CHAPTER XXX 

CURIOUS FACTS ABOUT AIRSHIPS 

     There are many strange things about airships. In the following paragraphs an effort is made to explain some of these. We are here consid­ering the America as if she were free in the air and had no equilibrator in connection with the earth. As a matter of fact, the resistance of the equilibrator in passing through the water to some extent modified the principles which we here explain; but the principles remain. 

     A steamship or sailing vessel is buffeted by wind and wave; but it is partly immersed in an ocean of fluid which offers great resistance and subjects it to violent shocks. 

     An airship is completely immersed in a me­dium which offers so little resistance that shocks are impossible. It is not an easy fact to grasp —but is still a fact—that the only resistance an airship offers to the wind, the only strain or pressure upon her parts, is that which she herself creates with her engine and propellers. 

     To understand this principle, imagine an air­ship far out over the ocean. It is calm; no engine is running; the crew, perchance, is asleep. Up springs a gale, fast gathering force till it reaches a velocity of fifty miles per hour. Noth­ing whatever in the motion, the vibration, the pitching or rolling of the ship alarms and awak­ens the crew. The ship being a "free balloon," because no engine is turning, simply becomes a part of the wind, so to speak, moves with the wind, offers no resistance to it, floats along as peacefully as if it still were dead calm. A mem­ber of the crew awakes, rubs his eyes, goes to his post ; but if it be night, and he cannot see the ocean, he has no idea whether the ship is stand­ing still or moving fifty miles per hour. If he strikes a match to light his pipe the flame curls straight upward, precisely as if he were in a closed room—an experience which many of us have had in ordinary spherical ballooning. 

     Suppose now the crew be roused. An engine is started, a pair of propellers set in motion. Then, and then only, does the ship offer resist­ance to the wind; and the measure of her re­sistance is the energy exerted by the propellers —just that, no more. Nor does t make any difference whether the velocity of the wind be ten or a hundred miles per hour; nor yet any difference whether the ship be headed into the wind, or with it—the result is the same. If the propellers exert a force sufficient to give the craft a movement of twenty /miles per hour in still air, that is her resistance to the wind, re­gardless of the wind's force or direction. 

     Steering into the eye of a wind of fifty miles per hour the airship would lose thirty miles per hour; running with such a gale she would make seventy miles per hour on her way. Obviously, as long as the ship has sea room—can keep herself up in the air—no storm that ever blew can hurt her. That is, if she be wholly free from contact with the earth through drag rope, equilibrator, or other trailing device. 

     Another vital factor in the endurance of an airship—its ability to make a long voyage—is its continued buoyancy under all conditions. We have already pointed out that the leakage of hydrogen through the envelope was expected to amount to a loss of less than 500 pounds of lifting force per day, while the load carried was to be lightened twice as much by the consumption of half a ton of gasoline in the engines every twenty-four hours. But this is not all of the story. 

     An airship is peculiarly sensitive to any change in meteorological conditions. It is af­fected by winds, sunshine, clouds, heat, cold, moisture, atmospheric pressure. All these mu­tations must be taken into account by the aero­nautic engineer, who, all things considered, may be regarded as rather a busy man, dealing with a wide range of the arts and sciences—physics, mechanics, chemistry, metallurgy, motors, me­teorology, nautical astronomy, seamanship, sex­tants, compasses, human nature, finance, fuels, and food, in fact almost everything under the sun. 

     The buoyant force of the airship America was nearly 24,000 pounds when the temperature of the surrounding air and of the gas within the reservoir was at a certain standard, say seventy degrees Fahrenheit, and the barometric pressure is normal, say 29.92 inches or 760 millimeters of mercury. Temperature and air pressure are constantly changing, and the volume of gas, and constantly the lifting power, changes with them. Both temperature and atmospheric pressure change not only from hour to hour and from day to day, due to general conditions, but change with every variation of the altitude of the ship. 

     The reader should first understand that all of the 345,000 cubic feet of gas in the balloon in our airship was contained in a single compart­ment or reservoir, tightly closed, and kept con­stantly under a small pressure that the form of the balloon may always be maintained. The only openings in this reservoir were three in number—one for putting gas in, two valves for letting gas out. One of the latter was at the top of the balloon, worked only by a cord from the car. The other was at the bottom, well aft, and set to open at a pressure of 12 millimeters of water, equal to 12 kilogrammes per meter square —oh, that the United States as an approximately civilized country could enjoy the blessing of the metric system of weights and measures !—or about 21/2 pounds per square foot. When the pressure rises to this height the valve opens automatically, gas blows out till the pressure is reduced to the standard, when it closes again. When the gas contracts and reduces the pressure below the standard, air is pumped in by a special apparatus installed in the car until the required pressure is regained.

     Within the balloon are six inner reservoirs for air. These are empty when the ship sets out upon a voyage, the desire being of course to take the largest possible quantity of gas and there­fore the maximum of lifting force. As hydro­gen escapes, or is let out, air is pumped into these six bags to replace the loss, always maintaining the integrity of the balloon's form. If the bal­loon should become "flat," that is, not be quite full and distended, "pockets" would be formed in its forward part, greatly increasing its resist­ance to the wind. 

     In the America six of these inner compart­ments were provided so that air—which is so much added weight or ballast—might be put in that part of the ship where it seems most needed, and to prevent the rolling to and fro of the rel­atively heavy air, as would be the case if it were introduced to a single great compartment. 

     An airship sets forth on a voyage. It is, say, the afternoon. As night comes on the tempera­ture of the air falls. The gas, which has been warmed by the sun during the day, rapidly cools by radiation. All gases expand or contract 1/491st part of their volume for each degree Fahrenheit of heat or cold. Gas within a bal­loon subjected to hours of warm sunshine will absorb heat, much as a greenhouse does, till it is far warmer than the surrounding air. If the temperature of the gas in the afternoon should be 100° Fahrenheit, and during the night it were to cool to 50° Fahrenheit, it would suffer a con­traction equal to about one-tenth of its volume. 

     This is an extreme case; but a contraction of one-twentieth of the volume is not improbable. With the America that would mean a loss of ap­proximately 1,200 pounds of lifting force. In other words, to keep the ship from going down to earth 1,200 pounds must be taken from the load which she carries. 

     At the same time the atmospheric pressure may increase,' still further contracting the gas. Rain may deposit 500 to 1,000 pounds of water upon the 4,000 square yards of the balloon and its appurtenances. To lighten ship under such circumstances ballast is usually carried in the form of sand or water to be thrown overboard. 

     Assuming that the ship by these means has weathered the night, the following morning the sun comes out bright and warm. The gas ab­sorbs heat and expands; air is expelled from the balloonets or inner balloons when the pressure passes the fixed point, and opens the valves of the air compartments; every cubic foot of air out lightens the ship 1.2 ounces; the accumulated moisture upon the envelope and the car is evapo­rated by the sun's heat, and under all these in­fluences the craft rapidly rises in the air. 

     For many reasons it is not desirable to go very high. Chief among these is the fact that if a ship is at great altitude when the gas begins to con­tract and starts her upon a descent she acquires momentum going down, and desperate measures must be adopted to avert disastrous collision with the earth's surface. To prevent rising to an un­desirable altitude the valve cord is pulled, and gas deliberately let out at the commencement of a period of expansion. 

     It is obvious that an airship operated by the usual means—throwing over ballast on the one hand or letting out hydrogen on the other—must not only carry a heavy weight of water or sand, thus reducing the quantity of fuel that can be taken for the engines, but that her vitality and endurance will be quickly exhausted by these al­ternating drafts upon her resources. For the reasons just set forth an airship depending upon the usual means could not hope to make a voyage of more than two or three days' duration.