The term "lift" will appear many times in the following pages, and it is necessary to understand what it really means. The difference between the weight of air displaced and the weight of gas in a balloon or airship is called the "gross lift." The term "disposable," or "nett" lift, is obtained by deducting the weight of the structure, cars, machinery and other fixed weights from the gross lift. The resultant weight obtained by this calculation determines the crew, ballast, fuel and other necessities which can be carried by the balloon or airship. The amount of air displaced by an airship can be accurately weighed, and varies according to barometric pressure and the temperature; but for the purposes of this example we may take it that under normal conditions air weighs 75 lb. per 1,000 cubic feet. Therefore, if a balloon of 1,000 cubic feet volume is charged with air, this air contained will weigh 75 lb. It is then manifest that a balloon filled with air would not lift, because the air is not displaced with a lighter gas. Hydrogen is the lightest gas known to science, and is used in airships to displace the air and raise them from the ground. Hydrogen weighs about one-fifteenth as much as air, and under normal conditions 1,000 cubic feet weighs 5 lb. Pursuing our analogy, if we fill our balloon of 1,000 cubic feet with hydrogen we find the gross lift is as follows: 1,000 cubic feet of air weighs 75 lb. 1,000 cubic feet of hydrogen weighs 5 lb. ------ The balance is the gross lift of the balloon 70 lb. It follows, then, that apart from the weight of the structure itself the balloon is 70 lb. lighter than the air it displaces, and provided that it weighs less than 70 lb. it will ascend into the air. As the balloon or airship ascends the density of the air decreases as the height is increased. As an illustration of this the barometer falls, as everyone knows, the higher it is taken, and it is accurate to say that up to an elevation of 10,000 feet it falls one inch for every 1,000 feet rise. It follows that as the pressure of the air decreases, the volume of the gas contained expands at a corresponding rate. It has been shown that a balloon filled with 1,000 feet of hydrogen has a lift of 70 lb. under normal conditions, that is to say, at a barometric pressure of 80 inches. Taking the barometric pressure at 2 inches lower, namely 28, we get the following figures: 1,000 cubic feet of air weighs 70 lb. 1,000 cubic feet of hydrogen weighs 4.67 " --------- 65.33 lb.