west of both sound-areas and of the silent zone, the air as a rule was almost still. It is well known that sounds carry but a short distance against the wind. The reason is that, for some distance above the ground, the velocity of the wind increases with the height. Taking the velocity of sound at 1100 feet per second, and that of the wind at ten feet per second near the surface and twenty feet per second at a height of eight feet, it is clear that the velocity of the sound-wave against the wind would be 1090 feet per second at the surface and 1080 feet per second at eight feet above the ground. The effect of this difference on the portion of the wave-front which is vertical at any moment is shown in the figure below. The slanting lines represent the positions of the wave-front at successive equal intervals of time. The result is evidently an upward tilting of the wave-front; and, as sound-waves always travel in a direction perpendicular to their front, it follows that the sound-waves in the direction against the wind take an upward course. The sound-vibrations in that direction thus become inaudible not so much from any enfeeblement in the vibrations as to their being lifted over the heads of observers on the surface of the ground. DIRECTION of Sound. DIRECTION OF WIND. As we have seen, the intensity of the sound in the inner area decreased rapidly outwards, showing that the sound-waves soon began to leave the ground. The inaudible air-waves also rose, but to a less extent, for they remained strong, even to the limits of the sound-area. Having once risen well above the ground, sound-waves and air-waves then continued their journey over the silent zone, the sound-waves taking the higher course. This seems evident, for the sound was rarely, perhaps not more than once, heard in the silent zone, while the air-waves, diverging downwards, shook windows and startled pheasants at several places within it. What heights the wave-paths attained we do not know, but they must have been least at the western end of the silent zone. For there the sound-waves returned to earth after their shortest aërial journey and the air-waves manifested their principal effects. Several interesting problems must remain unsolved owing to our ignorance of the essential atmospheric conditions. The reason why the sound-waves and airwaves came back to the ground in the outer area can only be surmised. There was probably some change in the direction or the velocity of the wind at a height of not more than a few thousand feet. Nor is it easy to explain why the inner sound-area was so abruptly limited towards the south, and so distorted in the directions of Canterbury and Northampton. The repetition of the reports in the outer area is another feature worthy of attention. It is easy to say that the sound-waves split up and crossed the silent zone by different paths, less easy to explain what made them take their several ways. For the solutions of these problems we must look to future investigations made in happier times. But many of the difficulties which attend such enquiries will remain. They are independent of official reticence. None can forecast an explosion nor provide us with a complete knowledge of the atmospheric structure at the moment when it shall occur. The best that we can do is to collect the facts when they present themselves, and thus provide the mathematical physicist with materials for the complete solution which he may some day give us. CHARLES DAVISON. Art. 4.-PRICES AS AFFECTED BY CURRENCY INFLATION. ALTHOUGH the enormous increases in the cost of living since the autumn of 1914 have been tabulated by the Board of Trade, debated in the House of Commons, and discussed in the Press, very few serious attempts have been made to discover whether the upward movement of prices is entirely due to unavoidable causes, or whether some part of it is attributable to circumstances which it is within our power to ameliorate. The price of food is the concern of every man and woman in the country; and it is, of all subjects of pressing importance, the one which the people might themselves most easily probe to the roots. Discussions on the subject have, however, generally been based on the assumption that high prices are either absolutely unavoidable or are, at the best, only capable of such mitigation as might result from 'profiteers' being compelled to forgo a portion of their extra gains. Probably this neglect of an important subject is due in part to the fact that some of the more serious evils of high prices are mitigated in consequence of unemployment having virtually ceased to exist, and of wages having risen considerably, even if not in the same proportion as the cost of living. In the course of the discussions which have taken place, the obvious reasons for prices rising have been fully explained. The most notable of these are restriction of production, caused by the withdrawal of great numbers of men for active service and for munition work; the necessity of feeding and clothing the fighting forces on a scale more liberal than that to which the men had, on the average, been accustomed in civil life; the narrowing of markets as a result of sources of supply being cut off; and the enormously increased difficulties of transport, particularly by sea. To these may be added the opportunities of effecting 'corners' in various classes of goods, which dislocation of trade has made possible, and also increased consumption on the part of the by no means insignificant number of civilians who have benefited financially by the war. These are the ostensible causes of high prices. They have always operated in the past when war has been waged on a considerable scale; and, just as the present colossal struggle transcends all previous wars in magnitude and intensity, so might the effects on prices of the factors named be expected to transcend the effects of the same factors on prices during other wars. But this phenomenon need not necessarily blind us to the fact that other very powerful influences may be at work. The late President of the Board of Trade was clearly not satisfied that the ostensible reasons were a sufficient explanation of the upward movement of prices. The currency of the world was,' he said, 'inflated, and values were not now what they appeared to be.' Although sovereigns and half-sovereigns have now practically been superseded in circulation by Treasury notes, our currency still remains on a gold basis, as also do the currencies of all the belligerents-of nearly the whole world in fact. It is an axiom of Political Economy that the price of anything is its value in relation to money. That being the case, a general rise in prices simply means that the value of money has decreased in relation to the value of all other articles. According to Adam Smith, 'gold and silver, like every other commodity, vary in their value; are sometimes cheaper and sometimes dearer, sometimes of easier and sometimes of more difficult purchase. The quantity of labour which any particular quantity of them can purchase or command, or the quantity of other goods which it will exchange for, depends always upon the fertility or barrenness of the mines which happen to be known about the time when such exchanges are made.' Cairnes, writing in 1859, argued the point more fully in hisEssay towards a solution of the gold question.' He explained the consequences which would ensue from the increased supply of gold then being poured into the world from the mines of Australia and California, on the assumption that all other things remained the same. The consequences which he feared were not, however, realised, because all other things' did not remain the same. Expanding trade throughout the world necessitated largely increased currencies; and, commencing with Germany in 1872, followed a year later by the United States, several countries of first-class importance changed their standards from silver to gold. For these two reasons, vast quantities of the more precious metal were required for coinage; and the anticipated surplus resulting from mining activity in California and Australia not only failed to materialise, but the output of the mines was actually insufficient to meet the world's growing demand for gold. Consequently, the general tendency was for prices to fall until 1896, when the output from the Transvaal turned the scale in the other direction. Since then prices have, in the main, always moved upwards. Bagehot, who was perhaps the most illuminating of all writers on currency, was very emphatic. Money,' he declared, 'is a commodity subject to great fluctuations of value caused by a slight excess or deficiency of quantity.' · A very great increase in what Adam Smith rather quaintly called 'the fertility of existing gold mines' would, according to that writer, inevitably result in the cheapening of gold, and, therefore, in a general rise of prices. It is perfectly certain that there has been no great increase in the fertility' of the gold mines since the autumn of 1914; but it is equally certain that there has been, since then, an output of gold substitutes vastly exceeding in nominal value the wildest dreams of the possibilities of mineral production. By force of law, these gold substitutes, in the shape of British, Russian, French, Italian, and German paper money, rank in the countries of issue on the same footing as gold, and have had almost exactly the same effect on prices as the introduction into the currencies of the world of an equivalent amount of gold. Since the autumn of 1914, paper money in excess of the gold held in reserve for it has been issued by the belligerent States to the extent of over 1,500,000,000. The gold won from nature during the last 500 years is estimated to be slightly less than 3,600,000,000l., of which all but 700,000,000l. has been produced since 1850. Actual gold coin now in existence could not possibly exceed 2,000,000,000l., so that the effect of the paper money recently brought into circulation has been to increase the gold currency of the world by something like 75 per cent. During the last thirty-three years the Rand has produced gold to the extent of 515,000,000l., or very little more than the |