current of two cables very much larger in sectional area is only one
instance in point. The two major cables carry currents running in
opposite directions, and as these currents are both caused to return
along the third and smaller wire their electro-motive forces balance
one another, with the result that the return wire needs only to carry
a small difference-current. The return wire, in fact, is analogous to
the Banking Clearing House, which deals with balances only, and which
therefore can sometimes adjust business to the value of many millions
with payments of only a few thousands. Later on it may fairly be
expected that duplicate and quadruplicate telegraphy will find its
counterpart in systems by which different series of electrical
impulses of high voltage will run along a wire, the one alternating
with the other and each series filling up the gaps left between the
others.

CHAPTER IV.

ARTIFICIAL POWER.

The steam-turbine is the most clearly visible of the revolutionary
agencies in motors using the artificial sources of power. In the first
attempts to introduce the principle the false analogy of the
water-turbine gave rise to much waste of inventive energy and of
money; but the more recent and more distinctly successful types of
machine have been constructed with a clear understanding that the
windmill is the true precursor of the steam-turbine. It is clearly
perceived that, although it may be convenient and even essential to
reduce the arms to pigmy dimensions and to enclose them in a tube,
still the general principle of the machine must resemble that of a
number of wind motors all running on the same shaft.

It has been proved, moreover, that this multiplicity of minute wheels
and arms has a very distinct advantage in that it renders possible the
utilisation of the expansive power of steam. The first impact is small
in area but intense in force, while those arms which receive the
expanded steam further on are larger in size as suited to making the
best use of a weaker force distributed over a greater amount of space.

The enormous speed at which steam under heavy pressure rushes out of
an orifice was not duly appreciated by the first experimenters in this
direction. To obtain the best results in utilising the power from
escaping steam there must be a certain definite proportion between the
speed of the vapour and that of the vane or arm aga