179. =Fire-Engine.=--The fire-engine has commonly two forcing-pumps, with a contrivance for making the water issue in a uniform stream. This contrivance can be explained in Fig. 125. The discharging pipe, L M, extends down into a large vessel, I K, which is filled with air. The uniformity of the stream depends upon the elastic force of compressed air, as you will see if I explain the operation of the machine. When the water is forced through the opening H, it rises to the level N O, compressing the air in I K, for the tube L M is too small to allow all the water to escape that comes from the larger tube, H E. Now the moment that the piston ceases to force the water through H, the elastic force of the compressed air operates, shutting down the valve H, and forcing the water up L M. The result, you see, is a continuous forcing up of the water through this tube, and therefore a uniform stream. CHAPTER X. MOTION. 180. =Universality of Motion.=--The world is full of motion. The rising and setting of the sun, the changes of the seasons, the falling of the rain, the running of rivers into the ocean, the ascent of water into the air by evaporation, the wind moving in silence or rushing on in its might, are familiar examples of motion constant and every where present. But with all this motion, sometimes in conflict and often variable, order and regularity reign. The causes of motion, though various in their operation, are kept by the Creator from producing confusion and disorganization by a few simple laws, which regulate the movements both of atoms and of worlds. The principal of these causes I will now briefly notice. 181. =Causes of Motion.=--Attraction is the most universal of the causes of motion in the universe. While it binds atom to atom, it also binds system to system throughout the immensity of space; and while it makes the stone fall to the ground, it moves the countless orbs forever onward in their courses. It is this which causes the tides to flow and the rivers to run down their slopes to the ocean, and thus by keeping up the never-ending motion of water all over the earth in seas, lakes, rivers, and the millions of little streamlets, diffuses life and beauty over the vegetable world, and gives to man the vast resources which we see developed in the numberless applications of water-power and navigation. Heat pervades all matter, and is every where uniting its influence with the other causes of motion. It is heat that produces all the motions of the air, termed winds. It is heat that causes the rise of the water all over the earth in evaporation, so that it may be collected in clouds, again to descend to moisten the earth and keep the ever-flowing rivers full. Heat applied to water gives to man one of his best means of producing motion in machinery.