468 
POPULAR SCIENCE REVIEW. 
egress for a distance of five, ten, or twenty miles, even such 
an expense might be worth its cost in actual warfare, while 
certainly in shorter lengths, within the region of a fortifica- 
tion, or between contiguous forts, it would undoubtedly have 
the highest claims to consideration. This, however, may be 
the last and least profitable of the many uses we shall see this 
system turned to. 
We mentioned incidentally, in our opening pages, the 
“ sighing ” and “ breathing ” of the despatch-tube after 
its work. This is very curious. As soon as the carriages 
have passed the gratings of the Euston tube, a deep 
sighing begins, occasioned by the wind rushing in and out. 
If, for example, it be after an “ exhaust,” the air rushes vio- 
lently into the tube, and, by its own impetus, crowds itself 
up and condenses itself within the tube, until its inherent 
elasticity reverses the current, and then an out-thrust of air 
takes place at such velocity that the tenacity and cohesion 
of the air within the tube is on the opposite hand pulled 
upon, and the new vacuum thus created again causes another 
sighing indraught of air. And so these waves of air sway to 
and fro, and in and out, for many seconds or even several 
minutes after the work of the tube is done, before quiescence 
is restored in its interior recesses. 
Does not this incident hint how much more we have yet to 
learn of oven the mechanical properties of air ? 
EXPLANATION OF PLATE. 
Fig. 1. The Pneumatic Despatch Tube, with valves, VV', at both ends. 
Fig. 2. The fan-wheel and stationary engine. 
Fig. 3. The fan-wheel, showing its central orifice at c, c, c, and the bearing- 
ring cl, cl, cl, which works against the in-draught pipe, E. 
Fig. 4. Section of fan- wheel in its inclosing case, showing its diminishing 
breadth from centre to periphery ; the stationary engine, C, by 
which it is worked by the crank, c, and the shaft, s s ; at K K and 
L L, the air enters from the in-draught chambers, E E' ; and is 
thrown off at the periphery, a, a, a. 
Fig. 5. One of the Euston pneumatic mail carriages. 
Fig. 6. Section of Euston pneumatic pipe, with rails and carriage within it. 
Fig. 7. End of Pneumatic Tube, showing by the increasing darkening of the 
tint the increasing pressure of air as the travelling carriage 
approaches the closed valve, V, and the effect of the piston, P, in 
allowing escape of air above the fixed pressure. 
Fig. 8. The double exhausted Pneumatic Despatch Tube in use at the Central 
Telegraph Station in Moorgate-street ; the exhaustion being 
continuous by a stationary engine at c ; the parcels delivering 
themselves at the valves a or c. 
