i Fan. 20, 1870]: . 
pressure due to a column of 409 or 500 feet of water 
—which is equal to about 2o0o0lb, per square inch— 
would not only cause infiltration through fissures and 
cracks, but would in a short time enlarge them, abrade 
the very rock, and so day by day increase the flow into 
the driftways. Asa general assertion, this evinces a con- 
fusion of the static with the dynamic laws of hydraulics. 
__ If we stop the flow from a crack, and if the water can- 
not be diverted, the full hydrostatic pressure due to the 
whole column will be established at the very margin of 
the crack where it was artificially closed. But if we do 
NATORE 
not stop the flow from it and allow the discharge, the | 
pressure at the margin within the tunnel will by no means 
be equal to the full hydrostatic column; for, in passing 
water along a fissure with a certain velocity, by far the 
largest part of that column will, under ordinary circum- 
stances, be destroyed or consumed in overcoming the 
friction of the water over the large area of the fissure, and, 
as a rule, the effective pressure at the end of a fine crack 
within the tunnel would be onlya small fraction of thewhole | 
column of water—probably not one per cent., so that no | 
abrasion could take place, and the flow of the water, 
whatever it might be, would not increase day by day. 
This being a very important question in reference to 
* the proposed tunnel, let us take an example.—Assume a 
fissure 1 yard long and 4,55 part of an inch wide—about 
the thickness of tissue paper. Let this fissure continue 
in a vertical direction for a distance of 100 yards, to the 
bottom of the channel, and assume above that fissure 30 
fathoms of water. What quantity of water will find its 
way through that fissure, and what will be the pressure at 
its margin within the Tunnel? We assume a clear 
opening throughout, and yet that fissure could only pass 4 
gallons per hour into the tunnel, and the effective pressure 
at its margin would only be 3}, 1b. per square inch. 
The effect of such a crack or opening would therefore 
be quite insignificant. Let all the circumstances remain 
the same, but assume the fissure Io times as wide, viz., 
,bo part of an inch in the clear; it would then pass 
-into the tunnel 136 gallons per hour, with a marginal 
pressure of 5; part of a pound per square inch; let it 
_ be 45 inch wide in the clear, and it will pass 4,392 
gallons per hour, with a marginal pressure of 2 of a pound 
-—and if we assume the fissure 1 inch wide in the clear, 
it will pass 139,905 gallons per hour, with a marginal | 
| position, to drive a tunnel under the Channel, but that 
pressure of 44 pounds per square inch. 
It then becomes chiefly a question of the wa¢éure of the 
fissures with which we may have to deal, and while a fine 
crack one yard long would only admit an insignificant 
volume of water into the tunnel, a similar crack one inch 
wide would be a serious matter, very few of which would 
drown the drift. But even in the latter case no abrasion 
of the rock could take place, for a pressure of four pounds 
per square inch has no effect upon chalk. 
But though the fissures within the driftways might be 
very fine, each passing or oozing out a comparatively 
small quantity of water, easily to be dealt with within a | 
moderate length of the driftways, their number within a 
distance of twenty-two miles would be legion, and, we 
believe, would overpower all appliances. These fissures 
should not be permitted jointly to send water into the 
driftways ; they should be closed as soon as practicable. 
How is that to be done? By keeping a water-tight main 
305 
tunnel close upon the face of the driftway.. As soon, 
however, as the cracks would be closed by the main 
water-tight tunnel, the full hydrostatic pressure of many 
hundred feet would be resting on it. The main tunnel 
would collapse unless it could bear a pressure of about 
fifteen tons per square foot on its superficial area. This 
circumstance determines the form and the material of the 
tunnel. 1¢ could not be an ordinary tunnel in any sense. 
Its form must be circular, and its principal material iron. 
No brickwork could stand that pressure at whatever 
thickness, within practicable limits, it might be assumed, 
because by increasing its thickness its superficial area 
would also increase. Nor could any brickwork be water- 
tight against such pressures, and, unless it be so, water 
will find its way into the tunnel very nearly as fast through 
the lining as without it. 
We cannot, therefore, help differing from the resolution 
of the engineers that— 
“Tn respect of the execution of the work itself, we consider it 
proper to drive preliminary driftways or headings under the 
channel, the ventilation of which would be accomplished by 
some of the usual modes adopted in the best coal-mines.”’ 
We consider this resolution fraught with danger. The 
driftways could never be accomplished without the aid of 
the main tunnel; nor would the proposed ventilation 
through the driftways be adequate. Why should the 
ventilation be similar to that adopted in our best coal- 
mines? Is the proposed tunnel to be driven through the 
coal measures? No, it is to be through the grey chalk, 
and a ventilation which may be adequate in coal measures 
would certainly fail in the chalk, from which a large 
amount of an irrespirable gas would exude. This 
another reason why the main tunnel should be kept 
closely behind the face of.the driftway, viz., to exclude the 
surface of the chalk.- And why have two driftways to 
begin with? Moreover, there is no difficulty in sending into 
the tunnel through ordinary piping, to the face of the head- 
ing, ten times as much pure air by mechanical power as the 
best mode of ventilating coal-mines could possibly ensure. 
The tunnel could only be worked by pneumatic pressure: 
this is obvious from recent investigations of eminent 
engineers ; it could not be for a double line, because 
that system is not applicable to it ; and the old atmospheric 
is 
| plan failed above ground. 
We are of opinion that it is not an unreasonable pro- 
in some measure it must be a venture. If we are 
to undertake such a venture to gain a magnificent 
prize, of immense value to the English and French 
nations, we must be prepared to meet all ordinary even- 
tualities, and we must not fail in the attempt by want 
of foresight, energy, and by dint of proper means. It 
will cost a great deal, but not too much, with proper 
management and a little good fortune. The first step 
towards accomplishing the object would be to obtain 
a geological section across the Channel, on one, or perhaps 
on several lines ; not a section constructed upon certain 
theories and assumptions, but one obtained from actual 
test of the materials which compose the. bed of the 
Channel, following Mr. Hawkshaw’s steps on shore also 
across the Channel, for which purpose the assistance of 
the Governments of England and France may be con- 
fidently expected. 
