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

Those who will give themselves the trouble of the calculations necessary to establish the truth of the preceding statements relative to the effect of momentum, it will be unnecessary to remind of any of the occurrences which prove it. But those who do not choose to take that trouble, may be reminded, that a circumstance often witnessed, gives practical demonstration of the accuracy of these statements.

During certain adhesive states of the crust of the road, it is frequently seen when travelling, that the pressure of the wheels causes particles of earth to adhere to, and rise from the ground, sticking to the tire of the wheel.

The adhesion of these particles of earth being, however, soon destroyed by the centrifugal force imparted by the revolution of the wheel, they become, the moment it loosens them from the wheel, and allows the other influence to operate, projected in directions varying according to the position of the part of the wheel to which they adhered, at the moment of their quitting it.

Some of them, being carried to the top of the wheel, fly forward; but the majority, leaving the wheel at about the height of the axle-tree, become projected vertically, and are seen bobbing up and down by the windows of the carriage, somewhat like motes in the sunbeam.

Their thus rising and falling may, perhaps, hitherto have been observed, without being regarded as demonstrative of any principle which may be rendered subservient to our purposes. But as they are, in point of fact, evidences, that the momentum imparted by the velocity at which the tire of the wheels is revolving, will cause bodies to rise to the height of three or four feet perpendicular, above the point of the wheel from which they fly; and as this velocity is exactly commensurate with that at which the carriage goes over the ground, they are unquestionable proofs, that, provided friction be annihilated as relates to counteractive effect, by the continued operation of the moving power, the vehicle itself would ascend an inclined plane of any rate of ascent, to the same height to which they rise above the position of the part of the wheel they adhered to, at the moment of their flying from it.

But, to leave this question relative to momentum, and return to that of the steam-engines and air-pumps.

It being the property of air to neutralise, or absorb, a smaller portion of whatever impulse may be imparted to it, than, perhaps, any other ponderable medium nature offers us, the power of the steam-engines which operated on the air-pumps that exhausted air from the tunnel, might be brought to bear,—and that too, without their energy being so diminished as even to approach an insuperable objection—on the vehicles in it; and an effect in consequence produced, which we cannot, at first, conceive to be possible.

It is evident, that it will not require the power of the engines (each equal to several hundred horses’ power), by which the air-pumps would be worked, to move one, or even many vehicles. What then will become of the surplus power? Will it be lost in overcoming the friction of the air, as adverted to at page [41]; or, rather, may it not operate to increase the rate at which the vehicles will move? And if so, how many times will the rate at which we may be conveyed, exceed that at which we now travel, and what is the limit that will be attained in this particular?

It is well known that air will rush into a vacuum at the rate of nearly a thousand miles an hour. Now although it is no more expected we should be conveyed at any such rate as that, than it is intended we should be placed in a vacuum, yet are, both this almost inconceivable velocity, and what is generally expressed by the term “vacuum,” so connected with the subject of consideration, that it becomes unavoidable to advert to them, injurious as they must prove, and strongly as they will array our preconceived notions and prejudices against the proposition.

It cannot be denied that we have the power of laying down a tunnel, such as has been referred to, and of adapting a railway to the inside of it, for any distance we please: and, though it may not be in our power so to connect the separate “lengths” or cylinders which compose it, as to render the joints perfectly air-tight against a vacuum, yet, with reference to the trivial degrees of exhaustion necessary for the purpose here contemplated, every joint may most easily be made “air-tight”: since, supposing the degree of exhaustion to be equal to the pressure at which gas is forced through the mains of a public company whose works I know, a load of above 100 tons would be carried along a tunnel of eight feet in diameter, at whatever rate the air was pumped out of it. Equally certain, as it therefore becomes, that we have the power of extending this tunnel at pleasure, is it, that the power of making and working any number of air-pumps, such as have been referred to, will enable us to exhaust from, and consequently cause air to rush through it, at rates so vastly exceeding any at which we now travel, that our preconceived notions and prejudices cause us to look on the proposition as both impossible and absurd.