Besides, although the work was colossal it did not exceed the limit of human capability.  Far from that.  How many works of much greater difficulty, and in which the elements had to be more directly contended against, had been brought to a successful termination!  Suffice it to mention the well of Father Joseph, made near Cairo by the Sultan Saladin at an epoch when machines had not yet appeared to increase the strength of man a hundredfold, and which goes down to the level of the Nile itself at a depth of 300 feet!  And that other well dug at Coblentz by the Margrave Jean of Baden, 600 feet deep!  All that was needed was a triple depth and a double width, which made the boring easier.  There was not one foreman or workman who doubted about the success of the operation.

An important decision taken by Murchison and approved of by Barbicane accelerated the work.  An article in the contract decided that the Columbiad should be hooped with wrought-iron—­a useless precaution, for the cannon could evidently do without hoops.  This clause was therefore given up.  Hence a great economy of time, for they could then employ the new system of boring now used for digging wells, by which the masonry is done at the same time as the boring.  Thanks to this very simple operation they were not obliged to prop up the ground; the wall kept it up and went down by its own weight.

This manoeuvre was only to begin when the spade should have reached the solid part of the ground.

On the 4th of November fifty workmen began to dig in the very centre of the inclosure surrounded by palisades—­that is to say, the top of Stony Hill—­a circular hole sixty feet wide.

The spade first turned up a sort of black soil six inches deep, which it soon carried away.  To this soil succeeded two feet of fine sand, which was carefully taken out, as it was to be used for the casting.

After this sand white clay appeared, similar to English chalk, and which was four feet thick.

Then the pickaxes rang upon the hard layer, a species of rock formed by very dry petrified shells.  At that point the hole was six and a half feet deep, and the masonry was begun.

At the bottom of that excavation they made an oak wheel, a sort of circle strongly bolted and of enormous strength; in its centre a hole was pierced the size of the exterior diameter of the Columbiad.  It was upon this wheel that the foundations of the masonry were placed, the hydraulic cement of which joined the stones solidly together.  After the workmen had bricked up the space from the circumference to the centre, they found themselves inclosed in a well twenty-one feet wide.

When this work was ended the miners began again with spade and pickaxe, and set upon the rock under the wheel itself, taking care to support it on extremely strong tressels; every time the hole was two feet deeper they took away the tressels; the wheel gradually sank, taking with it its circle of masonry, at the upper layer of which the masons worked incessantly, taking care to make vent-holes for the escape of gas during the operation of casting.