It seems hardly necessary in this day and generation to explain that a balloon rises because it is lighter than the air it displaces, but it does seem astonishing that a balloon weighing several tons may yet be lighter than an equal volume of air. We do not ordinarily think of air as having any weight. We know that the ocean of air bears on the earth with a pressure of about 14½ pounds per square inch, or about a ton per square foot at sea level. This amounts to 28 millions tons per square mile and, when we multiply this figure by the number of square miles of surface in the total area of the earth, we find that the whole ocean of air has a weight of 5,500,000,000,000,000 tons—a figure which is far beyond our conception, but it does impress us with the fact that air is a ponderable substance. Of course, the atmosphere that exerts a pressure of 14½ pounds per square inch is scores of miles deep, but even a small quantity of air has appreciable weight. If weighed in a vacuum a cubic foot of air would tip the scales at 1¼ ounces, and 100 cubic feet would weigh close to 8 pounds. The air in an empty room 10 feet square and with a 10-foot ceiling weighs nearly 80 pounds at a temperature of 32 degrees Fahrenheit. In other words, if we had a box measuring 10 feet on each side and weighing less than 80 pounds in a vacuum it would float in the atmosphere when the thermometer was down to the freezing point, provided there was no air in the box to weigh it down.
Vacuum balloons were proposed long before the time of the Montgolfier, but the problem was to construct a vessel strong enough to resist the crushing pressure of the atmosphere. The best bracing for the walls of the vessel is some gas dense enough to exert a pressure equal to that of the atmosphere but whose weight is less than that of the air. The weight of the vessel must then not exceed the difference between the weight of the gas and an equal volume of air. The Montgolfier brothers used heated air to keep their box or envelope distended. Air expanded by heat is lighter than cold air. However, it does not make a very good filler because 1,000 cubic feet of air heated to 212 degrees Fahrenheit weighs 59 pounds, leaving only 21 pounds on a freezing cold day for the absolute weight of our 1,000-cubic-foot box. Coal gas, on the other hand, weighs about 40 pounds per 1,000 cubic feet at 32 degrees Fahrenheit, while the same volume of hydrogen weighs only 5½ pounds. If coal gas were used to brace our box against the pressure of the air we should have a surplus of about 40 pounds for the weight of the box, while hydrogen would allow us 74½ pounds. Of course, the weight of a volume of gas depends upon its density. No matter how small a weight of gas we placed in the box it would fill the box completely, but when we speak of a 1,000 cubic feet of gas or hydrogen we mean a volume sufficiently dense to bear against the container with the same pressure that the atmosphere presses outside; i. e., 14½ pounds per square inch. Temperature also affects the pressure. In the case of fire balloons the hot air inside the envelope is less dense and hence lighter than cool air outside, but the pressure is equal because the former is expanded by heat. The volume of gas in a hydrogen or coal-gas balloon varies greatly with the temperature to which it is subjected. For instance, if on rising through a cloud or a blanket of fog, the balloon should encounter the rays of the sun, the heat would burst open the envelope were no vent provided. The mouth of the bag is kept open, however, for just such emergencies so that the surplus gas may escape. On the other hand, sudden chilling of the gas will contract it and send it down to earth, or the balloon might encounter a downward current of air, when the only salvation of the aeronaut is to throw out the ballast.
BALLOON NAVIGATION
Balloons seem like very helpless craft, and yet they are capable of skillful navigation at the hands of an experienced pilot. Although the balloonist has no means of self-propulsion and must drift with the winds, he is capable of controlling motion in the vertical direction and can choose the particular air currents on which he desires to ride. By throwing out sand ballast the bag may be made to rise and by letting out the gas it may be made to descend, and a pilot who is familiar with prevailing currents of the atmosphere or able to interpret meteorological indications, may locate the air stream that will carry him to his destination. Sand is the balloonist’s fuel; when that is gone the balloon may as well come to earth at once. Its course can no longer be directed and there is nothing to prevent it from being suddenly dashed to earth should it run into an “air hole,” which is another name for a downward air current. When a balloon comes to earth it is liable to be dragged by the wind and many accidents from dragging occurred in the early days of aeronautics until John Wise, an American, invented the rip panel by which the envelope may be ripped open by pulling a cord, thus freeing the gas and permitting the bag to collapse instantly.
A LOOM PROVIDED WITH A JACQUARD ATTACHMENT
Note the belt of cards that determine the pattern that is to be woven
A BATTERY OF MULE-SPINNING FRAMES
