“The fields of the dirigible and the air plane are separate and there is no conflict between the two. For light loads, great speed, and quick manœuvering, the airplane is supreme. For heavy loads, long distance, ability to remain in the air for great periods of time, the dirigible is the only air craft that can fulfill the requirements. Dirigibles will soon be in use which can start from Europe, sail over New York, and drop enough poison gas to kill thousands and make practically the whole city uninhabitable.” CHAPTER IV EARTH’S FOUR ATMOSPHERES The earth has four important atmospheres and others of less importance. The principal ones are oxygen, nitrogen, vapor of water, and carbon dioxide, each comporting itself as it would do if the others were not present. There is space between the molecules of each gas, and therefore it is easily compressed. A doubling of its pressure reduces its volume one half. =Composition of Atmospheric Air.= It is difficult for the mind to form a picture of the infinitely small molecules of the air. Let us therefore use terms and comparisons that will the more directly appeal to the human senses. First let us imagine each molecule enlarged to the size of a small grain of sand. Then with the molecules from one cubic inch of air transformed into grains of sand we could build a roadway ten feet deep and one hundred feet wide extending from New York to San Francisco. May one still further grasp the idea of the atom, many of which are required to make up the molecules? If so, the imagination has been stretched to its limits to enable the human mind to comprehend some of the simplest facts with regard to the wonderful fluid in which we live. Sir William Thomson, afterwards Lord Kelvin, in endeavoring to give relative values that would appeal to the imagination, said that if a drop of water were enlarged to the size of the earth, the molecules of which it is composed would be no larger than cricket balls, and the smallest about the size of small peas.