world, yet it is only a simple tunnel 8 miles long, while the Simplon
is a double tunnel, each bore of which is 12-1/4 miles. The chief
engineer of the Mont Cenis tunnel was M. Sommeiler, the man who devised
the first power drill ever used in such work. In addition to the power
drill the building of this tunnel induced the invention of apparatus
to suck up foul air, the air compressor, the turbine and several other
contrivances and appliances in use at the present time.

Great strides in modern tunneling developed the "shield" and brought
metal lining into service. The shield was invented and first used by
Sir M. I. Brunel, a London engineer, in excavating the tunnel under
the River Thames, begun in 1825 and finished in 1841. In 1869 another
English engineer, Peter Barlow, used an iron lining in connection with
a shield in driving the second tunnel under the Thames at London. From
a use of the shield and metal lining has grown the present system of
tunneling which is now universally known as the shield system.

Great advancement has been made in the past few years in the nature
and composition of explosives as well as in the form of motive power
employed in blasting. Powerful chemical compositions, such as
nitroglycerine and its compounds, such as dynamite, etc., have
supplanted gunpowder, and electricity, is now almost invariably the
firing agent. It also serves many other purposes in the work,
illumination, supplying power for hoisting and excavating machinery,
driving rock drills, and operating ventilating fans, etc. In this
field, in fact, as everywhere else in the mechanical arts, the electric
current is playing a leading part.

To the English engineer, Peter Barlow, above mentioned, must be given
the credit of bringing into use the first really serviceable circular