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. [Illustration: A LOOM PROVIDED WITH A JACQUARD ATTACHMENT Note the belt of cards that determine the pattern that is to be woven] [Illustration: A BATTERY OF MULE-SPINNING FRAMES] [Illustration: JACQUARD ATTACHMENT FOR LOOMS] SEVEN MILES ABOVE THE EARTH