for the purpose that can be found. But the billy country for twenty miles to the east of Bath, which is from 700 to 1000 feet high above the level of Bath, forms almost an insurmountable barrier to a railroad.
Mr. Brunel proposes a tunnel through those hills whose under strata is composed of chalk and freestone of easy and safe cutting. But there are decided objections to tunneling for railroads at the depth and length here required. First, the tunnel must be many miles long, without light and air, except artificial air and light, perhaps gas lights. The want of atmospheric air, combined with the combustion of gas, smoke, and steam, will render the tunnel almost incapable of human existence. And, secondly, the condensed or compressed air in the tunnel, if it is only a mile long, will give a formidable resistance to the speed of the propelling engine and train of carriages, that would require more than double the power to propel them, compared with the power on the same level in the open air; and if a carriage enter at the other end at the same time, it will act like two balls in a tube[1], the one would repel the other. Thirdly, in driving a tunnel, of the presumed depth of only 600 or 700 feet below the surface of that country, is considerably below the level of several great springs, that form the head of the rivers Isis, the Kennet, and the Avon; and cutting across the country by the projected tunnel would, no doubt, tap these powerful springs, which would drain all the high country, and convert the projected tunnel into a river.
The line from Bath to Bristol is about twelve miles
- ↑ I am aware that air shafts may be made, and that tunnels can be arched, in clay or sandy under strata at a great expense.
