A Letter to the Kensington Canal Company on the Substitution of the Pneumatic Railway for the Common Railway by Which They Contemplate Extending Their Line of Conveyance

by John Vallance

One of the circumstances which at first strikes us as fatal to the proposition, is the inability to respire, which we all feel we should be liable to, if conveyed rapidly through the air. A moment’s reflection will, however, enable us to see that this objection has no application whatever to the case. It is not proposed that we shall be conveyed rapidly through the air, but that we shall cause air, which we have first set in rapid motion, to convey us along with it, as fast as itself goes: a state of things so different from going through, or against, and meeting the air, that our supposed objection does not apply to the case.

Stating facts will, however, be the best way of settling this question; and for this purpose the experience of our aeronauts is referred to. Much as they have sometimes been inconvenienced from the rarity of the air, at the heights to which they have ascended, yet have we never heard them complain of being unable to breathe freely, owing to the velocity with which they were carried along over the earth’s surface, notwithstanding that they have been conveyed at rates of 70, 80, and, in one instance, 160 miles an hour. And why? because that which was the cause of motion went with them.—“I had not,” says Lunardi, in his account of the first ascent ever made in England, “the slightest sense of motion from the machine. I knew not whether I went swiftly or slowly—whether it ascended or descended—whether it was agitated or tranquil, but by the appearance or disappearance of objects on the earth.” Rapidly, therefore, as they have moved, yet have they felt as if in a calm. Now exactly similar in point of respiration, would be the feeling of those who might be conveyed in the proposed tunnel. The air, being the cause of motion, must go, at least, equally fast as it drove them, and necessarily be wherever they were. Let the rate of motion therefore, be what it might, the feeling of those who experienced it, must prove that of being in a perfect calm.

Nor are the objections we at first conceive, relative to the effect which pumping air from the tunnel, and producing what only the word vacuum (inapplicable as it is) will enable us to convey the idea of, at all more tenable. The degree to which air would be exhausted from the tunnel might scarcely ever be sufficient to sink a barometer two inches lower than one exposed to the atmosphere stood at; so that even were we exposed to it no inconvenience would be felt. [69] But we never shall be exposed to it, any more than those who witness the cruel experiment of putting a mouse under the receiver of an air-pump, and then exhausting it, are exposed to what the little animal suffers. Between those who see and the poor creature which feels the effect of the apparatus, is the side of the receiver. And between the part of the tunnel in which the exhaustion, or rather the difference of density is, and the passengers in the vehicle, would be the end of the vehicle; so that though close to them would be an atmosphere rarer than (we will suppose) it might prove pleasant to be in, yet would the atmosphere they actually were in be the same as that of the air at large. No inconvenience, therefore, can be experienced in this particular.

Equally untenable is the idea we take up, that it will be impossible so to adapt the ends of the vehicles to the inside of the tunnel, as to cause them to act as pistons in preventing the passage of the air by them, without occasioning friction to a degree which should deprive us of all the advantages the air would otherwise give, as a mean of communicating motion.

In the last carriage which I had for the tunnel I constructed at Brighton, there was a space of above an inch and a half in width left all round between the piston part of the carriage and the tunnel, through which air rushed unimpeded. Yet did not this “windage,” or leak, though equal in the aggregate, to an aperture of three square feet, prevent the carriage from springing forward to the impulse of the air-pumps, with a readiness I was surprised at. Nor did it ever cause the least perceptible diminution in their effect; owing to the small quantity of air that passed through it, in comparison with the immense quantity exhausted by the pumps.

When the Brighton Committee rode in that tunnel, one of them brought with him a mountain barometer, that he might ascertain the degree of “vacuum” or exhaustion necessary to move the carriage. This barometer was accordingly suspended in the part where the “vacuum” was to be produced, and the vernier adjusted with the greatest accuracy. But to his surprise the degree of exhaustion was not sufficient to lower the barometer in the least degree. Being aware of this, I had spirit gauges previously prepared, one of which was fixed in the end of the carriage. But even this gauge, though nearly fifteen times more sensitive than the barometer, was affected hardly enough to be visible, the amount of “vacuum” indicated by it, being only about ten grains per square inch, or less than the ten-thousandth part of a vacuum.

Nor would the quantity of air that rushed by the piston-end of the carriage be at all important, even when travelling at very great velocities, and with heavy loads. In a tunnel of the diameter which would be proper for such lines as those to Bristol, or South Wales, the pressure requisite to move a load of 100 tons would not be more than about 100 grains per square inch; which would cause air to rush past the piston-end of the carriage at the rate of about 30 feet per second. Therefore, even could no better adjustment of the piston-end of the carriage and the inside of the tunnel be effected, than took place with respect to that at Brighton, only 90 cubic feet of air per second would rush past, even were the carriages standing still; which is only one-tenth of what the air-pumps I used there were capable of exhausting in the same time; while, on such a line as the Bristol, or South Wales, it would not be one-hundredth of what the exhausting apparatus would take out in the same period; so that not one-hundredth of the power would be lost by it: and even this hundredth could easily be reduced to a thousandth: the space left between the piston-end of the carriage in the tunnel at Brighton being purposely an inch and a half in width, in order that I might shew, by actual proof, how utterly unimportant was that objection which engineers of the highest name and reputation had assured me must, inevitably, prove fatal to the motion of any carriage in any tunnel.

And as the carriage, instead of standing still, would be moving forward, the loss of power, which would, otherwise, result from the pressure requisite to give the velocity as well as move the load, would be equally unimportant as that arising from the pressure requisite to move the load alone.

With pressures so trivial as these capable of producing practical effects, and with it fully practicable so to adjust the “piston” part of the carriages to the tunnel, as to render this “windage,” or leak, perhaps less than one-hundredth of that which I purposely caused in the tunnel at Brighton, there can be no difficulty, either in preventing any important quantity of air from rushing past the carriages; or in so connecting the “lengths” of which the tunnel would be composed, as to render the joints air-tight.

And as there are no objections which the engineers can bring forward, that cannot be replied to in an equally satisfactory manner, I need not trouble you with any additional answer to them.