At a given signal from the foreman, two operatives, stationed at each of
the forty lifting points, with crowbars inserted in the holes provided for
the purpose, give the screws a simultaneous turn in the same direction. The
bars are then inserted in another hole higher up. The hoisting screws are
connected with the structure of the roof, and rise therewith. All that is
requisite for the hoisting from the next cross beam is to give a forward
turn to the screws. When the workmen had become accustomed to their task,
the hoisting to a distance of 1 meter occupied only about half to
three-quarters of an hour. At the outset, and merely by way of a trial, the
roof was lifted to a height of fully 2 meters, and left for some time
suspended in the air. The eighty men engaged in the operation carry on the
work with great regularity and steadiness, obeying the signal of the
foreman as soon as it was given.