ticket offering the finder a reward for its return, and giving instructions as to packing and shipping. Sooner or later it generally comes back. In fact, the large percentage of records recovered, even in sparsely settled countries, is not the least remarkable feature of this novel method of research. Thus, of seventy-two balloons sent up by a Franco-Swedish expedition in Lapland, forty-one were eventually recovered with their instruments. One of these fell into a lake and was found after three years.

No instruments are carried by the pilot balloon, which merely serves to show, by its observed drift, the speed and direction of the air currents at different levels. The pilot balloon is sighted, while in flight, through a special form of theodolite, or, preferably, two theodolites some distance apart. Several ingenious methods have been devised for computing and plotting its actual course through the air. Such balloons, apart from their use in scientific research, have become one of the principal adjuncts of aeronautical undertakings all over the world, and are also used by artillerists to enable them to make proper allowance for the deflective effect of the wind on the flight of projectiles. Hundreds of thousands of pilot balloons were sent aloft for military purposes during the world war.

Meteorological instruments are sent up attached to kites or captive balloons whenever—as in connection with weather forecasting—the observations must be obtained more promptly than would be possible with the aid of sounding balloons, but such devices can attain only moderate altitudes. Kites have been raised to about four and one-half miles above sea level, as compared with nearly twenty-two miles reached by a sounding balloon and twenty-four miles by a pilot balloon. The average height of sounding-balloon ascents is about ten miles. As already stated, balloonists have risen to 6.7 miles. This is a little higher than the best aeroplane record.

The use of the aeroplane for making meteorological observations is still quite limited, but will inevitably increase. One other device gives promise of yielding valuable aerological information, on account of its ability to rise to extraordinary altitudes. This is a special form of rocket, recently invented by Prof. R. H. Goddard, which is propelled by several successive discharges of an explosive in the course of its upward flight, and with which the inventor thinks it will be possible to explore the whole vertical extent of the atmosphere. Meteorological apparatus for use with the Goddard rocket has been planned by Mr. S. P. Fergusson of the Weather Bureau.

The atmosphere presses down upon the earth with a weight that, at sea level, amounts to about 14.7 pounds to the square inch, on an average. This pressure is, at any point, exerted equally in all directions; it acts, for example, on the whole surface of the human body, and this means that a man of average size lives under a burden of some seventeen tons of air. He is not incommoded because the pressure from without is balanced by that of the air that permeates his body.

The pressure of the atmosphere decreases upward at nearly the same rate as its density. Thus on mountains and plateaus it is considerably less than in lowlands. At no place is the pressure invariable, nor is there a constant relation between pressure and altitude, but, knowing approximately the average atmospheric pressure over the earth’s surface, and knowing also the area of the latter, we can compute in round numbers the total weight of the atmosphere—about 5,000,000,000,000,000 tons. This is about 1/1,200,000 of the entire weight of the earth.

CHAPTER II
THE RESOURCES OF THE ATMOSPHERE

In the economic stress of our times much is heard about “natural resources.” This phrase suggests to most people’s minds the store of minerals, fuels, and oil locked up in the ground; the waters available for drinking, washing, irrigation, power production, and navigation; the forests and other natural growths of useful vegetation; and the soil in which we raise our crops. A moment’s reflection, however, will show that this is a one-sided enumeration. The resources of the atmosphere are as essential to humanity as those of the land and the waters, if not more so.

The coal that is dug out of the earth consists mainly of carbon, which, in bygone ages, was extracted by plants from the air. Moreover, it would be of no use to us if we did not have the oxygen of the air in which to burn it. Neither could we smelt metallic ores without oxygen. All our forests and all our crops draw far more of their solid substance from the air than from the soil. Fuel and water are valuable sources of power, but so is the moving air that drives sailing ships and windmills, and the atmospheric pressure that helps to operate suction pumps. It is the moisture of the air that feeds our streams and, directly or indirectly, waters all plants that grow upon the land. Lastly, it is the atmospheric oxygen that we breathe that keeps us from very speedily becoming incapable of using any of the other resources of Nature.

Air and water together contain, in their oxygen, nitrogen, hydrogen, and carbon, all the major constituents of our foods in unlimited abundance. It is tantalizing to think of the slow and roundabout way in which these things are wrought into edible shape—and the prices we have to pay for them. No less tantalizing, when coal is scarce and costly, is the thought that every vagrant breeze is laden with the carbon dioxide from which the chemistry of living plants so readily extracts the chief element of fuels. The total carbon dioxide of the atmosphere amounts to something like 2,200,000,000,000 tons, equivalent to 600,000,000,000 tons of carbon.

We have spoken of the utility of the air as a source of power. It is, perhaps, even more useful as providing an easy means of storing and transmitting power. The engineer stores up energy in a mass of air by compressing it. When the air subsequently expands it gives up its energy, and, in so doing, may be made to perform a variety of useful tasks. By a somewhat analogous process energy is applied to creating a vacuum, in order that the ordinary pressure of the atmosphere may be made available for doing a particular piece of work. The suction pump, the siphon, and the vacuum cleaner furnish examples of this process; and so do such familiar operations as sucking beverages through a straw and filling a medicine dropper.

From crude types of bellows, with which, from remote antiquity, air was compressed for the purpose of blowing fires, have been developed a host of wonder-working appliances of the present day, such as the air brake, the pneumatic tube, the compressed-air locomotive, diving apparatus, the caisson, certain kinds of refrigerating machinery, and a long list of pneumatic tools. To cap the climax of ingenuity in this field, methods involving both the compression and the expansion of air have been discovered whereby this invisible, elusive substance may be changed to a visible liquid and a visible solid; a process having extremely valuable applications, as we shall presently see.

Compressed air, as a means of transmitting power, rivals such mechanical devices as gearing, belting, and rope drives, when it is applied near the compressor; or it may be conducted for many miles in pipes, thus competing with the electric current; or, finally, it may be transported in tanks to the place where it is to be used, a process analogous to the use of the electric storage battery. Compressed air has, moreover, certain advantages over other methods of transmitting