National Geographic : 1936 Jan
MAN'S FARTHEST ALOFT the gas inside. This was at 65,000 feet, as calculated by the designers. To go higher we must discharge ballast, so Captain An derson methodically tripped sack after sack of fine lead shot, while with stop-watch he calculated the rate of rise. Looking up into the interior of the bal loon, we could see the special small meteor ograph of the Bureau of Standards hanging 40 feet below the top. It had been placed there to record conditions inside the balloon. Between the two valves, and a few feet be low, inside the balloon, was a thermometer of circular design, three feet in diameter. The unaided eye could not read the position of the thermometer hand, so we brought our ten-power binoculars into use, and found that the temperature of the helium gas in the balloon was four below zero, Fahrenheit. Outside the balloon, the air temperature varied from 70 to 78 degrees below zero, Fahrenheit. We had some 74 degrees of superheat already. Superheat is the term that de scribes the excess in temperature of the gas inside a balloon over the outside air temper ature. At high altitudes, the amount of heat received by the balloon from the sun is very great. This heat expands the gas inside and gives greater lifting power. As we watched, the thermometer hand inside the balloon climbed steadily. Soon it was five degrees above, and then 14 de grees above. In less than an hour it got to 32 degrees, Fahrenheit. This thermometer was important to us, for every degree that it registered higher meant that we must reserve a certain extra amount of ballast for the descent, if we were to land the balloon without crashing it. One cannot prevent the gas from acquir ing superheat in sunlight; one cannot avoid cooling of the gas and loss of superheat on the descent-and loss of it is loss of lift. On the way down, ballast must be dis charged to compensate for this loss just as, on the ascent, it must be discharged to permit the balloon to rise. Near the top of our ascent Captain An derson discharged bag after bag of ballast, sending us higher and higher. AT THE TOP! When we had discharged all the ballast deemed advisable, the balloon stopped ris ing, and began moving gently up and down, each vertical oscillation being less than the preceding one. The huge appendices gently closed-opened-closed again. Finally, at 11:40 a. m. (MST) came the big moment when we were at the top. Our precision altimeter registered 272 milli meters of mercury. I opened the case of the Factograph (which automatically photographed the in strument readings) to look at the mercury column to see whether it could really be true that we were floating in air so thin that it would hold up only a little more than an inch of mercury. I opened the door quickly, for I did not dare let any moisture form on the barometer tube lest it spoil the photographic records. And, sure enough, the difference in the levels of the two arms of the U-shaped barometer was only a little over an inch. At sea level our atmosphere holds up a column of mercury approximately 30 inches high; here the air pressure was only enough to support a column less than a twenty-fifth as high. This meant that 24 of 25 parts of the total atmosphere of the world was beneath our level-only one twenty-fifth above us. Reference to the correction table showed that the altimeter had a 2-millimeter cor rection at the part of the scale now reached, and that our altitude should be that corre sponding to 292 millimeters, or 73,000 feet. This correction was a rough and ready one which could give us only an approximation of the truth. We knew that we would have to await the calibration of the instruments after the flight and corrections because of temperature in order to learn our true alti tude (see text, page 74). It was a temptation to both of us to dis charge more ballast and see the altimeter hand move to 26 or 25. At great altitudes it takes only a little more than a millimeter of mercury to cover a thousand feet of rise. We knew we could dump another thou sand pounds of ballast and go 2,000 feet higher. But would we have enough ballast left to keep us from crashing on landing? We could not afford to overlook the fact that the most vital function of ballast in a balloon flight, in so far as the safety of an expedition is concerned, is its release dur ing the downward trip, and, especially as the earth is approached, to reduce the speed of descent and bring the craft gently to its landing.