Sixty feet of the rise to be surmounted, occurring in the last half-mile of your line, I shall have nearly two miles to acquire the necessary velocity in: and as the continuation of the action of the power which overcame friction on the level, will neutralise, and, as relates to counteractive effect, annihilate the friction of the carriages while ascending these sixty feet, I have only to cause them to attain a velocity somewhat greater than has yet been attained on railways, that is, 42½ miles an hour during the two miles, to enable them to “swing” themselves up these sixty feet, in consequence of the momentum which that velocity will impart: while, let the height of Rodway Hill (which is adverted to as so desirable to avoid, in the Report of the Provisional Committee of the Bristol Railway) be what it may, all that would be requisite to obviate the necessity for the “inclined plane and stationary engine” spoken of as unavoidable there, would be to attain the velocity due to the altitude of said hill, to enable my vehicles to surmount it from their momentum.
Nor would the ascending power imparted by the vertical operation of the pressure of the atmosphere, be much less important with respect to diminishing the expense of bridging, on the line of this Bristol Railway, than would “momentum” as relates to the expense of cutting and embanking. From the map issued from the London office of that Company, it appears that that railway is to be carried five times across the Avon; twice across the Kennet and Avon Canal; three times across the Wilts and Berks Canal; and four times across the Thames. These various crossings are not for the sake of approaching places of magnitude, or commercial importance; but solely because the principle of railway transmission compels the level to be servilely adhered to: while, though the right line distance between London and Bristol is only 108 miles, yet is the line of railway there laid down, shewn as being 120 miles long; the 12 additional miles being added by the curves taken in thus crossing these waters for the sake of the level.
Now though I do not mean to say that it would be possible, by laying down a tunnel instead of this railway, to avoid all bridging whatsoever, yet owing to hills and rises being no impediment to the operation of this principle, the line for a tunnel might be several miles shorter than this line of the railway, and yet the whole of these bridges be saved, excepting one over the Avon; while not a quarter of the expense would be incurred for carrying a tunnel over the waters which its course must cross, which will be incurred in bridging the railway over those other waters that intersect its course, which are not laid down in the map shewing its line.
The estimated expense of bridging for the railway is 474,800l.; which, when increased by the per centages allowed by the Committees, amounts to 556,194l. as the whole estimated expense of bridging. What proportion of this amount is for bridging over waters, and what for bridging over roads, is not stated. On the Liverpool and Manchester Railway 108,565l. 11s. 9d. was expended on 63 bridges; of which only five were over waters: the other 58 being over roads, or to carry roads over the railway. On the Birmingham Railway the number of bridges is 300; of which only nine are stated to be over waters, the others being for roads. The estimated amount of them is 350,574l. One bridge alone (the Sankey viaduct) on the Liverpool and Manchester line, cost nearly 50,000l.
Now as the power of going up or down, imparted by the vertical operation of the pressure of the atmosphere, would render it wholly immaterial whether the level was preserved in the line of a tunnel; as burying it under ground, in the manner proposed at page [27], would equally do away with any occasion for the many hundreds of bridges, which, on the three lines I have mentioned, must be provided to carry those railways clear of roads, as it would save bridging over the roads on your line; and as a tunnel could have been carried under the Sankey, for almost one-tenth of the expense it cost to construct the viaduct by which the Liverpool and Manchester Railway is carried over that canal—as my principle offers facilities of this kind for obviating the necessity of bridging—I do not hesitate to say, that, on the whole three lines, and considering how much the actual, will exceed the estimated amounts, above one million sterling might be saved in the item of bridging alone, by substituting tunnels for railways; which, when added to, as it would be, by the almost equal amount that would be saved in the expense of the land, in consequence of my plan requiring a width of only ten or a dozen feet under ground, instead of from 60 to 300 on the surface, will admit of my saying that (in round numbers) nearly two millions might be saved by my plan, in these two items of bridging and land, on the lines of the Liverpool and Manchester, the London and Birmingham, and the London and Bristol Railways: while, if what my plan would save of the 398,286l. allowed for the cost of land, and of the 261,928l. allowed for that of the entrances to London, Bath, and Bristol, be added to the savings I have stated it would effect in bridging, cutting, embanking, and tunnelling, I may say that it would also save nearly two millions (of the present estimated expense) on the Bristol Railway alone.
The ten times greater heights than I have yet specified, which may be surmounted by combining the operation of the momentum of the air itself with that of the vehicles, it is not necessary for me to trouble you with, owing to the shortness of your line, and the small height to be ascended: though it may be permitted me to observe, that as attaining only equal velocities to those which have been spoken of as attainable by locomotive engines and steam-coaches, will enable my vehicles, of themselves, to surmount hills of many hundred feet in height; while combining with their momentum, the momentum of the air itself (that which is before the vehicles; the friction whereof will be overcome, and neutralised by the operation of the exhausting apparatus) in tunnels of proper length, and loads of corresponding weight, will enable me to ascend more thousands of feet, than the momentum of the vehicles alone will carry them up hundreds, I may be able to extend Louis le Grand’s exclamation, “Il n’y a plus des Pyrennées,” to “il n’y a plus des montagnes sur la terre,” so far as relates to their longer preventing intercourse between countries; and consequently render the whole earth level to us, in point of effect.
In reference to the force required to overcome the friction of the medium by which the moving power operated to impel the carriages, would a tunnel be also superior to a railway. From Messrs. R. Stephenson and Locke’s reply to Mr. Walker’s Report to the Directors of the Liverpool and Manchester Railway, it appears that the friction of the ropes by which stationary engines draw waggons up inclined planes, is one-twelfth of their weight: while, as the latter part of your line gives a sharper rise than that of the Liverpool tunnel, the weight of the rope you must use should not be less than 7lbs. per yard; the friction and gravitation of which would be 0.73231b. per yard, or 1289lbs. per mile. The line in the plan for the railway, which was laid before your meeting, being 2½ miles long, the whole resistance of friction and gravitation upon it would be 3222 lbs.
From experiments on the friction of air in tubes, I am enabled to state that both the inertia and friction of the air against the inside of an equal length of the tunnel I propose to you to lay down would not, when said air was moved by exhaustion, and conveying 50 tons at the same rate at which the same quantity is drawn up the tunnel of the Liverpool and Manchester Railway (i.e. ten miles an hour), be so much as one sixteenth part of this; while it would have this important advantage, that the heavier the load was, the less would be both the inertia and friction of the air. For instance: the degree of exhaustion requisite to admit of an equal load to what is drawn up the Liverpool tunnel (i.e. 50 tons) being moved up a tunnel of the same size as that I constructed at Brighton, and rising at the same rate your’s must rise (1 in 47) by the pressure of the atmosphere, would be about the 40th part of a vacuum.
But supposing ten times this load were to be raised, the degree of exhaustion must be ten times as great, or about the fourth of a vacuum. And, as the greater the exhaustion, the less the expansive power, and, consequently the less the inertia and friction of the air inside the exhausted part of the tunnel, this “rope of air” as it has, derisively, been called, possesses the important advantage of decreasing as relates to the density, inertia, and friction, which itself opposes, in proportion to the increase of the load drawn by it: while, as the valves I should place at every quarter, or half, or whole mile, to be opened by the carriages as they pass them, and admit air immediately behind said carriages, would prevent there being the inertia and friction of more than a few hundred yards of air of the natural density behind the carriages to be overcome, the impediment which presents an insuperable obstacle in the opinion of the numbers who have condemned the proposition (because they deemed operating by exhaustion the same as operating per plenum) diminishes, in point of fact, to a far less important hindrance, than that which is occasioned by the old system of drawing loads by means of stationary engines and ropes; since, in the present instance, the inertia and friction would not be the one-hundred-and-sixtieth part of what it would be, to move an equal quantity by the stationary engine, and rope system.
And notwithstanding that the superiority which the tunnel possesses over the locomotive system is not so great at this, yet is it important.