418 REPORT— 1882. 



horse-power required to ventilate the tunnel. The paper itself, and an 

 account of the discussion which followed, will be found in the Minutes of 

 the Proceedings of the Institution of Civil Engineers.^ 



If the attempt be made in a tunnel, 20 miles long, to create, artifici- 

 ally, a sufiicient velocity in the air to maintain it in a state of even 

 comparative purity, the difficulties will be found to be very great ; but, 

 if it be divided into sections, each 5 miles long, and these sections be 

 treated separately, the difBculties in a great measure disappear. 



The distance between the ventilating shafts of Line No. 1 would be 

 about 21 miles ; but, to simplify the calculations, assume a tunnel 20 

 miles long, with descending gradients of 1 in 80 to points distant 5 miles 

 from each shore, and rising gradients from thence of 1 in 1,000 for the 

 5 miles to the centre. (See Plate YII.) The drainage headings, each 

 with a falling gradient of 1 in 1,000 to the pumping shafts on the shore, 

 will begin at the lowest points of the tunnel, midway between the centre 

 and the shores where the two gradients meet. 



If the main tunnel be circular, with an internal diameter of 30 feet 

 and an area of 470 square feet above rail level, air-passages may be formed 

 below the rails, having an aggregate area of 106 square feet. The drain- 

 age heading may also be circular, with an internal diameter of 1 7 feet, 

 and a sectional area of 227 square feet. This will not be much in excess 

 of what is required during the construction of the tunnel, for the greater 

 part of the material excavated and materials for construction will be 

 taken out and in through the drainage heading. 



If air be now di'awn out of the drainage heading with a velocity of 10 

 miles an hour, it will produce a velocity in the tunnel of 2'2.5 miles an 

 hour, supposing the air exhausted from the tunnel to be replaced at the 

 shore ends from the shafts and at the centre from the air-passages below 

 the rails. If 48 trains pass through the tunnel in 24 hours, at intervals 

 of half an hour, the air will remain pure at the shore ends and in the 

 centre. Between those points, the quantity of cai-bonic acid, in excess of 

 that normally contained in air (3| parts per 10,000 of air), will gradually 

 increase, until it reaches a maximum at the points midway between the 

 centi'e of the tunnel and each shore, where it will amount to 12'68 parts 

 per 10,000 in excess, or 16-18 parts altogether. ^ The average condition 

 throughout the tunnel will be 634 parts in excess, or 9'84 parts in all. 

 Dr. Angus Smith, in his work on air and rain, states that in his own 

 study he found 10'4 parts of carbonic acid per 10,000 parts of air. In 

 theatres, it has been found to vary from 20 to 32 parts ; in the Chancery 

 Court, between 19 and 20 parts; and the air in a first-olass carriage, 

 between Gower Street and King's Cross, with the windows open, con- 

 tained 22-5 parts per 10,000 of air. Thus it will be seen that, if such a 

 state of things as is described above could be maintained in the tunnel, 

 there could be no cause of complaint. To do this in a tunnel 20 miles 

 long would require about 460 effective H.P., or 230 effective H.P. 

 in each country. In a tunnel in a direct line between Fan Hole and 

 Sangatte about 500 effective H.P. would suffice. The cost of keeping 



' Vol. XLIV. 



- In these calculations tlie quantity of carbonic acid produced per train mile has 

 been arrived at on the assumption that each pound of carbon consumed produces 31 1 

 cubic feet of carbonic acid, and that the average consumption of carbon would be 

 25 lbs. per train mile. 



