"No, no, Mr. Cobb; do not be displeased. We really meant no discourtesy; but your remark is not what you would have made had you thought a moment, for we know you to be a man of education. We do not use wheels on the pneumatic roads. These trains run upon the many little runners which you saw under the cars. Were we to use wheels," he continued, after a pause, "centrifugal force would tear them into pieces in no time. Take the case of a wheel four feet in diameter: the circumference of such a wheel is a little over twelve feet. At the rate of four miles per minute, it would have to revolve 1,760 times. No wheel that can be made would stand such a test. It would fly into fragments inside of the first mile. A wheel of the above dimensions and at that rate of revolution would have a centrifugal force equal to 1,000,000 pounds. Now, as the centripetal force is the tensile strength of the material only, and that of the best steel wire only 160,000 pounds, it will readily be seen that the centrifugal force would instantly cause the wheel to fly into fragments." "You are right," Cobb answered, going over the figures in his mind. "Wheels would never do; I can see it plainly." "Even were we to use a smaller wheel to decrease the centrifugal force, we would have to increase the number of revolutions, so there would be no gain in so doing. Our trains run upon two peculiarly constructed rails, and the runners are flanged to exactly fit the rail. There is, in addition, on either side of the tunnel, another rail of similar shape, while upon the upper part are two more. The car has runners for all of these rails, and the position of them is such that the car cannot jump the track, or swing or sway from side to side. It travels as if in a groove, and the little runners, separate from one another, conform to the curves of the tunnel." "It must take powerful engines to exhaust the air from such a long tunnel, does it not?" "Yes, very powerful ones. But what is different from any other mode of propulsion, the same engine can do as much service for a line 2,000 miles long as for one of 200 miles in length, rate of speed being the same. The reason for stations at intervals of about 500 miles, is because more trains can be kept in motion on medium short lines than on very long ones. There are at Salt Lake, at the receiving end of this line, fifteen engines of 5,000 horse-power each; ten at work all the time, with five in reserve." "A pretty strong set of engines for a single railroad, I would say; and a costly motive power, too." "Not so costly as you would think," he returned. "If you take into consideration that these engines are worked by electricity, and not by steam, and that the electricity is furnished by water-power, you will perceive that they can be worked quite cheaply." "Give me some of the statistics, please," said Cobb.