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 sh.o.r.e 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, a.s.sociated with the device of taking a cable from the buoy and turning it several times round a drum on sh.o.r.e, 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 sh.o.r.e 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 const.i.tuted 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 sh.o.r.e 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 sh.o.r.e, 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 sh.o.r.e 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 pa.s.s 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 pa.s.s 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 pa.s.s a given point hourly--then 360 waves will pa.s.s 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 circ.u.mstances, be rather more or rather less than 300 horse-power.
CHAPTER III.
STORAGE OF POWER.
The three princ.i.p.al forms of stored power which are now in sight above the horizon of the industrial outlook are the electric storage battery, compressed air, and calcium-carbide. The first of these has come largely into use owing to the demand for a regulated and stored supply of electricity available for lighting purposes. Indeed the storage battery has practically rendered safe the wide introduction of electric lighting, because a number of cells, when once charged, are always available as a reserve in case of any failure in the power or in the generators at any central station; and also because, by means of the storage cells or "acc.u.mulators," the amount of available electrical energy can be subdivided into different and subordinate circuits, thus obviating the necessity for the employment of currents of very high voltage and eluding the only imperfectly-solved problem of dividing a current traversing a wire as conveniently as lighting gas is divided by taking small pipes off from the gas mains.
Compressed air for the storage of power has. .h.i.therto been best appreciated in mining operations, one of the main reasons for this being that the liberated air itself--apart from the power which it conveyed and stored--has been so great a boon to the miner working in ill-ventilated stopes and drives. The cooling effects of the expansion, after close compression, are also very grateful to men labouring hard at very great depths, where the heat from the country rock would become, in the absence of such artificial refrigeration, almost overpowering. For underground railway traffic exactly the same recommendations have, at one period during the fourth quarter of the nineteenth century, given an advent.i.tious stimulus to the use of compressed air.
Yet it is now undoubted that, even in deep mining, the engineer"s best policy is to adopt different methods for the conveyance and storage of power on the one hand, and for the ventilation of the workings on the other. Few temptations are more illusory in the course of industrial progress than those presented by that cla.s.s of inventions which aim at "killing two birds with one stone". If one object be successfully accomplished it almost invariably happens that the other is indifferently carried out; but the most frequent result is that both of them suffer in the attempt to adapt machinery to irreconcilable purposes.
The electric rock-drill is now winning its way into the mines which are ventilated with comparative ease as well as into those which are more difficult to supply with air. It is plain, therefore, that on its merits as a conveyer and storer of power the electric current is preferable to compressed air. The heat that is generated and then dissipated in the compression of any gas for such a purpose represents a very serious loss of power; and it is altogether an insufficient excuse to point to the compensation of coolness being secured from the expansion. Fans driven by electric motors already offer a better solution of the ventilation difficulty, and the advantages on this side are certain to increase rather than to diminish during the next few years.
The electric rock-drill, which can already hold its own with that driven by compressed air, is therefore bound to gain ground in the future. This is a type and indication of what will happen all along the industrial line, the electric current taking the place of the majority of other means adopted for the transmission of power. Even in workshops--where it is important to have a wide distribution of power and each man must be able to turn on a supply of it to his bench at any moment--shafting is being displaced by electric cables for the conveyance of power to numerous small motors.
The loss of power in this system has already been reduced to less than that which occurs with shafting, unless under the most favourable circ.u.mstances; and in places where the works are necessarily distributed over a considerable area the advantage is so p.r.o.nounced that hardly any factories of that kind will be erected ten years hence without resort being had to electricity, and small motors as the means of distributing the requisite supplies of power to the spots where they are needed. It was a significant fact that at the Paris Exposition of 1900 the electric system of distribution was adopted.
In regard to compressed air, however, it seems practically certain that, notwithstanding its inferiority to electric storage of power, it is applicable to so many kinds of small and cheap installations that, on the whole, its area of usefulness, instead of being restricted, will be largely increased in the near future. There will be an advance all along the line; and although electric storage will far outstrip compressed air for the purposes of the large manufacturer, the air reservoir will prove highly useful in isolated situations, and particularly for agricultural work.
For example, as an adjunct to the ordinary rural windmill for pumping water, it will prove much more handy and effective than the system at present in vogue of keeping large tanks on hand for the purpose of ensuring a supply of water during periods of calm weather. Regarding a tank of water elevated above the ground and filled from a well as representing so much stored energy, and also comparing this with an equal bulk of air compressed to about 300 pounds pressure to the square inch, it would be easy to show that--unless the water has been pumped from a very deep well--the power which its elevation indicates must be only a small fraction of that enclosed in the air reservoir.
It will be one great point in favour of compressed air, as a form of stored energy for the special purpose of pumping, that by making a continuous small flow of air take the place of the water at the lowest level in the upward pipe, it is possible to cause it to do the pumping without the intervention of any motor.
One means of effecting this may be simply indicated. The air under pressure is admitted from a very small air pipe and the bubbles, as they rise, fill the hollow of an inverted iron cup rising and falling on a bearing like a hinge. Above and beneath the chamber containing this cup are valves opening upwards and similar to those of an ordinary force or suction pump. The cup must be weighted with adjustable weights so that it will not rise until quite full of air.
When that point is reached the stroke is completed, the air having driven upwards a quant.i.ty of water of equal bulk with itself, and, as the cup falls again by its own weight, the vacuum caused by the air escaping upwards through the pipe is filled by an inrush of water through the lower valve. The function of the upper valve, at that time, is to keep the water in the pipe from falling when the pressure on the column is removed. The expansive power of the air enables it to do more lifting at the upper than at the lower level, so that a larger diameter of pipe can be used at the former place.
Cheap motors working on the same principle--that is to say through the upward escape of compressed air, gas or vapour filling a cup and operating it by its buoyancy, or turning a wheel in a similar manner--will doubtless be a feature in the machine work of the future; and for motors of this description it is obvious that compressed air will be very useful as the form of power-storage. Excepting under very special conditions, steam is not available for such a purpose, seeing that it condenses long before it has risen any material distance in a column of cold water.
"The present acc.u.mulator," remarked Prof. Sylva.n.u.s P. Thompson in the year 1881, referring to the Faure storage batteries then in use, "probably bears as much resemblance to the future acc.u.mulator as a gla.s.s bell-jar used in chemical experiments for holding gas does to the gasometer of a city gasworks, or James Watt"s first model steam-engine does to the engines of an Atlantic steamer." When Faure, having in 1880 improved upon the storage battery of Plante, sent his four-cell battery from Paris to Glasgow, carrying in it stored electrical energy, it was found to contain power equal to close upon a million foot-pounds, which is about the work done by a horse-power during the s.p.a.ce of half an hour. This battery weighed very nearly 75 lb. It nevertheless represented an immense forward step in the problem of compressing a given quant.i.ty of potential power into a small weight of acc.u.mulator.
The progress made during less than twenty years to the end of the century may be estimated from the conditions laid down by the Automobile Club of Paris for the compet.i.tive test of acc.u.mulators applicable to auto-car purposes in 1899. It was stipulated that five cells, weighing in all 244 lb., should give out 120 ampere-hours of electric intensity; and that at the conclusion of the test there should remain a voltage of 17 volt per cell.
Very great improvements in the construction of electric acc.u.mulators are to be looked for in the near future. Hitherto the average duration of the life of a storage cell has not been more than about two years; and where impurities have been present in the sulphuric acid, or in the litharge or "minium" employed, the term of durability has been still further shortened. It must be remembered that while the princ.i.p.al chemical and electrical action in the cell is a circular one,--that is to say, the plates and liquids get back to the original condition from which they started when beginning work in a given period,--there is also a progressive minor action depending upon the impurities that may be present. Such a reagent, for instance, as nitric acid has an extremely injurious effect upon the plates.
During the first decade after Plante and Faure had made their original discoveries, the main drawback to the advancement of the electric acc.u.mulator for the storage of power owed its existence to the lack of precise knowledge, among those placed in charge of storage batteries, as to the destructive effects of impurities in the cells. It is, however, now the rule that all acids and all samples of water used for the purpose must be carefully tested before adoption, and this practice, in itself, has greatly prolonged the average life of the acc.u.mulator cell.
The era of the large electric acc.u.mulator of the kind foreshadowed by Prof. Sylva.n.u.s P. Thompson has not yet arrived, the simple reason being that electric power storage--apart from the special purposes of the subdivision and transmission for lighting--has not yet been tried on a large scale. For the regulation and graduation of power it is exceedingly handy to be able to "switch-on" a number of small acc.u.mulator cells for any particular purpose; and, of course, the degree of control held in the hands of the engineer must depend largely on the smallness of each individual cell, and the number which he has at command. This fact of itself tends to keep down the size of the storage cell which is most popular.
But when power storage by means of the electric acc.u.mulator really begins in earnest the cells will attain to what would at present be regarded as mammoth proportions; and the special purpose aimed at in each instance of power installation will be the securing of continuity in the working of a machine depending upon some intermittent natural force. Windmills are especially marked out as the engines which will be used to put electrical energy into the acc.u.mulators. From these latter again the power will be given out and conveyed to a distance continuously.
High ridges and eminences of all kinds will in the future be selected as the sites of wind-power and acc.u.mulator plants. In the eighteenth century, when the corn from the wheat-field required to be ground into flour by the agency of wind-power, it was customary to build the mill on the top of some high hill and to cart all the material laboriously to the eminence. In the installations of the future the power will be brought to the material rather than the material to the power. From the ranges or mountain peaks, and also from smaller hills, will radiate electrical power-nerves branching out into network on the plains and supplying power for almost every purpose to which man applies physical force or electro-chemical energy.
The gas-engine during the twentieth century will vigorously dispute the field against electrical storage; and its success in the struggle--so far as regards its own particular province--will be enhanced owing to the fact that, in some respects, it will be able to command the services of electricity as its handmaid. Gas-engines are already very largely used as the actuators of electric lighting machinery. But in the developments which are now foreshadowed by the advent of acetylene gas the relation will be reversed. In other words, the gas-engine will owe its supply of cheap fuel to the electric current derived at small expense from natural sources of power.
Calcium carbide, by means of which acetylene gas is obtained as a product from water, becomes in this view stored power. The marvellously cheap "water-gas" which is made through a jet of steam impinging upon incandescent carbons or upon other suitable glowing hot materials will, no doubt, for a long time command the market after the date at which coal-gas for the generation of power has been partially superseded.
But it seems exceedingly probable that a compromise will ultimately be effected between the methods adopted for making water-gas and calcium carbide respectively, the electric current being employed to keep the carbons incandescent. When power is to be sold in concrete form it will be made up as calcium carbide, so that it can be conveyed to any place where it is required without the a.s.sistance of either pipes or wires. But when the laying of the latter is practicable--as it will be in the majority of instances--the gas for an engine will be obtainable without the need for forcing lime to combine with carbon as in calcium carbide.
Petroleum oil is estimated to supply power at just one-third the price of acetylene gas made with calcium carbide at a price of 20 per ton.
This calculation was drawn up before the occurrence of the material rise in the price of "petrol" in the last year of the nineteenth century; while, concurrently, the price of calcium carbide was falling. A similar process will, on the average, be maintained throughout each decade; and, as larger plants, with cheaper natural sources of energy, are brought into requisition, the costs of power, as obtained from oil and from acetylene gas, will more and more closely approximate, until, in course of time, they will be about equal; after which, no doubt, the relative positions will be reversed, although not perhaps in the same ratio. Time is all on the side of the agent which depends for its cheapness of production on the utilisation of any natural source of power which is free of all cost save interest, wear and tear, and supervision.
Even the steam-engine itself is not exempt from the operation of the general law placing the growing advantage on the side of power that is obtainable gratis. One cubic inch of water converted into steam and at boiling point will raise a ton weight to the height of one foot; and the quant.i.ty of coal of good quality needed for the transformation of the water is very small. One pound of good coal will evaporate nine pounds of water, equal to about 250 cubic inches, this doing 250 foot-tons of work. But Niagara performs the same amount of work at infinitely less cost. However small any quant.i.ty may be, its ratio to nothing is infinity.
It has been the custom during the nineteenth century to inst.i.tute comparisons between the marvellous economy of steam power and the expensive wastefulness of human muscular effort. For instance, the full day"s work of an Eastern porter, specially trained to carry heavy weights, will generally amount to the removal of a load of from three to five hundred-weight for a distance of one mile; but such a labourer in the course of a long day has only expended as much power as would be stored up in about five ounces of coal.
Still the fact remains that one of the greatest problems of the future is that which concerns the reduction in the cost of power. Hundreds of millions of the human race pa.s.s lives of a kind of dull monotonous toil which develops only the muscular, at the expense of the higher, faculties of the body; they are almost entirely cut off from social intercourse with their fellow-men, and they sink prematurely into decrepitude simply by reason of the lack of a cheap and abundant supply of mechanical power, ready at hand wherever it is wanted.
Scores of "enterprises of great pith and moment" in the industrial advancement of the world have to be abandoned by reason of the same lack. In mining, in agriculture, in transport and in manufacture the thing that is needful to convert the "human machine" into a more or less intelligent brainworker is cheaper power. All the technical education in the world will not avail to raise the labourer in the intellectual scale if his daily work be only such as a horse or an engine might perform.
The transmission of power through the medium of the electric current will naturally attain its first great development in the neighbourhoods of large waterfalls such as Niagara. When the manufacturers within a short radius of the source of power in each case have begun to fully reap the benefit due to cheap power, compet.i.tion will a.s.sert itself in many different ways. The values of real property will rise, and population will tend to become congested within the localities" served.
It will be found, however, that facilities for shipment will to a large extent perpetuate the advantage at present held by manufactories situated on ports and harbours; and this, of course, will apply with peculiar force to the cases of articles of considerable bulk. Where a very great deal of power is needed for the making of an article or material of comparatively small weight and bulk proportioned to its value--such for instance as calcium carbide or aluminium--the immediate vicinity of the source of natural power will offer superlative inducements. But an immense number of things lie between the domains of these two cla.s.ses, and for the economical manufacture of these it is imperative that both cheap power and low wharf.a.ge rates should be obtainable.
An increasingly intense demand must thus spring up for systems of long distance transmission, and very high voltage will be adopted as the means of diminishing the loss of power due to leakage from the cables.
Similarly the "polyphase" system--which is eminently adapted to installations of the nature indicated--must demand increasing attention.
Taking a concrete example, mention may be made of the effects to be expected from the proposed scheme for diverting some of the headwaters of the Tay and its lakes from the eastern to the western sh.o.r.es of Scotland and establishing at Loch Leven--the western inlet, not the inland lake of that name--a seaport town devoted to manufacturing purposes requiring very cheap supplies of power. It is obvious that the owners of mills in and around Glasgow, and only forty or fifty miles distant, will make the most strenuous exertions to enable them to secure a similar advantage.
It is already claimed that with the use of currents of high voltage for carrying the power, and "step-down transformers" converting these into a suitable medium for the driving of machinery, a fairly economical transmission can be ensured along a distance of 100 miles.
It therefore seems plain that the natural forces derived from such sources as waterfalls can safely be reckoned upon as friends rather than as foes of the vested interests of all the great cities of the United Kingdom.
The possibilities of long distance transmission are greatly enhanced by the very recent discovery that a cable carrying a current of high voltage can be most effectually insulated by encasing it in the midst of a tube filled with wet sawdust and kept at a low temperature, preferably at the freezing point of water.