The chief use made of the wind to-day, as in ages past, is to propel sailing ships, and its use for this purpose is, of course, of less importance, in a relative sense, than it was before the introduction of steam. The importance of windmills has also greatly declined. This fact was strikingly brought out some years ago when the United States Bureau of Statistics collected, through American consuls abroad, detailed information concerning the use of the windmills in foreign countries. In most parts of Europe windmills are rapidly disappearing. In Holland, for example, the traditional home of the windmill, the perpetual task of draining the polders is now performed by steam pumps, and the total number of windmills is estimated to be only about one-tenth what it was centuries ago. Our own country is probably the only one in which the use of windmills is increasing. The modern American windmill, with its disklike assemblage of numerous light sails, and ingenious contrivances for veering, reefing, etc., is a much more efficient contrivance than the old-fashioned windmill; but its utility, like that of other windmills, is limited by the irregular force of the winds. For years the hope has been entertained that the windmill would eventually become a common means of generating electricity, but this hope has not yet been realized, though isolated installations of this character are in successful use. CHAPTER III THE ATMOSPHERE AS A HIGHWAY Within the last few years the atmosphere has assumed a new and tremendous importance in human affairs as a medium that affords facilities for travel and transportation far superior, in many respects, to those offered by the land or the water. The aerial highways are now open for business and pleasure. This is a fact that the majority of people find it difficult to realize. The navigation of the air on a general scale has so long been looked upon as a dream of the future that we cannot readily adjust our minds to the reality. The story of the slow steps by which this momentous fact has been brought to pass is far too long to be told here. What we purpose to do in the present chapter is to sketch the multifarious uses to which man is now applying the aeronautical knowledge and skill that he has acquired. At the same time we shall anticipate, to some extent, the developments of the near future; for the lines of progress are so clearly marked out that it is possible to do this without giving too much rein to the imagination. In a subsequent chapter, dealing with Aeronautical Meteorology, we shall touch briefly upon the mechanical principles that underlie aerial navigation, by way of preface to a more detailed description of the conditions of wind and weather encountered by aircraft, and of the services that the meteorologist is rendering to the aeronaut.