the other it is possible to steer the machine. In fact it can be made to turn around in its own length by stopping the belt on one side and driving the belt on the other. Such a machine is ideal for plowing and other are hauled with ease over the soft plowed ground. For ordinary farming purposes speed is not essential, but the possibility of rapid travel with tread-belt traction was demonstrated in the World War when small “tanks” were built which could run at the rate of twelve miles per hour. CHAPTER XIV INVASION OF THE SKY FROM the day when man began to assert his superiority over other animals he began to cast longing eyes at the sky. Fired with ambition and stirred with a spirit of mastery he chafed at the bonds that bound him to earth; but the heavens remained a forbidden kingdom to him. Many a bold adventurer who dared to emulate the birds paid the penalty of his temerity with a broken limb and even with his life. It seemed as if man were destined forever to grovel on the bed of the atmospheric ocean with never a chance to rise except in dreams and fancy. It was not until near the close of the eighteenth century that a means of rising off the earth was discovered. Two brothers, Stephen and Joseph Montgolfier, of Annonay, France, were sitting before a fire, watching the smoke curl up the chimney, when it occurred to one of them that smoke might serve as a vehicle to carry them up into the air. They belonged to a prominent paper-manufacturing family and naturally turned to that material as the most suitable for trapping and harnessing the smoke. They began their experiments with a large bag of thin paper which they filled with smoke and floated up to the ceiling. The next step was to fasten a dish filled with burning embers to the bag, so that the balloon carried its own smoke generator. The experiment was tried in the open air and the balloon arose to a great height. Larger bags were made of linen and paper and on the fifth clay of June, 1782, a public exhibition was given. A pit was dug in the ground in which a fire was lighted and over this was placed a huge balloon which weighed 300 pounds. Eight men were required to hold it down while it was filling with heated air and, when released, it shot up to an elevation of about 6,000 feet and came to earth about a mile and a half away. THE FIRST HYDROGEN BALLOON