WHAT sources of energy are available to mankind, and how do we stand if we attempt roughly to take stock? In approaching the consideration of this subject the first thing which strikes us is the enormous energy in nature which is unavailable to us, at least at present. The Energy of the' Earth. Consider, for instance, the energy involved in the earth 's motion, both orbital and rotational. The earth flies along in its orbit with a velocity of about twenty miles per second, or 1,200 times that of an express train. Also it rotates in twenty-four hours. It has, therefore, energy of rotation measured by one half I w, where I is its moment of inertia and w its angular velocity and also Introductory Lecture to the Engineering Glasses at University College, London. energy of motion represented by one half MV2 Where M is its mass and V its orbital velocity. The moment of inertia of a sphere round an axis is equal to 2/5 of its mass multiplied by the square of its radius. Taking the foot, pound and second as units, it is easily found that the rotational energy of the earth is 3/16 103” foot-pounds, or a hundred thousand million billion horse-power hours (= 102s horse-power hours). But the orbital energy or energy of motion in its orbit is ten thousand times greater (= 1027 horse-power hours). The earth, therefore, is a great fly-wheel which, in virtue of its diurnal rotation, has this enormous energy stored up in it. Suppose we could in some way or other slow down its rotation so as to make the day just five minutes longer, and give us all five minutes more in bed. I do.not.thinkjt would give rise to any great inconvenience. This would decrease the earth's angular velocity by about 1/3 of 1 per cent, and decrease the angular energy by about 2/3 of 1 per cent, or say by 1/150 part. If then we could capture and store up the difference in the rotational energy in the two cases, it would give us about six million billion horse-power hours, or a billion horse-power for seventy thousand years. The energy we can obtain by the combustion of ail the one thousand million tons of coal at present raised per year, sinks into siginficance compared with the enormous energy which would be set free by an almost imperceptible lengthening of the earth's diurnal time of rotation. As matter of fact the tidal friction due to motion of the tide-wave round the earth is reducing its speed of rotation, bu t only by a small fraction of a second per century. Since our earth is one of the smaller planets, these figuresgive us some faint conception of the colossal energy associated with the axial and orbital rotations of all the planets, and when we add to that the energy of axial and progressive motion of the sun, the mass of which is far greater than that of all the planets taken together, we see that the kinetic energy which is represented by the motions of the solar system is something far beyond the grasp cf our minds to appreciate, even although it may be set down easily in arithmetic form. Solar Radiation. Another case of apparently illimitable power which is captured only in small degree is that of the solar radiation. - Nevertheless it is the original source of nearly all our available energy. A very important constant in cosmical physics is the so-called “solar constant.” This means the heat delivered per minute by solar radiation to one square centimeter of surface held normally to the sun's rays and corrected for atmospheric absorption. The best results give two gramme calories per minute per square centimeter, or nearly one million gramme calories per year per square centimeter. In other words, on each square centimeter 8.4 joules or 6.3 foot-pounds of energy would be expended per minute. This is equal to 1.6 horse-power per square yard, or 7,000 horse-power per acre. 'As a matter of fact, owing to atmospheric absorption and clouds and the varying inclination of the sun's rays, experiment shows that for Central Europe only about 50,000-gramme calories are received per year per square centimeter. If there were no absorption the earth would receive 250 X 1012 horse-power. Prof. Very has calculated that 100 square meters receiving per annum solar energy, actually receive as follows: Central Europe, 4 to 6 X 10° kilowatt-hours, or B.T.U. North, U. S. A., 5 to 7 X 106 kilowatt-hours, or B.T.U. S. W., U. S. A., 10 to 15 X 106 kilowatt-hours, or B.T.U. The earth receives only two thousand millionth part of all the energy sent out from the sun. Hence it is easy to show that tho total power emitted by the sun is one half billion billion horse-power. Hence the actual amount absorbed at the earth's surface is only about 4 to 10 per cent of the solar delivered. From the value of the solar constant it can be shown that the solar radiation is equivalent to 80,000 horse-power per square yard of the sun's surface. If, therefore, the sun were a furnace burning coal it would have to be supplied with seven tons of coal per square yard per hour to produce by complete combustion the output of energy as regards mere quantity of heat, though it could not produce the actual solar temperature. All but an insignificant fraction of this enormous energy output ever reaches the earth. Of that fraction which does reach it all but an extremely small part appears to be wasted. Nature seems to be inexpressibly extravagant in scattering energy through space or storing it up in inaccessible places.but the amount of it which we can actually control, transform and utilize, is remarkably small. Nevertheless, out of that small fraction of solar energy we have received or do receive nearly all our present energy stores have been produced. If, however, we endeavor to classify all those sources which either are available now or perhaps may be available to mankind in consequence of future advances in scientific -knowledge, we find we can arrange them in four great classes as follows: 1. Food-stuffs and all materials which are combustible; for example, wood, coal, peat, oil, natural gas and such organic combustibles as oils and alcohols of various •kinds which can be prepared from plants. 2. Sources of kinetic energy such as wind, waterfalls and rivers, or moving water generally, and sources of potential energy, such as lakes at a level higher than the sea. Tidal energy may be included in this class, though really derived from the earth's energy of rotation. 3. Solar radiation or direct-radiant heat. Volcanic or subterranean heat. 4. Energy stored. up in atomic structures and released in radio-active changes. The utilization of this is only remotely possible. There may be some other small sources of energy due to oxidizable or combustible materials, such as sulphur, native metals, bitumen deposits, which have small utility. The energy sources in Classes 3 and 4 have scarcely yet been tapped but as we shall see are enormous in amount. The Energy of Food and of Combustibles. The energy sources in Class I depend essentially upon the presence of oxygen in our atmosphere. If all the oxygen were suddenly to escape from the earth's atmosphere these sources ..would cease to represent available energy. As we chiefly require our energy for creating kinetic energy in the form of moving bodies or to expend it in producing potential energy as in lifting up masses of -matter in building, we have in addition to possessing various sources of energy to provide ourselves with suitable transforming devices which shall create the necessary transformations of energy. Such a device is usually called an engine. The oldest kind of engine employed byman is man himself, or some other man in the form of a slave, and the subsequently domesticated animals, such as the horse, ox, elephant, camel or dog. The animal engine is supplied with potential energy in the form of food and air, and wiil then transform some of it into external mechanical work by lifting weights or setting in motion heavy bodies. Hence an initial source of energy is found in the various food materials. Every kind of food-stuff has a certain energy value which may be determined by burning it completely in a calorimeter. The result may be expressed either in units of heat, such as the calorie large or small, the British thermal unit, or better still in foot-pounds per pound of edible. It is convenient to remember that the so-called large calorie or energy required to raise 1 kilogramme of water 1 deg. Cent; at or near 15 deg. Cent., is equal to 3,085 or nearly 3,100 foot-pounds. The human body can, however, only appropriate a portion of the total energy value of any given food. It is stated in works on dietetics that 1 pound of beef eaten by a man supplies his system with energy equal to about 3,000,000 foot-pounds, 1 pound of white bread with 3,600,000 foot-pounds, 1 pound of cheese with 5,700,000 foot-pounds. An ordinary helping of meat (say beef) and potatoes represents an energy donation of about 1,000,000 foot-pounds given to the body. A full-grown man doing hard muscular work requires from 25 to 40 ounces of food per day, according to climate and habit, reckoned dry, apart from the necessary water which is present in a considerable quantity in the food as eaten. This food must be taken partly in the form of protein, partly as fat, and partly as carbohydrates, and the total energy extractable from this twenty-five ounces by the human body is equivalent to about 3,000 large calories, or from nine to ten million foot-pounds. This energy is partly used in performing the internal work of - the body and in supplying the heat lost and latent heat of vaporization and partly can be recovered as external work. It is doubtful whether any man can continuously perform muscular work equal to 1 horse-power hour per day, or 2,000,000 foot-pounds of mechanical work per diem. Probably not, as a rule, half or a quarter of one horse-power hour, or say 500,000 footpounds can be done as a regular thing. This is equivalent to lifting his own, weight, say 140 pounds (3,571 feet high), say to the top of Ben Nevis. It is estimated thus about 6,000,000 foot-pounds are expended each day in keeping the circulation, respiration and digestive organs in motion. Also that if a man is called upon to do, say 700,000 foot-pounds of external mechanical work, the body must provide in addition another 3,500,000 footpounds, which is thrown off as heat due to the exertion. Hence apart from the internal work required to keep the bodily machine going. the efficiency for external work is about 20 per cent, which is more than the steam engine but less than that of the best internal combustion engines. The human body is, in fact, an internal combustion engine which burns (not oil) but food of some kind more'or less imperfectly. But there is a steady stand-by energy consumption of some five or six million footpounds. Nevertheless this human engine in the form of slave labor was for long ages the only engine, in conjunction with similar animal engines, of transforming the energy of combustion of food-stuffs into mechanical” work of various kinds, and we may say that even now it is one of the most common forms of engine. It was supplemented at best by a few rudimentary forms of wind engine or windmill, and watermill or engine, in performing such work as pumping, grinding corn, or raising weights. The Steam Engine. The steam engine and boiler are far from being an efficient means of transforming the poten tial energy of coal and air into kinetic-energy of motion of masses. A pound of good coal, when completely burnt, gives heat equal to about fourteen thousand British thermal units, equivalent nearly to ten or eleven million foot-pounds. The best engines and boilers do not enable us to obtain from it much more than one or one and one half million footpounds of external mechanical work. Part of this inefficiency is due to the essential properties of the working fluid, viz., steam, and to -its relatively high condensing temperature, which in accordance with the second law of thermodynamics puts a limit on the fraction of the heat taken from the source which we can convert into work. The other cause is the unavoidable and large heat losses owing to the surface exposed by boiler, steam pipes and cylinders, and losses in products of combustion. Then further there are - special disadvantages or difficulties involved in handling and storing the bulky dirty combustible coal, which at most has a total calorific energy of 28TJ horse-power hours per cubic foot of coal in the lump, and not mbre than 30 to 40 horse-power hours of this can be extracted by present methods in mechanical form. Hence, although the steam engine held sway as the chief heat engine of the nineteenth century, the invention of the internal combustion engine in which the fuel, gaseous or liquid, is burnt in the engine cylinder or closely adjacent chamber and used to create an explosion, has been the beginning of a new era. The Internal Combustion Engine. The modern internal combustion engine is more efficient as a thermo-mechanical machine than the steam engine, transforming a greater fraction of the total calorific energy of the fuel into mechanical energy. As much as 25 to perhaps 30 per cent of the whole energy of the fuel is yielded as mechanical work even in comparatively small engines. An interesting event in this con- ' nection was the invention of the Diesel engine, in which all necessity for electric ignition of the explosive mixture of air and oil vapor is obviated, and also the use of a highly volatile oil or spirit. Coal ' Distillates as a Source of Power. For those countries like Norway, Switzerland and Canada, where coal is dear and waterfalls numerous, the question of competition with coal does not arise, but for Great Britain the basis of comparison is the cost of power obtained as at present by the combustion of raw coal. The important question is, therefore, whether we can with advantage gradually substitute for the steam -engine and boiler burning raw coal in the furnace the employment of internal combustion engines using oil distilled from coal or else some form of producer-gas, or combinations of oil, gas and steam engines, consistently with capital outlay on plant which shall ' enable us to obtain more energy from a ton of coal than we do at present, and with accompanying financial advantage. A letter recently appeared in The Times' Engineering Supplement, from Mr. Gilbert R. Redgrave, stating that distillation of coal at a lower temperature than is usual with coke ovens will reduce the quantity of gas evolved but increase that of the tar oil from which a good fuel can be prepared available for use for the production of power. To be economical there must be no waste products. It is .clear, however, that a new era is about to rise in which the present use of raw coal will be disfavored. Nevertheless, it will remain the chief source of British energy. The present output of coal in the world is. I believe, about eleven hundred or twelve hundred million tons a year, and that of oil from forty to sixty million tons. Hence, even though one ton of oil can in internal combustion engines generate three or four times the power that one ton of coal used in the ordinary way with steam engine and boiler can give, it is clearly seen that oil is a long way from displacing coal as a prime source of energy. There is still another method in which the more economical use of coal for power generating may, perhaps, be developed, viz., in the coal engine of Mr. A. M. Low. In this case coal in a finely divided form is led from a hopper along pipes in which it is heated and when mixed with air gives an explosive mixture. Steam can also be used jointly with the explosive vapor. The heat generated by the explosions is caused to heat the incoming fuel to volatilization. Hence, the process is, to some extent, regenerative. Experiments with a 100 horse-power engine have shown an efficiency of 1 horse-power per half pound of coal per hour. The experiment has been tried of using in place of vaporized or atomized heavy oils finely divided coal directly in a Diesel internal combustion engine. The explosive character of coal dust and air or even flour dust and air are well known. The difficulty which occurs is due to the accumulation of ash in the cylinder or combustion space. The future of the gas turbine is a subject which was discussed by Dr. Dugald Clerk, who is a great authority on this subject, at Dundee, at the British Association, and is of very considerable interest. It affords another possibility for utilization of products of coal distillation. The problem, therefore, is the establishment on a much larger scale than anything yet attempted of power stations in contiguity to our coal mines in which power is - generated by some form of large internal combustion engines utilizing a heavy coal oil or coal gas assisted, perhaps, by steam engines utilizing coke as fuel, the energy being transmitted electrically at high tension to the centers of consumption. One of the difficulties at present is the small power, relatively speaking, of the internal combustion engines which yet can be built as compared with steam engines. It is now quite feasible to transmit at electric pressures up to 100,000 or even 150,000 volts, and at 75,000 volts a current of only 1 ampere conveys 100 horsepower. There is nothing impossible to present-day engineering in the proposal to generate 50,000 horsepower in the form of electric current at 75,000 volts pressure, and distribute it in different directions by suitable overhead conductors over areas of 100 square miles. Wind and Tidal Power. In certain regions of the earth there are regular winds which blow for months at a time and more might be done to utilize them. The irregularity in force is, however, the chief obstacle to usage except for pumping up water to reservoirs. As regards the utilization of tides, it is a very popular subject for speculation but the very large reservoir area. and constructions necessary are likely to make it anything but a free gift. The immense extension of hydro-electric stations for utilizing ordinary waterfalls or high level water of late years in Switzerland, Norway, and Canada are well known. It has been estimated that there is available in' Sweden 3,800,000 horse-power, in Norway 4,800,00 horse-power, and in Canada 17,000,000 horse-power in water-power. In Canada about 1,000,00 horse-power is now utilized and in Sweden and Norway about 1,000,00. but Grea.t Britain, is unfortunately, deficient in this natural source of power. The Direct Use of Solar Radiation. As regards the direct use of sun power or sun heat, an interesting attempt has been made in the solar engine of Mr. Shuman of Tacony in Philadelphia. He allows the sun heat on hot, bright days to ' heat water placed in pipes in shallow trenches covered with glass plates and employs this hot water to actuate a turbine worked with a vacuum in the rotor chamber so that 10 per cent of the nearly boiling water explodes into steam and ejects the rest against the rotor, -thus producing mechanical power. The experiments . show that an engine of 1 horse-power required 160 square feet of absorber surface. This shows that only a small fraction of the incident solar energy is really converted to mechanical power. It may be even better to utilize solar light and hea.t to force growth of vegetables from which oil or alcohol can be procured to use in internal combustion engines of the Diesel type. Atomic Energy. I turn, however, in conclusion to consider a possible source of energy as yet quite untapped, but which is almost limitless in amount. and perhaps not quite beyond human ability to appropriate in some degree. I mean the energy locked up in atoms of matter in the form of structural potential energy. Until 1896 it was generally assumed that the eighty or so different kinds of atoms of matter were perfectly incapable of being broken up or altered. Clerk Maxwell had spoken of them in a famous lecture as the “foundation stones of the material universe,” which remain, as he said, “unbroken and unworn.” But at that date Becquerel made the initial discovery that minerals containing uranium had peculiar properties of affecting a photographic plate. M. and Mme. Curie followed up this discovery with astonishing skill, and ended by isolating from uranium ores the new element, radium, having startling properties. These are chiefly as follows: Compounds of i-adiurn produce brilliant fluorescence when held near certain substances such as platinocyanides of barium, the mineral willemite, and other bodies such as zinc sulphide and diamond.' Secondly, they cause the air near them to become conductive so that a charged electroscope loses its charge when radium compounds are brought near it. Thirdly, they maintain themselves continuously at a higher temperature than surrounding bodies -in such fashion as to show that each gramme of radinni emits 100 gramme-calories of heat per hour. last they emit continuously a torrent of atoms of helium, of electrons and of ether waves, which are called respectively the a, /J and y radiation. The researches of numerous physicists have made it clear that the explanation which best fits the facts of these phenomena is that the radium atoms are in a continual state of disruption. It may be compared with a magazine full of cartridges which from some cause or other are in an unstable state. Every now and again a cartridge explodes and fires off its bullet. In the case of the radium atom the bullets are either particles the size of atoms, probably of helium, or stili smaller masses called electrons. The helium atoms or <* particles are shot 'off with a velocity equal to about 1/18 or 1/20 thai of light and the electrons with a velocity approaching that of light. The a particles are charged with positive1 electricity and the electrons carry a negative charge. These a particles or helium atoms are not all ejected at once. The first change which takes place when a 'radium atom fires off or expels one atom of helium is that the remainder becomes a kind of non-valent gas resembling in its general chemical behavior the neutral atmospheric gases argon, neon, etc., discovered by Sir William Ramsay. This gas is called the emanation. The emanation in turn loses more atoms of helium and becomes converted into substances called radium A, B, C, D, E, and F. Radium itself is believed to be produced in the same manner from uranium, and the final product obtained from radium when all these changes are complete is very probably lead. It appears not improbable that an atom of uranium is a complicated structure from which, when 8 atoms of helium have been expeUed, the residue is an atom of lead. Hence, the dreams of the alchemists have in a sense come true, only it is not lead transformed into gold but uranium which is transformed into lead. It would take far too much time to tell the whole story of these sensational discoveries. What is of importance to us now is the energy evolved in these changes. It has been shown that 4 parts in 10,000 of any mass of radium disintegrates per year, and hence the average life of a radium atom must be 2,500 years. Since 1 gramme of radium evolves 100 gramme calories per hour, a gramme of radium must give out during its whole life energy equal to that produced by the combustion of of a ton of coal. Now, ton is nearly 250,000 grammes. Hence, the potential energy of radium is a quarter of a million times greater than that of an equal mass of coal. . We must, therefore, regard the atoms of these radio-active elements, uranium, thorium, radium, and actinium, as structures which are gradualy breaking up and evolving heat. At each rupture they give out energy just as when a house falls down the potential energy represented by the elevated masses of brick or stone is' converted into heat. This discovery has greatly modified our views as to the cause of solar and terrestrial temperature. Until lately the most probable hypothesis as to the cause of solar heat was that it was due to a continual shrinking of the sun's diameter, compressing materials and raising their temperature. It was calculated that a shrinkage of 40 miles in every thousand years in the sun's diameter or something of the order of 6 inches a day (an amount quite imperceptible to measurement) would account for the solar radiation. Moreover, the earth was considered to be continualy-cooling, and from data obtained by the temperature gradient in the earth's crust it was asserted that the earth must have been in a state of incandescence or at least - hotter than boiling water not much more than one hundred million years ago. But the geologists always declared that this time was- not sufficient to alow of the development • of the earth to its present ' condition, and to afford time for evolution to produce ' the flora and fauna now found on it. Experiments by Prof. Jolly; Prof. Strutt, and others, have shown, however, that a large number of the earth crust materials , if not all, contain radium, roughly speaking, from two to twenty-five parts in a billion, by- weight. If this amount were distributed uniformly through' the globe it would not only suffice to account for al the terrestrial heat lost, but at the same time would even cause the earth's temperature to be rising. In the same way Rutherford has calculated that if there were present in the sun two and one half parts by weight, of radium in a million of other matter it would suffice to maintain the present rate of radiation by the sun. Hence, although the actual amounts of radium already extracted Jrom uranium ores is small, probably not more than an ounce or even half an ounce altogether hasever been prepared, yet, nevertheless, the total amount contained in the whole earth may even total up to one thousand million tons, which is equivalent in heat-producing power to two hundred and fifty billion tons of coal. A problem of surpassing interest then in connection with human progress is whether we shall be able to tap this enormous source'of energy in any way. At the present moment we have not any idea of how this may be done. A hint as to a possible solution of the problem may be obtained as foUows: It is now pretty gener-aUy recognized that an atom is a complicated structure, a sort of solar system in miniature composed of revolving electrons. - It may bo possible to break down the structure by the action of impulses due to concentrated electric-waves of the right period, setting up vibration, which are resonant with some natural period of the atom, just as it is possible to break down a suspension bridge by a number of men jumping on it in time with its natural period of oscillation. If, then, the atom were to break down the energy liberated might be far greater than that applied to it in the form of the resonant impulses. All this, however, is a long way from realizations. As far as concerns the sources of energy which are available at present, the moral to. be drawn is that they are not inexhaustible. Our stores of coal and oil are large but not indefinite. Water-power and food will continue as long as the sun shines on us, but population and energy demands ever increase. This slight sketch of the position of our energy problem wili be sufficient to show you that while we are no doubt a long way yet from an energy famine, the world has arrived at a stage in which we cannot afford to treat our available sources at ail wastefully. Hence, the engineer is more than ever the arbiter of the world's destinies. The fate of population, and indeed of civilization itself, depends a great deal more on the engineer than it does on most of our statesmen and politicians with their quack or doubtful “remedies for human ills. Hence, there is a wide field for useful work • in al branches of engineering, provided we bring to its prosecution initiative, originality and a high scientific training.-