It will be noticed that the blades H on the intake side are so disposed that their concave surfaces are on the advance sides while those in the discharge end of the shell have their convex faces in the retreating side. ACTION OF PUMP.--The hub has inlet ports J below each blade, and discharge ports K between each of the blades I. When rotating the points of the blades H catch the water at the inlet and drive it inwardly through the ports J, from which it passes through the hub to the ports K, and is then violently thrown by centrifugal motion, and by the action of the blades I to the discharge opening C. Should the pump cease working there is always a free passage way for the natural circulation of water through the pump. CHAPTER XI CARBURETERS In considering carbureters it would be well to have an understanding of what is meant by this term. It is the practice to call the vaporized fuel from the carbureter, a gas; but this is a misnomer. It is not a gas, but a vapor, being merely air which is charged with small particles of gasoline. CARBURETED AIR.--It has been frequently termed also a _carbureted fuel_. This is a wrong term. What is meant is _carbureted air_, because the air carries the fuel with it, and is impregnated with a carbon charge. COMPOSITION OF GASOLINE.--Gasoline contains, approximately, 82 per cent. carbon, and 15 per cent. of hydrogen. This mixture of the two fuel elements requires about two parts of oxygen to one part of the gasoline, but as common air is only one-fifth oxygen and four-fifths nitrogen, which does not aid in combustion, it is necessary to supply five times the amount of air, which would mean at least fifteen parts of air to one of the gasoline. In speaking of _parts_ it must not be understood, that reference is made to parts in a liquid form, but it is necessary for the gasoline to be put into the form of a gas, and this gas becomes the measure from which we determine the parts. GASOLINE EXPANSION.--If a cubic inch of gasoline is converted into a gas, it will occupy a space equal to about one cubic foot, which means that it now has a volume, or bulk of 1728 cubic inches. Now, for every 1728 inches, there must be about 30,000 cubic inches of air, in order to make a combustible fuel out of the mixture. REQUIREMENTS OF A CARBURETER.--A carbureter is designed to do several well-defined things: First; it must be able to comminute, or break up the liquid fuel into infinitesimally small particles. Second; it must be able to properly mingle the vapor thus produced. Third; it should be so constructed that it will automatically check the inflow of gasoline, and prevent flooding, or waste of the fuel.