of the line and at other intermediate points. The fan is 5 ft. wide and 20 ft. in diameter, and makes 145 revolutions a minute, its capacity being 100,000 cub. ft. a minute. It is operated from 1 to 4 a.m., and the openings at all the intermediate stations being closed it draws fresh air in at the Bank station. The tunnel is thus cleared out about 2¾ times each night and the air is left in the same condition as it is outside. The fan is also worked during the day from 11 a.m. to 5 p.m., the intermediate doors being open; in this way the atmosphere is improved for about half the length of the line and the cars are cleared out as they arrive at Shepherd's Bush. Samples of the air in the tunnel taken when the fan was not running contained 7·07 parts of carbonic acid in 10,000, while the air of a full car contained 10·7 parts. The outside air at the same time contained 4·4 parts. A series of tests made for the London County Council in 1902 showed that the air of the cars contained a minimum of 9·60 parts and a maximum of 1447 parts. In some of the later tube railways in London-such as the Baker Street and Waterloo, and the Charing Cross and Hampstead lines-electrically driven exhaust fans are provided at about half-mile intervals; these each extract 18,500 cub. ft. of air per minute from the tunnels, and discharge it from the tops of the station roofs, fresh air being conveyed to the points of suction in the tunnels.

The Boston system of electrically operated subways and tunnels is ventilated by electric fans capable of completely changing the air in each section about every fifteen minutes. Air admitted at portals and stations is withdrawn midway between stations. In the case of the East Boston tunnel, the air leaving the tunnel under the middle of the harbour is carried to the shore through longitudinal ducts (fig. 3) and is there expelled through fan-chambers. In the southerly 5 m. of the New York Rapid Transit railway, which runs in a four-track tunnel of rectangular section, having an area of 650 sq. ft., and built as close as possible to the surface of the streets, ventilation by natural means through the open staircases at the stations is mainly relied upon, with satisfactory results as regards the proportions of carbonic acid found in the air. But when intensely hot weather prevails in New York the tunnel air is sometimes 5° hotter still, due to the conversion of electrical energy into heat. This condition is aggravated by the fine diffusion through the air of oil from the motors, dust from the ballast and particles of metal ground off by the brake shoes, &c. Volume of Air Required for Ventilation.-The consumption of coal by a locomotive during the passage through a tunnel having been ascertained, and 29 cub. ft. of poisonous gas being allowed for each pound of coal consumed, the volume of fresh air required to maintain the atmosphere of the tunnel at a standard of purity of 20 parts of carbon dioxide in 10,000 parts of air is ascertained as follows: The number of pounds of fuel consumed per mile, multiplied by 29, multiplied by 500, and divided by the interval in minutes between the trains, will give the volume of air in cubic feet which must be introduced into the tunnel per minute. As an illustration, assume that the tunnel is a mile in length, that the consumption of fuel is 32 lb per mile, and that one train passes through the tunnel every five minutes in each direction; then the volume of air required per minute will be - —l—752 fb X 29 wb” ft' X 500=185,600 cub. ft. 2% minutes.-Corrosion

of Rails in Tunnels.-Careful tests made in the Box and Severn tunnels of the Great Western railway, to ascertain if possible the loss that takes place in the weight of rails owing to the presence of corrosive gases, gave the following results:- Box TUNNEL (1 m. 66 chains in length).

Percentage of Wear per annum.

lb per yard

Down line, gradient falling 1 in 100- % per annum. At east mouth ....... . 0'439= 0'377

28 chains from east mouth

48 chains from east mouth

1 m. 8 chains from east mouth

At west mouth .... .

I'800=I'540

2'IIO= 1 ~810

2 -880 =2.480

0-64e=e~==1

Up line,, gradient rising 1 in 100- »

0~620 =o-575

1 -500 = 1 ~380

I 520~= 1 '3I0

o~680=o~587

Percentage of Wear per annum.

lb per yard

Down line, outside and quite clear of tunnel, % per annum. Bristol end, gradient falling 1 in 100   0'28O=0'24O Up line, outside and quite clear of tunnel, Newport end, gradient falling 1 in 90  O'440=O'390 At Bristol mouth, gradient falling 1 in 100 1 -200=1 -020 33 chains from Bristol mouth, gradient falling 1 in 100. . . -  .. 2-160=1-860

3 m. 15% chains from Bristol mouth, gradient rising 1 in 90. ..... . 1 ~900= 1 -630 At Newport mouth ..... 0'3I0=0'270

Down and up line under main-shaft level  3 -200=2'750 At east mouth. ..

I m. 8 chains from east mouth

I m. 28 chains from east mouth

At west mouth. .

It will be seen that the maximum wear and corrosion together reached the extraordinary weight of 2% lb per yard of rail per yeara very serious amount that involved reat expenditure The wear occurred over the whole of the rail, Tut the top, over which the engine and train passed, wore at a greater rate, presumably on account of the surface being kept bright and the gases being able to act on it. The Great Vlfestern Company tried the experiment in the Severn tunnel of boxing up the rails, so that the ballast approached their surface within 1 in. or 1% in. It was found, however, that-in the case, at any rate, of the limestone ballast the cure was almost worse than the disease, the result being a maximum wear of 2% lb and an average wear of just under 2 Ib per ard of rail per year. The average on the open line would be about 0.25 lb in the same time.

See ''Proc. Inst. Civ. Eng.''; also works on tunnelling by Drinker, Simms, Stauffer and Prelini, and on tunnel shields, &c., by Copperthwaite.

TUNNEL VAULT, the term in architecture given to the semicircular or elliptical vault over underground passages, in contradistinction to the wagon or barrel vault of edifices above ground.

TUNNY (Thunnus thynnus), one of the largest fishes of the family of mackerels, belongs to the genus of which the bonito (Th. pelamys) and the albacores (Th. albacora, Th. alalonga, &c.) are equally well-known members. From the latter the tunny is distinguished by its much shorter pectoral fins, which reach backwards only to, or nearly to, the end of the first dorsal fin. It possesses nine short inlets behind the dorsal, and eight behind the anal fin. Its colour is dark bluish above, and greyish, tinged and spotted with silvery, below. The tunny is a pelagic fish, but periodically approaches the shore, wandering in large shoals, within well-ascertained areas along the coast. It not infrequently appears in small companies or singly in the English Channel and in the German Ocean, probably in pursuit

Tunny.

of the shoals of pilchards and herrings on which it feeds. The regularity of its appearance on certain parts of the coasts of the Mediterranean has led to the establishment of a systematic fishery, which has been carried on from the time of the Phoenieians to the present day. Immense numbers of tunnies were caught on the Spanish coast and in the Sea of Marmora, where, however, this industry has much declined. The Sardinian tunnies were considered to be of superior excellence. The greatest number is now caught on the north coast of Sicily, the fisheries of this island supplying most of the preserved tunny which is exported to other parts of the world. In ancient times the fish were preserved in salt, and that coming from Sardinia, which was specially esteemed by the Romans, was known as