NATURAL POWER.
"Nature," remarked James Watt when he set to work inventing his
improved steam-engine, "has always a weak side if we can only find it
out." Many invaluable secrets have been successfully explored through
the discovery of Nature's "weak side" since that momentous era in the
industrial history of the world; and the nineteenth century, as Watt
clearly foresaw, has been emphatically the age of steam power. In the
condenser, the high pressure cylinder and the automatic cut-off, which
utilises the expansive power of steam vapour, mankind now possesses
the means of taming a monster whose capacities were almost entirely
unknown to the ancients, and of bringing it into ready and willing
service for the accomplishment of useful work. Vaguely and loosely it
is often asserted that the age of steam is now giving place to that of
electricity; but these two cannot yet be logically placed in
opposition to one another. No method has yet been discovered whereby
the heat of a furnace can be directly converted into an electric
current. The steam-engine or, as Watt and his predecessors called it,
the "fire-engine" is _par excellence_ the world's prime motor; and by
far the greater proportion of the electrical energy that is generated
to-day owes its existence primarily to the steam-engine and to other
forms of reciprocating machinery designed to utilise the expansive
power of vapours or gases acting in a similar manner to steam.
The industrial revolutions of the coming century will, without doubt,
be brought about very largely through the utilisation of Nature's
waste energy in the service of mankind. Waterfalls, after being very
largely neglected for two or three generations, are now commanding
attention as valuable and highly profitable sources of power. This is
only to be regarded as forming the small beginning of a movement
which, in the coming century, will "acquire strength by going," and
which most probably will, in less than a hundred years, have produced
changes in the industrial world comparable to those brought about by
the invention of the steam-engine.
Lord Kelvin, in the year 1881, briefly, but very significantly,
classified the sources of power available to man under the five
primary headings of tides, food, fuel, wind, and rain. Food is the
generator of animal energy, fuel that of the power obtained from steam
and other mechanical expansive engines; rain, as it falls on the
hill-tops and descends in long lines of natural force to the sea
coasts, furnishes power to the water-wheel; while wind may be utilised
to generate mechanical energy through the agency of windmills and
other contrivances. The tides as a source of useful power have hardly
yet begun to make their influence felt, and indeed the possibility of
largely using them is still a matter of doubt. The relative advantages
of reclaiming a given area of soil for purposes of cultivation, and of
converting the same land into a tidal basin in order to generate power
through the inward and outward flow of the sea-water, were contrasted
by Lord Kelvin in the statement of a problem as follows: Which is the
more valuable--an agricultural area of forty acres or an available
source of energy equal to one hundred horse-power? The data for the
solution of such a question are obviously not at hand, unless the
quality of the land, its relative nearness to the position at which
power might be required, and several other factors in its economic
application have been supplied. Still, the fact remains that very
large quantities of the coastal land and a considerable quantity of
expensive work would be needed for the generation, by means of the
tides, of any really material quantity of power.
It is strange that, while so much has been written and spoken about
the possibility of turning the energy of the tides to account for
power in the service of man, comparatively little attention has been
paid to the problem of similarly utilising the wave-power, which goes
to waste in such inconceivably huge quantities. Where the tidal force
elevates and depresses the sea-water on a shore, through a vertical
distance of say eight feet, about once in twelve hours, the waves of
the ocean will perform the same work during moderate weather once in
every twelve or fifteen seconds. It is true that the moon in its
attraction of the sea-water produces a vastly greater sum total of
effect than the wind does in raising the surface-waves, but reckoning
only that part of the ocean energy which might conceivably be made
available for service it is safe to calculate that the waves offer
between two and three thousand times as much opportunity for the
capture of natural power and its application to useful work as the
tides could ever present. In no other form is the energy of the wind
brought forward in so small a compass or in so concrete a form. A
steam-ship of 10,000 tons gross weight which rises and falls ten times
per minute through an average height of 3.3 feet is thereby subjected
to an influence equal to 22,400 horse-power. In this estimate the unit
of the horse-power which has been adopted is Watt's arbitrary standard
of "33,000 foot pounds per minute". The work done in raising the
vessel referred to is equal to ten horse-power multiplied by the
number of pounds in a ton, or, in other words, 22,400 horse-power, as
stated.
Wind-power, again, has been to a large extent neglected since the
advent of the steam-engine. The mightiest work carried out in any
European country in the early part of the present century was that
which the Dutch people most efficiently performed in the draining of
their reclaimed land by means of scores of windmills erected along
their seaboard. Even to the present day there are no examples of the
direct employment of the power of the wind which can be placed in
comparison with those still to be found on the coasts of Holland. But,
unfortunately for the last generation of windmill builders, the
intermittent character of the power to which they had to trust
completely condemned it when placed in competition with the handy and
always convenient steam-engine. The wind bloweth "where it listeth,"
but only at such times and seasons as it listeth, and its vagaries do
not suit an employer whose wages list is mounting up whether he has
his men fully occupied or not. The storage of power was the great
thing needful to enable the windmill to hold its own. The electrical
storage battery, compressed air, and other agencies which will be
referred to later on, have now supplied this want of the windmill
builder, but in the meantime his trade has been to a large extent
destroyed. For its revival there is no doubt that, as Lord Kelvin
remarked in the address already quoted, "the little thing wanted to
let the thing be done is cheap windmills."
This, however, leads to another part of the problem. The costliness of
the best modern patterns of windmill as now so extensively used,
particularly in America, is mainly due to the elaborate, and, on the
whole, successful attempts at minimising the objection of the
intermittent nature of the source of power. To put the matter in
another way, it may be said that lightness, and sensitiveness to the
slightest breeze, have had to be conjoined with an eminent degree of
safety in the severest gale, so that the most complicated
self-regulating mechanisms have been rendered absolutely imperative.
Once the principle of storage is applied, the whole of the conditions
in this respect are revolutionised. There is no need to attempt the
construction of wind-motors that shall run lightly in a soft zephyr of
only five or six miles an hour, and stability is the main desideratum
to be looked to.
The fixed windmill, which requires no swivel mechanism and no vane to
keep it up to the wind, is the cheapest and may be made the most
substantial of all the forms of wind-motor. In its rudimentary shape
this very elementary windmill resembles a four-bladed screw steam-ship
propeller. The wheel may be constructed by simply erecting a high
windlass with arms bolted to the barrel at each end, making the shape
of a rectangular cross. But those at one end are fixed in such
positions that when viewed from the side they bisect the angles made
by those at the other side. Sails of canvas or galvanised iron are
then fastened to the arms, the position of which is such that the
necessary obliquity to the line of the barrel is secured at once.
Looking at this elementary and at one time very popular form of
windmill, and asking ourselves what adaptation its general principle
is susceptible of in order that it may be usefully employed in
conjunction with a storage battery, we find, at the outset, that,
inasmuch as the electric generator requires a high speed, there is
every inducement to greatly lengthen the barrel and at the same time
to make the arms of the sails shorter, because short sails give in the
windmill the high rate of speed required.
We are confronted, in fact, with the same kind of problem which met
the constructors of turbine steam-engines designed for electric
lighting. The object was to get an initial speed which would be so
great as to admit of the coupling of the dynamo to the revolving shaft
of the turbine steam-motor, without the employment of too much
reducing gear. In the case of the wind-motor the eighteenth century
miller was compelled to make the arms of his mill of gigantic length,
so that, while the centre of the wind pressure on each arm was
travelling at somewhere near to the rate of the wind, the axis would
not be running too fast and the mill stones would never be grinding so
rapidly as to "set the _tems_--or the lighter parts of the corn--on
fire."
The dynamo for the generation of the electric current demands exactly
the opposite class of conditions. We may therefore surmise that the
windmill of the future, as constructed for the purposes of storing
power, will have a long barrel upon which will be set numerous very
short blades or sails. Reducing this again to its most convenient
form, it is plain that a spiral of sheet-metal wound round the barrel
will offer the most convenient type of structure for stability and
cheapness combined. At the end of this long barrel will be fixed the
dynamo, the armature of which is virtually a part of the barrel
itself, while the magnets are placed in convenient positions on the
supporting uprights. From the generating dynamo the current is
conveyed directly to the storage batteries, and these alone work the
electric motor, which, if desired, keeps continually in motion,
pumping, grinding, or driving any suitable class of machinery.
It is rather surprising to find how relatively small is the advantage
possessed by the vane-windmill over the fixed type in the matter of
continuity of working. During about two years the Author conducted a
series of experiments with the object of determining this point, the
fixed windmill being applied to work which rendered it a matter of
indifference in which way the wheel ran. With the prevailing winds
from the west it ran in one direction, and with those of next degree
of frequency, namely from the east, it turned in the reverse
direction. The mill, however, was effective although the breeze might
veer several points from either of the locations mentioned. It was
found that there were rather less than one-fourth of the points of the
compass, the winds from which would bring the wheel to a standstill or
cause it to swing ineffectively, but as these were the directions in
which the wind least frequently blew it might safely be reckoned that
not one-eighth of the possible working hours of a swivel-windmill were
really lost in the fixed machine.
With the type adapted to the working of a dynamo as already described,
it will, in most cases, be convenient to construct two spirals on
uprights set in three holes in the ground, forming lines at right
angles to each other, but both engaging, by suitable gearing, with the
electric current generator situated at the angle. This will be found
cheaper than to go to the expense of constructing the mill on a swivel
so that it may follow the direction of the wind. At the same time it
should be noticed that the adoption of the high speed wind-wheel,
consisting of some kind of spiral on a very long axis, may be made
effective for improving even the swivel windmill itself, so as to
adapt it for electric generation and conservation of power through the
medium of the storage battery. Supposing that a number of small
oblique sails be set upon an axis lying in the direction of the wind,
the popular conception of the result of such an arrangement is that
the foremost sails would render those behind it almost, if not
entirely, useless.
The analogy followed in reaching this conclusion is that of the sails
of a ship, but, as applied to wind-motors, it is quite misleading,
because not more than one-third or one-fourth of the energy of the
wind is expended upon the oblique sails of an ordinary wind-wheel.
Moreover, in the case of a number of such wheels set on a long axis,
one behind the other as described, the space within which the shelter
of the front sail is operative to keep the wind from driving the next
one is exceedingly minute.
The elasticity of the air and its frictional inertia when running in
the form of wind cause the current to proceed on its course after a
very slight check, which in point of time is momentary and in its
effects almost infinitesimal. This being the case, and the principal
expense attendant upon the construction of ordinary wind-engines being
due to the need for providing a large diameter of wind-wheel, with all
the attendant complications required to secure such a wheel from
risk, it is obvious that as soon as the long axis and the very short
sail, or the metallic spiral, have been generally introduced as
adjuncts to the dynamo storage battery, an era of cheaper wind-motors
will have been entered upon,--in fact, the "little want" of which Lord
Kelvin spoke in 1881 will have been supplied. The high speed which the
dynamo requires, and the more rapid rate at which windmills
constructed on this very economical principle must necessarily run,
both mark the two classes of apparatus as being eminently suited for
mutual assistance in future usefulness.
The anemometer of the "Robinson" type, having four little
hemispherical cups revolving horizontally, furnishes the first hint of
another principle of construction adapted to the generation of
electricity. Some years ago a professor in one of the Scottish
Universities set up a windmill which was simply an amplified
anemometer, and connected it with several of Faure's storage batteries
for the purpose of furnishing the electric light to his residence. His
report regarding his experience with this arrangement showed that the
results of the system were quite satisfactory.
In this particular type of natural motor the wind-wheel, of course, is
permanently set to run no matter from what direction the wind may be
blowing. Tests instituted with the object of determining the pressure
which the wind exerts on the cup of a "Robinson" anemometer have shown
that when the breeze blows into the concave side of the cup, its
effect is rather more than three times as strong as when it blows
against the convex side. At any given time the principal part of the
work done by a windmill constructed on this principle is being carried
out by one cup which has its concave side presented to the wind,
while, opposite to it, there is another cup travelling in the opposite
direction to that of the wind but having its convex side opposed.
The facts that practically only one sail of the mill is operative at
any given time, and that even the work which is done by this must be
diminished by nearly one-third owing to the opposing "pull" of the cup
at the opposite side, no doubt must detract from the merits of such a
wind-motor, judged simply on the basis of actual area of sail
employed. But when the matter of cost alone is taken as the standard,
the advantages are much more evenly balanced than they might at first
sight seem to be.
The cup-shaped sail may be greatly improved upon for power-generating
purposes by adopting a sail having a section not semicircular but
triangular in shape, and by extending its length in the vertical
direction to a very considerable extent. Practically this cheap and
efficient wind-motor then becomes a square or hexagonal upright axis
of fairly large section, to each side of which is secured a board or a
rigid sheet-metal sail projecting beyond the corners. The side of the
axis and the projecting portion of the sail then together form the
triangular section required.
For the sake of safety in time of storm, an opening may be left at the
apex of the angle which is closed by a door kept shut through the
tension of a spring. When the wind rises to such a speed as to
overbalance the force of the spring each door opens and lets the blast
pass through. One collateral advantage of this type of windmill is
that it may be made to act virtually as its own stand, the only
necessity in its erection being that it should have a collar fitting
round the topmost bearing, which collar is fastened by four strong
steel ropes to stakes securely set in the ground. The dynamo is then
placed at the lower bearing and protected from the weather by a metal
shield through which the shaft of the axis passes.
For pumping, and for other simple purposes apart from the use of
the dynamo, a ready application of this form of wind-engine with
a minimum of intricacy or expense may be worked out by setting the
lower bearing in a round tank of water kept in circular motion by
a set of small paddles working horizontally. Into the water a
vertically-working paddle-wheel dips, carrying on its shaft a crank
which directly drives the pump. This simple wind-motor is particularly
safe in a storm, because on attaining a high speed it merely "smashes"
the water in the tank.
Solar heat is one of the principal sources of the energy to be derived
from the wind. Several very determined and ingenious attempts at the
utilisation of the heat of sunshine for the driving of a motor have
been made during the past century. As a solution of a mechanical and
physical puzzle, the arrangement of a large reflector, with a small
steam-boiler at the focus of the heat rays thrown by it, is full of
interest. Yet, when a man like the late John Ericsson, who did so much
to improve the caloric engine, and the steam-ship as applied to
war-like purposes, meets with failure in the attempt to carry such an
idea to a commercially successful issue, there is at least _prima
facie_ evidence of some obstacle which places the proposed machine at
a disadvantage in competition with its rivals.
The solar engine, if generally introduced, would be found more
intermittent in its action than the windmill--excepting perhaps in a
very few localities where there is a cloudless sky throughout the
year. The windmill gathers up the power generated by the expansion of
the air in passing over long stretches of heated ground, while a solar
engine cannot command more of the sun's heat than that which falls
upon the reflector or condenser of the engine itself. The latter
machine may possibly have a place assigned to it in the industrial
economy of the future, but the sum total of the power which it will
furnish must always be an insignificant fraction.
The wave-power machine, when allied to electric transmission, will,
without doubt, supply in a cheap and convenient form a material
proportion of the energy required during the twentieth century for
industrial purposes. Easy and effective transmission is a _sine qua
non_ in this case, just as it is in the utilisation of waterfalls
situated far from the busy mart and factory. Hardly any natural source
of power presents so near an approach to constancy as the ocean
billows. Shakespeare takes as his emblem of perpetual motion the
dancing "waves o' th' sea".
But the ocean coasts--where alone natural wave-power is constant--are
exactly the localities at which, as a rule, it is the least
practicable to build up a manufacturing trade. Commerce needs smooth
water for the havens offered to its ships, and inasmuch as this
requirement is vastly more imperative during the early stages of
civilisation than cheap power, the drift of manufacturing centres has
been all towards the calm harbours and away from the ocean coasts. But
electrical transmission in this connection abolishes space, and can
bring to the service of man the power of the thundering wave just as
it can that of the roaring torrent or waterfall.
The simplest form of wave-motor may be suggested by the force exerted
by a ferry boat or dinghy tied up to a pier. The pull exerted by the
rope is equal to the inertia of the boat as it falls into the trough
of each wave successively, and the amount of strain involved in rough
weather may be estimated from the thickness of the rope that is
generally found necessary for the security of even very small craft
indeed. A similar suggestion is conveyed by the need for elaborate
"fenders" to break the force of the shock when a barge is lying
alongside of a steamer, or when any other vessel is ranging along a
pier or jetty.
A buoy of large size, moored in position at a convenient distance
from a rock-bound ocean coast, will supply the first idea of a
wave-motor on this primary principle as adapted for the generation of
power. On the cliff a high derrick is erected. Over a pulley or wheel
on the top of this there is passed a wire-rope cable fastened on the
seaward side to the buoy, and on the landward side to the machinery in
the engine-house. The whole arrangement in fact is very similar in
appearance to the "poppet-head" and surface buildings that may be seen
at any well-equipped mine. The difference in principle, of course, is
that while on a mine the engine-house is supplying power to the other
side of the derrick, the relations are reversed in the wave-motor, the
energy being passed from the sea across into the engine-house. The
reciprocating, or backward and forward, movement imparted to the cable
by the rising and falling of the buoy now requires to be converted
into a force exerted in one direction. In the steam-engine and in
other machines of similar type, the problem is simplified by the
uniform length of the stroke made by the piston, so that devices such
as the crank and eccentric circular discs are readily applicable to
the securing of a rotatory motion for a fly-wheel from a reciprocating
motion in the cylinders. In the application of wave-power provision
must be made for the utilisation of the force derived from movements
of _differing lengths_, as well as of _differing characters_, in the
force of impact. Every movement of the buoy which imparts motion to
the pulley on top of the derrick must be converted into an additional
impetus to a fly-wheel always running in the same direction.
The spur-wheel and ratchet, as at present largely used in machinery,
offer a rough and ready means of solving this problem, but two very
important improvements must be effected before full advantage can be
taken of the principle involved. In the first place it is obvious that
if a ratchet runs freely in one direction and only catches on the
tooth of the spur-wheel when it is drawn in the other, the power
developed and used is concentrated on one stroke, when it might, with
greater advantage, be divided between the two; and in the second place
the shock occasioned by the striking of the ratchet against the tooth
when it just misses catching one of the teeth and is then forced along
the whole length of the tooth gathering energy as it goes, must add
greatly to the wear and tear of the machinery and to the unevenness of
the running.
Taking the first of these difficulties into consideration it is
obvious that by means of a counterbalancing weight, about equal to
half that of the buoy, it is possible to cause the wave-power to
operate two ratchets, one doing work when the pull is to landwards and
the other when it is to seawards. Each, however, must be set to catch
the teeth of its own separate spur-wheel; and, inasmuch as the
direction of the motion in one case is different from what it is in
the other, it is necessary that, by means of an intervening toothed
wheel, the motion of one of these should be reversed before it is
communicated to the fly-wheel. The latter is thus driven always in the
same direction, both by the inward and by the outward stroke or pull
of the cable from the buoy.
Perhaps the most convenient development of the system is that in which
the spur-wheel is driven by two vertically pendant toothed bands,
resembling saws, and of sufficient length to provide for the greatest
possible amplitude of movement that could be imparted to them by the
motion of the buoy. The teeth are set to engage in those of the
spur-wheel, one band on each side, so that the effective stroke in one
case is downward, while in the other it is upward. These toothed bands
are drawn together at their lower ends by a spring, and they are also
kept under downward tension by weights or a powerful spring beneath.
The effect of this is that when both are drawn up and down the
spur-wheel goes round with a continuous motion, because at every
stroke the teeth of one band engage in the wheel and control it, while
those of the reversed one (at the other side) slip quite freely.
The shock occasioned by the blow of the ratchet on the spur-wheel, or
of one tooth upon another, may be reduced almost to vanishing point by
multiplying the number of ratchets or toothed bands, and placing the
effective ends, which engage in the teeth of the wheel successively,
one very slightly in advance of the other. In this way the machine is
so arranged that, no matter at what point the stroke imparted by the
movement of the buoy may be arrested, there is always one or other of
the ratchets or of the teeth which will fall into engagement with the
tooth of the spur-wheel, very close to its effective face, and thus
the momentum acquired by the one part before it impinges upon the
other becomes comparatively small.
The limit to which it may be practicable to multiply ratchets or
toothed bands will, of course, depend upon the thickness of the
spur-wheel, and when this latter has been greatly enlarged, with the
object of providing for this feature, it becomes virtually a steel
drum having bevelled steps accurately cut longitudinally upon its
periphery.
The masts of a ship tend to assume a position at right angles to the
water-line. When the waves catch the vessel on the beam the greatest
degree of pendulous swing is brought about in a series of waves so
timed, and of such a length, that the duration of the swing coincides
with the period required for one wave to succeed another. The
increasing slope of the ship's decks, due to the inertia of this
continuous rhythmical motion, often amounts to far more than the angle
made by the declivity of the wave as compared with the sea level; and
it is, of course, a source of serious danger in the eyes of the
mariner.
But, for the purposes of the mechanician who desires to secure power
from the waves, the problem is not how to avoid a pendulous motion but
how to increase it. For each locality in which any large wave-power
plant of machinery is to be installed, it will therefore be advisable
to study the characteristic length of the wave, which, as observation
has proved, is shorter in confined seas than in those fully open to
the ocean. It is advisable then to make the beam width of the buoy,
no matter how it may be turned, of such a length that when one side is
well in the trough of a wave the other must be not far from the crest.
Practically the best design for such a floating power-generator will
be one in which four buoys are placed, each of them at the end of one
arm of a cross which has been braced up very firmly. From the angle of
intersection projects a vertical mast, also firmly held by stays or
guys. The whole must be anchored to the bottom of the sea by
attachment to a large cemented block or other heavy weight having a
ring let into it, from which is attached a chain of a few links
connecting with an upright beam. It is the continuation of the latter
above sea-level which forms the mast. On this beam the framework of
the buoy must be free to move up and down.
At first sight it might seem as if this arrangement rendered nugatory
the attempt to take advantage of the rise and fall of the buoy; but it
is not so when the relations of the four buoys to one another are
considered. Although the frame is free to move up and down upon the
uprising shaft, still its inclination to the vertical is determined by
the direction of the line drawn from a buoy in the trough of a wave to
one on the crest. In order to facilitate the free movement, and to
render the rocking effect more accurate and free from vibration, sets
of wheels running on rails fixed to the beam are of considerable
advantage.
The rise and fall of the tides render necessary the adoption of some
such compensating device as that which has been indicated. Of course
it would be possible to provide for utilising the force generated by a
buoy simply moored direct to a ring at the bottom by means of a common
chain cable; but this latter would require to be of a length
sufficient to provide for the highest possible wave on the top of the
highest tide. Then, again, the loose chain at low tide would permit
the buoy to drift abroad within a very considerable area of sea
surface, and in order to take advantage of the rise and fall on each
wave it would be essential to provide at the derrick on the shore end
of the wave-power plant very long toothed bands or equivalent devices
on a similarly enlarged scale.
By providing three or four chains and moorings, meeting in a centre at
the buoy itself but fastened to rings secured to weights at the bottom
at a considerable distance apart, the lateral movement might, no
doubt, be minimised; and for very simple installations this plan,
associated with the device of taking a cable from the buoy and turning
it several times round a drum on shore, could be used to furnish a
convenient source of cheap power. The drum may carry a crank and
shaft, which works the spur-wheel and toothed bands as already
described, so that no matter at what stage in the revolution of the
drum an upward or downward stroke may be stopped, the motion will
still be communicated in a continuous rotary form to the fly-wheel.
But the beam and sliding frame, with buoys, give the best practical
results, especially for large installations. It is in some instances
advisable, especially where the depth of the water at a convenient
distance from the shore is very considerable, not to provide a single
beam reaching the whole distance to the bottom, but to anchor an
air-tight tank below the surface and well beneath the depth at which
wave disturbance is ever felt. From this submerged tank, which
approximately keeps a steady position in all tides and weathers, the
upward beam is attached by a ring just as would be done if the tank
itself constituted the bottom.
One main reason for this arrangement is that the resistance of the
beam to the water as it rocks backwards and forwards wastes to some
extent the power generated by the force of the waves; and the greater
the length of the beam, the longer must be the distance through which
it has to travel when the buoys draw it into positions vertical to
that of the framework. A thin steel pipe offers less resistance than a
wooden beam of equal strength, besides facilitating the use of a
simple device for enabling the frame and buoys to slide easily up and
down.
The generally fatal defect of those inventions which have been
designed in the past with the object of utilising wave-power has
arisen from the mistake of placing too much of the machinery in the
sea. The device of erecting in the water an adjustable reservoir to
catch the wave crests and to use the power derived from them as the
water escaped through a water-wheel was patented in 1869. Nearly
twenty years later another scheme was brought out depending upon the
working of a large pump fixed far under the surface, and connected
with the shore so that, when operated by the rising and falling of
floats upon the waves, it would drive a supply of water into an
elevated reservoir on shore, from which, on escaping down the cliff,
the pressure of the water would be utilised to work a turbine.
Earlier devices included the building of a mill upon a rocking barge,
having weights and pulleys adjusted to run the machinery on board; and
also a revolving float so constructed that each successive wave would
turn one portion, but the latter would then be held firm by a toothed
wheel and ratchet until another impulse would be given to it in the
same direction. This plan included certain elements of the simple
system already described; but it is obvious that some of its floating
parts might with advantage have been removed to the shore end, where
they would not only be available for ready inspection and adjustment,
but also be out of harm's way in rough weather.
Different wave-lengths, as already explained, correspond to various
periods in the pendulous swing of floating bodies. Examples have been
cited by Mr. Vaughan Cornish, M. Sc., in _Knowledge_, 2nd March, 1896,
as follows: "A wave-length of fifty feet corresponds to a period of
two and a half seconds, while one of 310 feet corresponds to five and
a half seconds. It is mentioned that the swing of the steam-ship
_Great Eastern_ took six seconds." Other authorities state that during
a storm in the Atlantic the velocity of the wave was determined to be
thirty-two miles an hour, and that nine or ten waves were included in
each mile; thus about five would pass in each minute. But in average
weather the number of waves to the mile is considerably larger, say,
from fifteen to twenty to the mile; and in nearly calm days about
double those numbers.
One interesting fact, which gives to wave-power a peculiarly enhanced
value as a source of stored wind-power, is that the surface of the
ocean--wild as it may at times appear--is not moved by such extremes
of agitation as the atmosphere. In a calm it is never so inertly
still, and in a storm it is never so far beyond the normal condition
in its agitation as is the wind. The ocean surface to some extent
operates as the governor of a steam-engine, checking an excess in
either direction. In very moderate weather the number of waves to the
mile is greatly increased, while their speed is not very much
diminished. Indeed the rate at which they travel may even be
increased.
This latter phenomenon generally occurs when long ocean rollers pass
out of a region of high wind into one of relative calm, the energy
remaining for a long time comparatively constant by reason of the
multiplication of short, low waves created out of long, high ones. On
all ocean coasts the normal condition of the surface is governed by
this law, and it follows that, no matter what the local weather may be
at any given time, there is always plenty of power available.
An attempt was made by M. C. Antoine, after a long series of
observations, to establish a general relation between the speed of the
wind and that of the waves caused by it, the formulae being published
in the _Revue Nautique et Coloniale_ in 1879. The rule may be taken as
correct within certain limits, although in calm weather, when the
condition of the ocean surface is almost entirely ruled by distant
disturbances, it has but little relevancy. Approximately, the velocity
of wave transmission is seven times the fourth root of the wind-speed;
so that when the latter is a brisk breeze of sixteen miles an hour the
waves will be travelling fourteen miles an hour, or very nearly as
fast as the wind. When, on the other hand, a light breeze of nine
miles an hour is driving the waves, the latter, according to the
formula, should run about twelve and a half miles an hour; but, in
point of fact, the influence of more distant commotion nearly always
interferes with this result.
As a matter of experience, the waves on an ocean coast are usually
running faster than the wind, and, being so much more numerous in
calm than they are in rough weather, they maintain comparatively a
uniform sum total of energy. It is obvious that, so far as practical
purposes are concerned, three waves of an available height of three
feet each are as effective as one of nine feet. If the state of the
weather be such that the average wave length is 176 feet there will be
exactly thirty waves to the mile, and if the speed be twelve miles an
hour--that is to say, if an expanse of twelve miles of waves pass a
given point hourly--then 360 waves will pass every sixty minutes, or
six every minute. In the wave-power plant as described, each buoy of
one hundred tons displacement when raised and depressed, say, three
feet by every wave will thus be capable of giving power equal to three
times 600, or 1,800 foot-tons per minute.
The unit of nominal horse-power being 33,000 foot-pounds or about
fifteen foot-tons per minute, it is evident that each buoy, at its
maximum, would be capable of giving about 120 horse-power. Supposing
that half of the possible energy were exerted at the forward and half
at the backward stroke and that each buoy were always in position to
exert its full power upon the uprising shaft without deduction, the
total effective duty of a machine such as has been described would be
480 horse-power. In practice, however, the available duty would
probably, according to minor circumstances, be rather more or rather
less than 300 horse-power.