iron, with strong points in front to act as picks in
loosening the earth, and its bottom hung by a hinge at _d_, so that, by
detaching a catch, it may fly open and discharge the material raised. To
operate the machine, suppose the shovel _D_ to be in the position shown in
the cut; it is lowered by the chains _o_, _o_, and thrown forward or
backward, if necessary, by the drum _B_, and handle _S_, till the picks in
the front of the shovel are brought in proper contact with the face of the
cut; motion forward is now given to the shovel by the drum _B_ and handle
_S_, and at the same time it is raised by the chains _o_, _o_. These two
motions can be so adjusted to each other, as to give movement to the shovel
to enable it to loosen and scrape up a shovelful of earth. The handle _S_
is now left free, and the shovel _D_ is raised vertically by the chains
_o_, _o_. The crane is now turned round, till the shovel comes over a rail
car on a side track; the bottom of the shovel is opened, and the dirt
deposited in the car. All these motions are performed by the aid of a steam
engine, and are controlled by a man who stands on a platform at _f_.

[Illustration: Fig 76.]

692. _Q._--Having now described the most usual and approved forms of
engines applicable to numerous miscellaneous purposes for which a moderate
amount of steam power is required, will you briefly recapitulate what
amount of work of different kinds an engine of a given power will perform,
so that any one desiring to employ an engine to perform a given amount of
work, will be able to tell what the power of such engine should be?

_A._--It will of course be impossible to recapitulate all the purposes to
which engines are applicable, or to specify for every case the amount of
power necessary for the accomplishment of a given amount of work; but some
examples may be given which will be applicable to the bulk of the cases
occurring in practice.

693. _Q._--Beginning, then, with the power necessary for threshing,--a 4
horse power engine, with cylinder 6 inches diameter, pressure of steam 45
lbs., per square inch, and making 140 revolutions per minute, will thresh
out 40 quarters of wheat in 10 hours with a consumption of 3 cwt. of coals.

_A._--Although this may be done, it is probably too much to say that it can
be done on an average, and about three fourths of a quarter of wheat per
horse power would probably be a nearer average. The amount of power
consumed varies w