[Illustration: EXPLORING THE UPPER AIR. Left: Beginning of a pilot-balloon flight. Right: Sending up a sounding-balloon. Note the parachute, which wafts the basket of instruments gently to the ground after the balloon bursts. (_Photographs from U. S. Weather Bureau._)] Although, when air is liquefied, the oxygen and nitrogen are condensed simultaneously, the latter has a lower boiling point than the former and therefore passes off more rapidly when the liquid is allowed to evaporate. This fact makes it possible to separate the two substances, by the process known as “fractional distillation,” and hence liquid air plants have been established for the special purpose of manufacturing oxygen and nitrogen, for both of which there is a large and growing commercial demand. Scores of millions of cubic feet of oxygen are used every year in the wonderfully efficient process of welding metals with the oxyacetylene blowpipe, the flame of which has a temperature of about 6,000° Fahrenheit. Most of the supply now comes from liquid air. An equally large amount is used in a recently introduced method of cutting metal. The object to be cut is first heated to incandescence, after which a jet of oxygen is played upon it. The metal actually burns away in the stream, and a clean cut is made like that of a saw. It is interesting to reflect, when we fill our lungs with oxygen in order to keep our bodily machinery in operation, that the same atmospheric gas is applied to the building of motor cars, bicycles, safes, boilers, and battleships. Cartridges made of lampblack, dipped for a few moments in liquid oxygen and then primed with a fulminate cap, constitute an explosive as powerful as dynamite and much cheaper to produce. A small percentage of oxygen added to the air supplied to blast furnaces has been found to effect a great saving of fuel used in the furnace. [Illustration: METEOROGRAPH FOR USE WITH SOUNDING BALLOON. (Fergusson pattern. U. S. Weather Bureau, 1919.) The aluminum case, surrounded by hoops of rattan to protect the apparatus when it falls to the ground at the end of the flight, contains a set of very light self-registering meteorological instruments. (_Photograph from U. S. Weather Bureau._)] [Illustration: KITE METEOROGRAPH. (U. S. Weather Bureau Pattern.) The four pens record the barometric pressure, temperature, humidity, and wind-force on a sheet of paper wound around the large cylinder, which is turned by clockwork. Note the fan wheel inside the tube, for measuring the force of the wind. The apparatus is made chiefly of aluminum and is inclosed in an outer case of aluminum when sent aloft attached to the kite. (_Photograph from U. S. Weather Bureau._)]