Petroleum is an important source of energy, but though less is known of the sources of supply, it is almost certain that these are not being renewed, and it a certain that the total amount available is not to be compared with that of coal, and that it will be exhausted still sooner. In the eastern states of America the supply is diminishing rapidly, and though the output is increas ing from the states west of the Mississippi, yet even at the present rate the supply will be exhausted within a century, and, if the rate continues to increase, within this generation.
What other sources of energy have we ? For a thousand years and more the peoples of North-West Europe have used the energy of the tidal rise twice a day to carry shipping far inland against wind and river flow, and the energy saved has been of extra ordinary use ; it might seem possible to use this energy, which is running to waste, for all kinds of useful purposes, but except in a few favoured spots it cannot compete with goal ; even with the exhaustion of coal it does not appear likely that it would be used except as a last resource, and even then there would be little return for great outlay, while storms would be likely to damage the necessary extensive works.
The energy of the wind and of falling water, like that of the tides and unlike coal, is continually being renewed. The energy of the former is, however, also like that of the tides, in that it gives little return for outlay, and the total amount of the latter is probably by no means equal to that necessary to take, the place of coal energy if coal should fail. In the United States, for example, the water-power is estimated to be able to produce from 36 to 66 million H.-P. This, even if all utilized, is certainly less than a half of the H.-P. actually to be obtained from coal now mined in the United States, and may be a good deal less. The energy of falling water has, however, the advantage of being more economical than either wind or tidal energy, in other words, more may be obtained for a given outlay, and it is probable that we may see high and rainy regions taking a more prominent place in the world system.
It is, of course, possible that some agent may be discovered by which it may be possible to use the energy given out by certain forms of matter, of which radium is the chief example, or that the internal heat of the earth might be tapped; but it is thought to be very improbable that any considerable amount of energy may be obtained from either of these sources.
Thus the changes that appear likely are those depend ing on the exhaustion of coal and on the more extensive use of water-power, modified by increased ability .to use supplies of energy more economically. That is to say, other things being equal, it is likely that regions where coal remains unexhausted longest, and where large sup plies of water-power are available, may keep or gain an importance at the expense of others not so fortunate.
But we may take more fundamental distributions into account. Stores of coal and petroleum are of the nature of capital which has been accumulated long ages past, and in using them we are not really accumulating energy at all ; they are on a somewhat different footing from the energy which man makes his own, in almost the only way possible till 130 years ago, by eating food which has grown by the sun's energy within a few days or months of its consumption. The use of coal energy is something in the nature of an incident. In the midst of the changes which the Industrial Revolution has brought, we are in danger of forgetting that it is an incident, and that solar radiation is the final source of by far the greatest amount of energy available on the earth's surface, and especially that vegetation now growing supplies the energy in the most convenient forms ; culture of the soil, horticulture and agriculture and arboriculture, whether it be the oldest trade or not, is certainly the most fundamental.
Advance in saving energy is being made by the use of the best machinery and by organization of all kinds, so that there may be as little waste as possible. And further advance is being made by obtaining greater crops as the result of researches on all sorts of problems, the solution of which does not seem at first sight likely to bring about a saving of energy. As the results of studies on heredity, wheats are being bred which will resist disease, which will ripen in a shorter time, which will give a better bread than was possible before. As the results of research on bacteria which live in soils and elsewhere, means have been found of removing from the soil those organisms which prey on the particular bacteria that give energy in a form available for use by plants. As the results of observations of barometric height and rainfall at places -so widely separated as South America and the east coast of Africa—observations in themselves purely scientific, and discussed by means of pure mathe matics—some indication is given to the farmers in India of the amount of rain that the monsoon is likely to bring them. Advance is being made now in ability to obtain more energy from the soil and in allowing less to be wasted, and those lands accumulate most energy where there are most men either capable of showing how advance may be made, or capable of utilizing the new knowledge. Such areas for the most part are where there is most stored energy, so that some may be set apart to promote those researches ; that is to say, in the lands where there is most coal. Thus at present the results of advance in soil-cultivation are rather masked by the advance due to the use of coal.