MAN'S WAY OF HELPING HIMSELF
149. Labor-saving Devices. To primitive man belonged more especially
the arduous tasks of the out-of-door life: the clearing of paths
through the wilderness; the hauling of material; the breaking up of
the hard soil of barren fields into soft loam ready to receive the
seed; the harvesting of the ripe grain, etc.
[Illustration: FIG. 91.--Prying a stone out of the ground.]
The more intelligent races among men soon learned to help themselves
in these tasks. For example, our ancestors in the field soon learned
to pry stones out of the ground (Fig. 91) rather than to undertake the
almost impossible task of lifting them out of the earth in which they
were embedded; to swing fallen trees away from a path by means of rope
attached to one end rather than to attempt to remove them
single-handed; to pitch hay rather than to lift it; to clear a field
with a rake rather than with the hands; to carry heavy loads in
wheelbarrows (Fig. 92) rather than on the shoulders; to roll barrels
up a plank (Fig. 93) and to raise weights by ropes. In every case,
whether in the lifting of stones, or the felling of trees, or the
transportation of heavy weights, or the digging of the ground, man
used his brain in the invention of mechanical devices which would
relieve muscular strain and lighten physical labor.
If all mankind had depended upon physical strength only, the world
to-day would be in the condition prevalent in parts of Africa, Asia,
and South America, where the natives loosen the soil with their hands
or with crude implements (Fig. 94), and transport huge weights on
their shoulders and heads.
[Illustration: FIG. 92.--The wheelbarrow lightens labor.]
Any mechanical device (Figs. 95 and 96), whereby man's work can be
more conveniently done, is called a machine; the machine itself never
does any work--it merely enables man to use his own efforts to better
advantage.
[Illustration: FIG. 93.--Rolling barrels up a plank.]
150. When do we Work? Whenever, as a result of effort or force, an
object is moved, work is done. If you lift a knapsack from the floor
to the table, you do work because you use force and move the knapsack
through a distance equal to the height of the table. If the knapsack
were twice as heavy, you would exert twice as much force to raise it
to the same height, and hence you would do double the work. If you
raised the knapsack twice the distance,--say to your shoulders
instead of to the level of the table,--you would do twice the work,
because while you would exert the same force you would continue it
through double the distance.
[Illustration: FIG. 94.--Crude method of farming.]
Lifting heavy weights through great distances is not the only way in
which work is done. Painting, chopping wood, hammering, plowing,
washing, scrubbing, sewing, are all forms of work. In painting, the
moving brush spreads paint over a surface; in chopping wood, the
descending ax cleaves the wood asunder; in scrubbing, the wet mop
rubbed over the floor carries dirt away; in every conceivable form of
work, force and motion occur.
A man does work when he walks, a woman does work when she rocks in a
chair--although here the work is less than in walking. On a windy day
the work done in walking is greater than normal. The wind resists our
progress, and we must exert more force in order to cover the same
distance. Walking through a plowed or rough field is much more tiring
than to walk on a smooth road, because, while the distance covered may
be the same, the effort put forth is greater, and hence more work is
done. Always the greater the resistance encountered, the greater the
force required, and hence the greater the work done.
The work done by a boy who raises a 5-pound knapsack to his shoulder
would be 5x4, or 20, providing his shoulders were 4 feet from the
ground.
The amount of work done depends upon the force used and the distance
covered (sometimes called displacement), and hence we can say that
Work = force multiplied by distance,
or _W = f × d_.
151. Machines. A glance into our machine shops, our factories, and
even our homes shows how widespread is the use of complex machinery.
But all machines, however complicated in appearance, are in reality
but modifications and combinations of one or more of four simple
machines devised long ago by our remote ancestors. These simple
devices are known to-day, as (1) the lever, represented by a crowbar,
a pitchfork; (2) the inclined plane, represented by the plank upon
which barrels are rolled into a wagon; (3) the pulley, represented by
almost any contrivance for the raising of furniture to upper stories;
(4) the wheel and axle, represented by cogwheels and coffee grinders.
[Illustration: FIG. 95.--Primitive method of grinding corn.]
Suppose a 600-pound bowlder which is embedded in the ground is needed
for the tower of a building. The problem of the builder is to get the
heavy bowlder out of the ground, to load it on a wagon for
transportation, and finally to raise it to the tower. Obviously, he
cannot do this alone; the greatest amount of force of which he is
capable would not suffice to accomplish any one of these tasks. How
then does he help himself and perform the impossible? Simply, by the
use of some of the machine types mentioned above, illustrations of
which are known in a general way to every schoolboy. The very knife
with which a stick is whittled is a machine.
[Illustration: FIG. 96.--Separating rice grains by flailing.]
[Illustration: FIG. 97.--The principle of the lever.]
152. The Lever. Balance a foot rule, containing a hole at its middle
point _F_, as shown in Figure 97. If now a weight of 1 pound is
suspended from the bar at some point, say 12, the balance is
disturbed, and the bar swings about the point _F_ as a center. The
balance can be regained by suspending an equivalent weight at the
opposite end of the bar, or by applying a 2-pound weight at a point 3
inches to the left of _F_. In the latter case a force of 1 pound
actually balances a force of 2 pounds, but the 1-pound weight is twice
as far from the point of suspension as is the 2-pound weight. The
small weight makes up in distance what it lacks in magnitude.
Such an arrangement of a rod or bar is called a lever. In any form of
lever there are only three things to be considered: the point where
the weight rests, the point where the force acts, and the point called
the fulcrum about which the rod rotates.
The distance from the force to the fulcrum is called the force arm.
The distance from the weight to the fulcrum is called the weight arm;
and it is a law of levers, as well as of all other machines, that the
force multiplied by the length of the force arm must equal the weight
multiplied by the length of the weight arm.
Force × force arm = weight × weight arm.
A force of 1 pound at a distance of 6, or with a force arm 6, will
balance a weight of 2 pounds with a weight arm 3; that is,
1 × 6 = 2 × 3.
Similarly a force of 10 pounds may be made to sustain a weight of 100
pounds, providing the force arm is 10 times longer than the weight
arm; and a force arm of 800 pounds, at a distance of 10 feet from the
fulcrum, may be made to sustain a weight of 8000 pounds, providing the
weight is 1 foot from the fulcrum.
153. Applications of the Lever. By means of a lever, a 600-pound
bowlder can be easily pried out of the ground. Let the lever, any
strong metal bar, be supported on a stone which serves as fulcrum;
then if a man exerts his force at the end of the rod somewhat as in
Figure 91 (p. 154), the force arm will be the distance from the stone
or fulcrum to the end of the bar, and the weight arm will be the
distance from the fulcrum to the bowlder itself. The man pushes down
with a force of 100 pounds, but with that amount succeeds in prying up
the 600-pound bowlder. If, however, you look carefully, you will see
that the force arm is 6 times as long as the weight arm, so that the
smaller force is compensated for by the greater distance through which
it acts.
At first sight it seems as though the man's work were done for him by
the machine. But this is not so. The man must lower his end of the
lever 3 feet in order to raise the bowlder 6 inches out of the ground.
He does not at any time exert a large force, but he accomplishes his
purpose by exerting a small force continuously through a
correspondingly greater distance. He finds it easier to exert a force
of 100 pounds continuously until his end has moved 3 feet rather than
to exert a force of 600 pounds on the bowlder and move it 6 inches.
By the time the stone has been raised the man has done as much work as
though the stone had been raised directly, but his inability to put
forth sufficient muscular force to raise the bowlder directly would
have rendered impossible a result which was easily accomplished when
through the medium of the lever he could extend his small force
through greater distance.
154. The Wheelbarrow as a Lever. The principle of the lever is
always the same; but the relative position of the important points may
vary. For example, the fulcrum is sometimes at one end, the force at
the opposite end, and the weight to be lifted between them.
[Illustration: FIG. 98.--A slightly different form of lever.]
Suspend a stick with a hole at its center as in Figure 98, and hang a
4-pound weight at a distance of 1 foot from the fulcrum, supporting
the load by means of a spring balance 2 feet from the fulcrum. The
pointer on the spring balance shows that the force required to balance
the 4-pound load is but 2 pounds.
The force is 2 feet from the fulcrum, and the weight (4) is 1 foot
from the fulcrum, so that
Force × distance = Weight × distance,
or 2 × 2 = 4 × 1.
Move the 4-pound weight so that it is very near the fulcrum, say but 6
inches from it; then the spring balance registers a force only one
fourth as great as the weight which it suspends. In other words a
force of 1 at a distance of 24 inches (2 feet) is equivalent to a
force of 4 at a distance of 6 inches.
[Illustration: FIG. 99.--The wheelbarrow lightened labor.]
One of the most useful levers of this type is the wheelbarrow (Fig.
99). The fulcrum is at the wheel, the force is at the handles, the
weight is on the wheelbarrow. If the load is halfway from the fulcrum
to the man's hands, the man will have to lift with a force equal to
one half the load. If the load is one fourth as far from the fulcrum
as the man's hands, he will need to lift with a force only one fourth
as great as that of the load.
[Illustration: FIG. 100.--A modified wheelbarrow.]
This shows that in loading a wheelbarrow, it is important to arrange
the load as near to the wheel as possible.
[Illustration: FIG. 101.--The nutcracker is a lever.]
The nutcracker (Fig. 101) is an illustration of a double lever of the
wheelbarrow kind; the nearer the nut is to the fulcrum, the easier the
cracking.
[Illustration: FIG. 102.--The hand exerts a small force over a long
distance and draws out a nail.]
Hammers (Fig. 102), tack-lifters, scissors, forceps, are important
levers, and if you will notice how many different levers (fig. 103)
are used by all classes of men, you will understand how valuable a
machine this simple device is.
155. The Inclined Plane. A man wishes to load the 600-pound bowlder
on a wagon, and proceeds to do it by means of a plank, as in Figure
93. Such an arrangement is called an inclined plane.
The advantage of an inclined plane can be seen by the following
experiment. Select a smooth board 4 feet long and prop it so that the
end _A_ (Fig. 104) is 1 foot above the level of the table; the length
of the incline is then 4 times as great as its height. Fasten a metal
roller to a spring balance and observe its weight. Then pull the
roller uniformly upward along the plank and notice what the pull is on
the balance, being careful always to hold the balance parallel to the
incline.
When the roller is raised along the incline, the balance registers a
pull only one fourth as great as the actual weight of the roller. That
is, when the roller weighs 12, a force of 3 suffices to raise it to
the height _A_ along the incline; but the smaller force must be
applied throughout the entire length of the incline. In many cases, it
is preferable to exert a force of 30 pounds, for example, over the
distance _CA_ than a force of 120 pounds over the shorter distance
_BA_.
[Illustration: FIG. 103.--Primitive man tried to lighten his task by
balancing his burden.]
Prop the board so that the end _A_ is 2 feet above the table level;
that is, arrange the inclined plane in such a way that its length is
twice as great as its height. In that case the steady pull on the
balance will be one half the weight of the roller; or a force of 6
pounds will suffice to raise the 12-pound roller.
[Illustration: FIG. 104.--Less force is required to raise the roller
along the incline than to raise it to _A_ directly.]
The steeper the incline, the more force necessary to raise a weight;
whereas if the incline is small, the necessary lifting force is
greatly reduced. On an inclined plane whose length is ten times its
height, the lifting force is reduced to one tenth the weight of the
load. The advantage of an incline depends upon the relative length and
height, or is equal to the ratio of the length to the height.
156. Application. By the use of an inclined plank a strong man can
load the 600-pound bowlder on a wagon. Suppose the floor of the wagon
is 2 feet above the ground, then if a 6-foot plank is used, 200 pounds
of force will suffice to raise the bowlder; but the man will have to
push with this force against the bowlder while it moves over the
entire length of the plank.
Since work is equal to force multiplied by distance, the man has done
work represented by 200 × 6, or 1200. This is exactly the amount of
work which would have been necessary to raise the bowlder directly. A
man of even enormous strength could not lift such a weight (600 lb.)
even an inch directly, but a strong man can furnish the smaller force
(200) over a distance of 6 feet; hence, while the machine does not
lessen the total amount of work required of a man, it creates a new
distribution of work and makes possible, and even easy, results which
otherwise would be impossible by human agency.
157. Railroads and Highways. The problem of the incline is an
important one to engineers who have under their direction the
construction of our highways and the laying of our railroad tracks. It
requires tremendous force to pull a load up grade, and most of us are
familiar with the struggling horse and the puffing locomotive. For
this reason engineers, wherever possible, level down the steep places,
and reduce the strain as far as possible.
[Illustration: FIG. 105.--A well-graded railroad bed.]
The slope of the road is called its grade, and the grade itself is
simply the number of feet the hill rises per mile. A road a mile long
(5280 feet) has a grade of 132 if the crest of the hill is 132 feet
above the level at which the road started.
[Illustration: FIG. 106.--A long, gradual ascent is better than a
shorter, steeper one.]
In such an incline, the ratio of length to height is 5280 ÷ 132, or
40; and hence in order to pull a train of cars to the summit, the
engine would need to exert a continuous pull equal to one fortieth of
the combined weight of the train.
If, on the other hand, the ascent had been gradual, so that the grade
was 66 feet per mile, a pull from the engine of one eightieth of the
combined weight would have sufficed to land the train of cars at the
crest of the grade.
Because of these facts, engineers spend great sums in grading down
railroad beds, making them as nearly level as possible. In mountainous
regions, the topography of the land prevents the elimination of all
steep grades, but nevertheless the attempt is always made to follow
the easiest grades.
158. The Wedge. If an inclined plane is pushed underneath or within
an object, it serves as a wedge. Usually a wedge consists of two
inclined planes (Fig. 107).
[Illustration: FIG. 107.--By means of a wedge, the stump is split.]
A chisel and an ax are illustrations of wedges. Perhaps the most
universal form of a wedge is our common pin. Can you explain how this
is a wedge?
159. The Screw. Another valuable and indispensable form of the
inclined plane is the screw. This consists of a metal rod around which
passes a ridge, and Figure 108 shows clearly that a screw is simply a
rod around which (in effect) an inclined plane has been wrapped.
[Illustration: FIG. 108--A screw as a simple machine.]
The ridge encircling the screw is called the thread, and the distance
between two successive threads is called the pitch. It is easy to see
that the closer the threads and the smaller the pitch, the greater the
advantage of the screw, and hence the less force needed in overcoming
resistance. A corkscrew is a familiar illustration of the screw.
160. Pulleys. The pulley, another of the machines, is merely a
grooved wheel around which a cord passes. It is sometimes more
convenient to move a load in one direction rather than in another, and
the pulley in its simplest form enables us to do this. In order to
raise a flag to the top of a mast, it is not necessary to climb the
mast, and so pull up the flag; the same result is accomplished much
more easily by attaching the flag to a movable string, somewhat as in
Figure 109, and pulling from below. As the string is pulled down, the
flag rises and ultimately reaches the desired position.
If we employ a stationary pulley, as in Figure 109, we do not change
the force, because the force required to balance the load is as large
as the load itself. The only advantage is that a force in one
direction may be used to produce motion in another direction. Such a
pulley is known as a fixed pulley.
[Illustration: FIG. 109.--By means of a pulley, a force in one
direction produces motion in the opposite direction.]
161. Movable Pulleys. By the use of a movable pulley, we are able to
support a weight by a force equal to only one half the load. In Figure
109, the downward pull of the weight and the downward pull of the hand
are equal; in Figure 110, the spring balance supports only one half
the entire load, the remaining half being borne by the hook to which
the string is attached. The weight is divided equally between the two
parts of the string which passes around the pulley, so that each
strand bears only one half of the burden.
We have seen in our study of the lever and the inclined plane that an
increase in force is always accompanied by a decrease in distance, and
in the case of the pulley we naturally look for a similar result. If
you raise the balance (Fig. 110) 12 feet, you will find that the
weight rises only 6 feet; if you raise the balance 24 inches, you will
find that the weight rises 12 inches. You must exercise a force of
100 pounds over 12 feet of space in order to raise a weight of 200
pounds a distance of 6 feet. When we raise 100 pounds through 12 feet
or 200 pounds through 6 feet the total work done is the same; but the
pulley enables those who cannot furnish a force of 200 pounds for the
space of 6 feet to accomplish the task by furnishing 100 pounds for
the space of 12 feet.
[Illustration: FIG. 110.--A movable pulley lightens labor.]
162. Combination of Pulleys. A combination of pulleys called block
and tackle is used where very heavy loads are to be moved. In Figure
111 the upper block of pulleys is fixed, the lower block is movable,
and one continuous rope passes around the various pulleys. The load is
supported by 6 strands, and each strand bears one sixth of the load.
If the hand pulls with a force of 1 pound at _P_, it can raise a load
of 6 pounds at _W_, but the hand will have to pull downward 6 feet at
_P_ in order to raise the load at _W_ 1 foot. If 8 pulleys were used,
a force equivalent to one eighth of the load would suffice to move
_W_, but this force would have to be exerted over a distance 8 times
as great as that through which _W_ was raised.
[Illustration: FIG. 111.--An effective arrangement of pulleys known as
block and tackle.]
163. Practical Application. In our childhood many of us saw with
wonder the appearance and disappearance of flags flying at the tops
of high masts, but observation soon taught us that the flags were
raised by pulleys. In tenements, where there is no yard for the family
washing, clothes often appear flapping in mid-air. This seems most
marvelous until we learn that the lines are pulled back and forth by
pulleys at the window and at a distant support. By means of pulleys,
awnings are raised and lowered, and the use of pulleys by furniture
movers, etc., is familiar to every wide-awake observer on the streets.
164. Wheel and Axle. The wheel and axle consists of a large wheel
and a small axle so fastened that they rotate together.
[Illustration: FIG. 112.--The wheel and axle.]
When the large wheel makes one revolution, _P_ falls a distance equal
to the circumference of the wheel. While _P_ moves downward, _W_
likewise moves, but its motion is upward, and the distance it moves is
small, being equal only to the circumference of the small axle. But a
small force at _P_ will sustain a larger force at _W_; if the
circumference of the large wheel is 40 inches, and that of the small
wheel 10 inches, a load of 100 at _W_ can be sustained by a force of
25 at _P_. The increase in force of the wheel and axle depends upon
the relative size of the two parts, that is, upon the circumference of
wheel as compared with circumference of axle, and since the ratio
between circumference and radius is constant, the ratio of the wheel
and axle combination is the ratio of the long radius to the short
radius.
For example, in a wheel and axle of radii 20 and 4, respectively, a
given weight at _P_ would balance 5 times as great a load at _W_.
165. Application. _Windlass, Cogwheels._ In the old-fashioned
windlass used in farming districts, the large wheel is replaced by a
handle which, when turned, describes a circle. Such an arrangement is
equivalent to wheel and axle (Fig. 112); the capstan used on shipboard
for raising the anchor has the same principle. The kitchen coffee
grinder and the meat chopper are other familiar illustrations.
Cogwheels are modifications of the wheel and axle. Teeth cut in _A_
fit into similar teeth cut in _B_, and hence rotation of _A_ causes
rotation of _B_. Several revolutions of the smaller wheel, however,
are necessary in order to turn the larger wheel through one complete
revolution; if the radius of _A_ is one half that of _B_, two
revolutions of _A_ will correspond to one of _B_; if the radius of _A_
is one third that of _B_, three revolutions of _A_ will correspond to
one of _B_.
[Illustration: FIG. 113.--Cogwheels.]
Experiment demonstrates that a weight _W_ attached to a cogwheel of
radius 3 can be raised by a force _P_, equal to one third of _W_
applied to a cogwheel of radius 1. There is thus a great increase in
force. But the speed with which _W_ is raised is only one third the
speed with which the small wheel rotates, or increase in power has
been at the decrease of speed.
This is a very common method for raising heavy weights by small force.
Cogwheels can be made to give speed at the decrease of force. A heavy
weight _W_ attached to _B_ will in its slow fall cause rapid rotation
of _A_, and hence rapid rise of _P_. It is true that _P_, the load
raised, will be less than _W_, the force exerted, but if speed is our
aim, this machine serves our purpose admirably.
An extremely important form of wheel and axle is that in which the two
wheels are connected by belts as in Figure 114. Rotation of _W_
induces rotation of _w_, and a small force at _W_ is able to overcome
a large force at _w_. An advantage of the belt connection is that
power at one place can be transmitted over a considerable distance and
utilized in another place.
[Illustration: FIG. 114.--By means of a belt, motion can be
transferred from place to place.]
166. Compound Machines. Out of the few simple machines mentioned in
the preceding Sections has developed the complex machinery of to-day.
By a combination of screw and lever, for example, we obtain the
advantage due to each device, and some compound machines have been
made which combine all the various kinds of simple machines, and in
this way multiply their mechanical advantage many fold.
A relatively simple complex machine called the crane (Fig. 116) maybe
seen almost any day on the street, or wherever heavy weights are being
lifted. It is clear that a force applied to turn wheel 1 causes a
slower rotation of wheel 3, and a still slower rotation of wheel 4,
but as 4 rotates it winds up a chain and slowly raises _Q_. A very
complex machine is that seen in Figure 117.
[Illustration: FIG. 115.--A simple derrick for raising weights.]
[Illustration: FIG. 116.--A traveling crane.]
167. Measurement of Work. In Section 150, we learned that the amount
of work done depends upon the force exerted, and the distance
covered, or that _W_ = force × distance. A man who raises 5 pounds a
height of 5 feet does far more work than a man who raises 5 ounces a
height of 5 inches, but the product of force by distance is 25 in each
case. There is difficulty because we have not selected an arbitrary
unit of work. The unit of work chosen and in use in practical affairs
is the foot pound, and is defined as the work done when a force of 1
pound acts through a distance of 1 foot. A man who moves 8 pounds
through 6 feet does 48 foot pounds of work, while a man who moves 8
ounces (1/2 pound) through 6 inches (1/2 foot) does only one fourth of
a foot pound of work.
[Illustration: FIG. 117.--A farm engine putting in a crop.]
168. The Power or the Speed with which Work is Done. A man can load
a wagon more quickly than a growing boy. The work done by the one is
equal to the work done by the other, but the man is more powerful,
because the time required for a given task is very important. An
engine which hoists a 50-pound weight in 1 second is much more
powerful than a man who requires 50 seconds for the same task; hence
in estimating the value of a working agent, whether animal or
mechanical, we must consider not only the work done, but the speed
with which it is done.
The rate at which a machine is able to accomplish a unit of work is
called _power_, and the unit of power customarily used is the horse
power. Any power which can do 550 foot pounds of work per second is
said to be one horse power (H.P.). This unit was chosen by James Watt,
the inventor of a steam engine, when he was in need of a unit with
which to compare the new source of power, the engine, with his old
source of power, the horse. Although called a horse power it is
greater than the power of an average horse.
An ordinary man can do one sixth of a horse power. The average
locomotive of a railroad has more than 500 H.P., while the engines of
an ocean liner may have as high as 70,000 H.P.
169. Waste Work and Efficient Work. In our study of machines we
omitted a factor which in practical cases cannot be ignored, namely,
friction. No surface can be made perfectly smooth, and when a barrel
rolls over an incline, or a rope passes over a pulley, or a cogwheel
turns its neighbor, there is rubbing and slipping and sliding. Motion
is thus hindered, and the effective value of the acting force is
lessened. In order to secure the desired result it is necessary to
apply a force in excess of that calculated. This extra force, which
must be supplied if friction is to be counteracted, is in reality
waste work.
If the force required by a machine is 150 pounds, while that
calculated as necessary is 100 pounds, the loss due to friction is 50
pounds, and the machine, instead of being thoroughly efficient, is
only two thirds efficient.
Machinists make every effort to eliminate from a machine the waste due
to friction, leveling and grinding to the most perfect smoothness and
adjustment every part of the machine. When the machine is in use,
friction may be further reduced by the use of lubricating oil.
Friction can never be totally eliminated, however, and machines of
even the finest construction lose by friction some of their
efficiency, while poorly constructed ones lose by friction as much as
one half of their efficiency.
170. Man's Strength not Sufficient for Machines. A machine, an inert
mass of metal and wood, cannot of itself do any work, but can only
distribute the energy which is brought to it. Fortunately it is not
necessary that this energy should be contributed by man alone, because
the store of energy possessed by him is very small in comparison with
the energy required to run locomotives, automobiles, sawmills, etc.
Perhaps the greatest value of machines lies in the fact that they
enable man to perform work by the use of energy other than his own.
[Illustration: FIG. 118.--Man's strength is not sufficient for heavy
work.]
Figure 118 shows one way in which a horse's energy can be utilized in
lifting heavy loads. Even the fleeting wind has been harnessed by man,
and, as in the windmill, made to work for him (Fig. 119). One sees
dotted over the country windmills large and small, and in Holland, the
country of windmills, the landowner who does not possess a windmill is
poor indeed.
For generations running water from rivers, streams, and falls has
served man by carrying his logs downstream, by turning the wheels of
his mill, etc.; and in our own day running water is used as an
indirect source of electric lights for street and house, the energy of
the falling water serving to rotate the armature of a dynamo (Section
310).
[Illustration: FIG. 119.--The windmill pumps water into the troughs
where cattle drink.]
A more constant source of energy is that available from the burning of
fuel, such as coal and oil. The former is the source of energy in
locomotives, the latter in most automobiles.
In the following Chapter will be given an account of water, wind, and
fuel as machine feeders.