182. In the first lecture I showed how a large weight could be raised
by a smaller weight, and I stated that this subject would again occupy
our attention. I now fulfil this promise. The questions to be discussed
involve the most advantageous methods of employing a small force to
overcome a greater. Here is a subject of practical importance. A man
of average strength cannot raise more than a hundredweight without
great exertion, yet the weights which it is necessary to lift and move
about often weigh many hundredweights, or even many tons. It is not
always practicable to employ numerous hands for the purpose, nor is a
steam-engine or other great source of power at all times available.
But what are called the mechanical powers enable the forces at our
disposal to be greatly increased. One man, by their aid, can exert as
much force as several could without such assistance; and when they
are employed to augment the power of several men or of a steam-engine,
gigantic weights, amounting sometimes to hundreds of tons, can be
managed with facility.
183. In the various arts we find innumerable cases where great
resistances have to be overcome; we also find a corresponding number
and variety of devices contrived by human skill to conquer them. The
girders of an iron bridge have to be lifted up to their piers; the
boilers and engines of an ocean steamer have to be placed in position;
a great casting has to be raised from its mould; a railway locomotive
has to be placed on the deck of a vessel for transit; a weighty anchor
has to be lifted from the bottom of the sea; an iron plate has to be
rolled or cut or punched: for all of these cases suitable arrangements
must be devised in order that the requisite power may be obtained.
184. We know but little of the means which the ancients employed in
raising the vast stones of those buildings which travellers in the East
have described to us. It is sometimes thought that a large number of
men could have transported these stones without the aid of appliances
which we would now use for a similar purpose. But it is more likely
that some of the mechanical powers were used, as, with a multitude of
men, it is difficult to ensure the proper application of their united
strength. In Easter Island, hundreds of miles distant from civilised
land, and now inhabited by savages, vast idols of stone have been found
in the hills which must have been raised by human labour. It is useless
to speculate on the extinct race by whom this work was achieved, or on
the means they employed.
185. The mechanical powers are usually enumerated as follows:—The
pulley, the lever, the wheel and axle, the wedge, the inclined plane,
the screw. These different powers are so frequently used in combination
that the distinctions cannot be always maintained. The classification
will, however, suffice to give a general notion of the subject.
186. Many of the most valuable mechanical powers are machines in
which ropes or chains play an important part. Pulleys are usually
employed wherever it is necessary to change the direction of a rope
or chain which is transmitting power. In the present lecture we shall
examine the most important mechanical powers that are produced by the
combination of pulleys.