In addition to this advantage another would arise from the power always operating more truly in the line of motion than it can do on any other railway; since the operation of the air can be so arranged, as for every carriage, let the string, or line, be ever so numerous, to be moved forward, not in consequence of contact, or connection with those before or behind it, so as for curves to interpose an angle between the first and last carriages of a line or string; but by the action of the air between it and those immediately preceding

smallest protuberance on the surface, if it be sufficient to change the direction of the motion, is an impediment." "The resistance from dust is greater (Tredgold says) than would be expected. Mr. Palmer made an experiment to ascertain its effect on the Cheltenham tram road, from whence it appears that it required 19 per cent. more power to draw the same carriages, when the rails were slightly covered with dust, than when they were swept clean." To avoid the impediment thus arising from dust, Palmer states, that on the edge rail-way at the Penrhyn slate quarries, the rails of which are convex on the surface, it is usual to carry a butt of water before the train of carriages, which sprinkles the rails in its progress, and washes their surfaces," and "the same practice is not uncommon on tram ways, but those require it in a greater degree." Now not only would the single line of rail-way I use in the tunnel, be sheltered from the impediments common rail-ways are subject to, in consequence of "the surface of the rails being near the ground, they are necessarily exposed to dirt and other extraneous substances lying upon them;" but also, owing to the rate at which the air would move in the tunnel, so strong a wind would be raised as to prevent even dust from settling on it.

or succeeding it, in consequence of which, the line of motion and the line of impulse (of immediate impulse, i. e.) will be always the same, a circumstance that cannot possibly take place on other railways; and with reference to which, Palmer's principle has the disadvantage of being trivially more exposed to it than any other. Owing to these things, lateral friction will be more than made up for, and the whole of the important balance in my favor, arising from the degree in which the diameter of the wheels can be increased, may be carried to account.

I sum up, therefore, on this part of the question, by saying, that if the calculations you have made, as to the power necessary to move any weight along the single line of rail I propose in the tunnel, and the inferences you have drawn from them, are rectified by being divided by 14, the result will express the real force of traction necessary to draw the loads you have adverted to, along that rail-way.

I do not proceed to apply this diminishing number to the results and aggregates you have given, any further than to observe, that as only one-fourteenth of the power you have stated will be required, only one-fourteenth of the degree of exhaustion (or vacuum) need be produced; and, in adverting to these

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two points, I have, therefore, a less difficult task to perform than I acknowledge would have been my lot, had the case really been as you state it to be.

From a misconception I should be surprised at, but that the astonishment it excited merged in other more astounding mistakes, you charge me, at page 16, with encouraging the delusion that, "under a very considerable less degree of exhaustion than a complete vacuum, any load may travel with any velocity you please;" for proof of which you refer to page 47 of my pamphlet.

Now, as it happens, the degree of exhaustion I there refer to, is specific, (equal only to the three-thousandth part of a vacuum) and as it is well known that, under the pressure I mention, (37 grains per square inch) air will move at the rate of only about 12 miles an hour; while my words are "a load of ten tons, is what scarcely any waggon would bear, and hardly any team draw; yet would a load of this kind be carried rapidly along by a pressure so trivial as to be imperceptible"-with misconception thus guarded against, by the pressure being distinctly specified, and with the term "rapidly" evidently used in comparison with the rate at which a waggon loaded with ten tons would be drawn by

horses, I cannot but surmise, whether the exposition you have there so kindly favoured the world with, as to the "motions produced by atmospheric pressure in exhaustion," may not be read with equal benefit by yourself, as by any one; particularly when, after totally omitting to take into consideration the important effect which momentum (as well of the air itself, as of the vehicle) would have in modifying the motion, and preventing the stoppage of the carriage, in the way you describe at page 21, you exclaim "this then is a true philosophical explanation of what will take place in the action of a carriage impelled by atmospheric pressure."

Against such philosophy as this, I protest, in justice both to myself and the public. As the basis of lectures delivered at your Mechanics' Institution, where

-"words of learned length, and thund'ring sound,

Amaze the operatives' rang'd around,"

it may have sufficed. But when held up as a criterion by which the public mind is to take its tone for my condemnation, I am obliged to pronounce it philosophy of which you ought to be ashamed.

The velocity consequent on any degree of exhaustion, can be assigned with precision; and had the question reference only to the

passage of air through an aperture, might be exhibited, as tables of the rarefaction incident to any particular altitude are given. But when the passage of air under different degrees of rarefaction through tubes, is involved in the consideration, I feel it best becomes me to proclaim, that as I found the experiments I considered necessary to the full investigation of the question were beyond the means of an individual, I know nothing on the subject (applicable to high velocities) that I can depend on; and should be under obligations to any one who can inform me whether there is extant any thing with reference to it, that will admit of correct inferences being drawn, as relates to velocities exceeding five or six miles an hour.1

! Since this was written, I have had the pleasure of Professor Leslie's company for three days. Not having seen him since I began my practical illustration, I was solicitous to have the benefit of his witnessing its operation; and have the satisfaction of finding he considers the practicability of the principle sufficiently proved to render it a simple money question; and the only point for present consideration to be, whether it will pay to put it in practice between Brighton and Shoreham : just as a railway or canal question would be considered.

Although aware of what were his previous views, as to the friction of air in tubes, yet, as he might have obtained additional information on the subject, I asked him about it. His reply, however, was to the same effect as one I had previously received from Mr. Davies Gilbert-" that we have no data from which to calculate respecting it."