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640 Letter on Affairs mn general. ‘(Dse. 
In proportion as such a current of air as this may be confined or coni- 
pressed, its force will, as is obvious, be very greatly increased., And 
such a current may at once be created, by exhausting the air wholly or 
partially from any vessel—as in the case of drawing up the piston of.a 
common syringe—when the air from without rushes in by every acces- 
‘sible passage, to supply the void which is produced. Suppose, for 
instance, a common gun-barrel to be exhausted of the air contained in 
it, by the help of an air pump fixed at the mouth ; and fresh air then to be 
suddenly admitted by withdrawing a plug from the breech; a ball of 
cork, or of any other light substance, lying within—say against the 
touch-hole (which-however must be closed) —would be carried forward by 
the rushing in of the outward air, and probably thrown out to a con- 
siderable distance from the muzzle. And as what is true upon.a small 
scale (according to M. Vallance) will be true upon a large one, it occurs 
to him that a brick cylinder, of the same form as a gun-barrel, might be 
constructed to extend, say from London to Brighton, with a stage-coach, 
moving upon a tram road, and fitted with a sail (instead of a “ cork- 
ball”), torun within it. And that such coach, loaded with passengers, 
being placed, for example, at the London end of the cylinder, and det go 
(with an admission of fresh air behind it) from that point, just as the air 
contained within was exhausted by a pump worked by a steam engine, 
‘placed at the Brighton end, it would start off with a velocity little in- 
ferior to that of a cannon-ball, and deliver ‘its fare the full. fifty-two 
miles off (stoppages included) in Jess than half an hour. 
Now this is very foolish and very impracticable; but not. perhaps 
quite so visionary as at first sight it seems. The principle of the plan is 
perfectly unquestionable. The exhaustion or even rarefaction. of air at any 
given point, will no doubt create a strong current of air in that particular 
direction. The stage-coach projected is meant to run upona sort of tram 
road within the long tunnel or cylinder, provided with a fan or sail, to catch 
the whole force of the air which would be admitted behind it; and I have 
no doubt that the thing may be done which M, Vallance is now. attempt- 
ing; that is, that a model machine may be made in miniature—say a tin 
cylinder provided of a hundred feet long, with a toy coach to.run within 
it—which, by having the air exhausted, say at the north end, and suddenly 
admitted again at the south, would fully exemplify and perform all that 
he imagines, weap 
Then the peculiar excellence of this invention—say that it were pos- 
sible ever practically to carry it into execution—is not confined to the 
benefit of enabling us to travel without the aid of horses :—the grand 
gain is the speed. The velocity of a coach, impelled by air through a 
vacuum, is capable. of being carried as far as A THOUSAND. MILES AN 
HouR! It is true that the rate proposed for practice is not to exceed.a 
HUNDRED miles an hour; and I believe there is a slow. coach talked of, 
which ,;would not run more than rirty—for timid ladies or invalids. 
But, at starting, the going from London to Edinburgh between breakfast 
and dinner, is a thing calculated upon. ophit S241) Laie ae 
But the error here is one with which engineers are perfectly familiar— 
the mistake of fancying that the same results:can always be obtained 
upon an extended scale of action that we get upon a small one. .Prac- 
tically this is not the case: we lay a needle upon the surface of a glass 
of water, and it swims; but we could not therefore lay.a poker on the 
surface of a pond without its sinking. We see mouse thrown froma 
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