Intermit'tent, coming by fits, not constant. THE science of Hydraulics teaches how to estimate the velocity and force of fluids in motion. Upon the principle of this science all machines worked by water are constructed, as engines, mills, pumps, and others. Water can be set in motion by its own gravity, as when it is allowed to descend from a higher to a lower level; and by an increased pressure of the air, or by removing the pressure of the atmosphere, it will rise above its natural level. In the former case it will seek the lowest situation, and in the latter, it may be forced to almost any height. The syphon is a pipe used to draw off water, wine, or other fluids, from vessels which it would be inconvenient to move from the place in which they stand. It is made of tin or copper, and bent in such a manner that one limb may reach down through the hole in the top of the vessel to be emptied, to its very bottom; the other limb should be the longest, so that when filled it may contain a heavier body of fluid than that within the vessel. The pressure of the atmosphere being taken off from that part of the surface of the liquor within the tube, the liquor rises above its natural level, and flows through the longer limb, and the contents of the vessel are drawn off to the last. There are intermittent springs in various places of the world, which have been explained on the principle of the syphon. A passage for the water may have been formed in the soil, and when the internal cavity has been filled with water, so as to begin to run off by this passage, the pressure of the atmosphere will make the water flow till all is carried off. Of course the spring then ceases until the cavity is again filled with water, when the same phenomenon is repeated. Fluids may be conveyed over hills and valleys in bent pipes, to any height which is not greater than the level of the spring whence they flow. The Romans, either from their ignorance of the pressure of fluids, or from their love of magnificence, conveyed water across valleys by straight-lined aqueducts, which were supported by immense arches or columns. The common pump consists of a large tube or pipe, called the barrel, whose lower end is immersed in the water which it is designed to raise. A kind of stopper, called a piston, is fitted to this tube, and is made to slide up and down by means of a metallic or wooden rod. In the piston, there is a valve, or little door, which opening upwards, admits the water to rise through it, but prevents its returning. A similar valve is fixed in the body of the pump. When the pump is in a state of inaction, the two valves are closed by their own weight; but when the piston is made to ascend, it raises a column of air which rested upon it, and produces a vacuum between itself and the lower valve; the air beneath this valve expands and forces its way through it; and the water, relieved from the pressure of air, ascends into the pump, being forced up by the weight of the surrounding atmosphere. When the piston now descends it is forced into the water, which, as it cannot repass through the lower valve, must rise through the valve of the moveable piston, by the ascent of which, it is lifted up and runs off at the spout. There must never be so great a distance as thirty-three feet from the level of the water in the well, to the valve in the piston, for in that case, the water would not rise through the valve, because the pressure of the atmosphere will not sustain a column of water above that height. But when the water has passed the valve in the moveable piston, it is not the pressure of the air on the reservoir which makes it ascend; it is raised by lifting it up, as you would raise it in a bucket, of which the piston formed the bottom. The forcing pump is not only used to raise water from a well to the surface of the earth, but likewise to force it into reservoirs on the tops of buildings, from which pipes are laid to convey it to different parts as conveniency requires. It differs from the common pump by having the upper piston solid, and a pipe joined to the barrel just above the lower piston, through which the water passes into what is termed the air vessel. In the pipe which leads to the air vessel there is a fixed valve, which opens upwards and prevents the return of the water. Through the upper part of the air vessel a tube is inserted, which reaches nearly to its bottom. Now the air which is above the water in the vessel being confined, and condensed into a smaller bulk than its natural space, presses by its elasticity upon the surface of the water, and forces it violently up the tube in a continual stream. It is upon this principle that the engine for extinguishing fires is constructed. QUESTIONS.-1. What does the science of hydraulics teach? 2. What machines are constructed on the principles of this science? 3. What are the different ways in which water may be set in motion? 4. What is the syphon? 5. Describe the manner of its conveying fluids. 6. How are intermittent springs caused? 7. Describe the common pump and show how it raises water. 8. How high can water be raised in a common pump ? 9. Describe the forcing pump. 10. What engine is constructed on the principle of the forcing pump? 11. Describe the common pump by fig. 21. and show its action. 12. Forcing pump by fig. 22. and show how it acts in forcing up water. LESSON 26. The Diving Bell, and Steam Engine. Vertically, in a direction perpendicular to the horizon. If you take a glass tumbler, and plunge it in water with the mouth downwards, you will perceive that very little water will enter into it. The air which fills the glass prevents the entrance of the water; but as air is compressible, it cannot entirely exclude the water, which, by its pressure, condenses the air in a slight degree. Upon this simple principle machines have been invented, by which people have been able to walk about at the bottom of the sea, with as much safety as upon the surface of the earth. The original instrument of this kind was much improved by Dr. Halley, more than a century ago. The machine was made of copper in the shape of a bell. The diameter of the bottom was five feet, that of the top three feet, and it was eight feet high. To make the vessel sink vertically in water, the bottom was loaded with a quantity of leaden balls. Light was let into the bell by means of strong spherical glasses fixed in the top. Barrels, filled with fresh air, were made sufficiently heavy, and sent down, from which a leathern pipe communicated with the inside of the bell, and a tube with a stop at the upper part let out the air which had become unfit for breathing. The divers are generally let down from a ship, and taking a rope with them, to which is fixed a bell in the vessel, they have only to pull the string, and the people in the ship draw them up; but if business requires it, they will stay several hours at the bottom of the sea without the smallest difficulty. By means of a strong globular cap with circular glasses in front to give light, it has been found practicable for a diver to go out of the engine to the distance of eighty or a hundred yards, the air being conveyed to him in a continued stream by small flexible pipes. Accidents, which through carelessness have sometimes occurred, may be readily prevented, by a proper degree of attention, and people may descend to very great depths without danger. The diving bell has often been used in bringing up the goods from a vessel which has sunk in deep water, and in blowing rocks which impeded navigation. The Steam Engine is one of the most useful, curious, and important machines that have ever been invented. It consists of a large cylinder or barrel, in which is fitted a solid piston like that of the forcing pump. Steam is supplied from a large boiler, which in forcing up the piston, instantly opens a valve, through which cold water rushes, on the principle of the common pump. Other steam is then introduced above the piston, which forces it down, and drives the water out of the pipe. Steam raises the piston again, and again makes it fall, and thus produces an alternate motion, which is communicated, by an upright iron rod, to a large beam or lever, that is lifted up and pulled down with wonderful precision and force. This regular and powerful motion is easily applied by the mechanic to all kinds of machinery. The apparatus has been varied by different persons, and for different objects; but the principle remains the same. By the admirable contrivances of Watt and Fulton, the steam-engine has become a thing stupendous alike for its force and flexibility,-for the prodigious power which it can exert, and the ease, and precision, and ductility with which it can be varied, distributed, and applied. The trunk of an elephant, that can pick up a pin or rend an oak, is nothing to it. It can engrave a seal, and crush masses of obdurate metal before it,-draw out, without breaking, a thread as fine as gossamer, and lift up a ship of war like a bauble in the air. It can embroider muslin and forge anchors,-cut 56 NATURE AND PROPERTIES OF air. steel into ribands; and impel loaded vessels against the fury of the winds and waves. It has armed the feeble hand of man, in short, with a power to which no limits can be assigned; completed the dominion of mind over the most refractory qualities of matter; and laid a sure foundation for all those future miracles of mechanic power which are to aid and reward the labour of after generations. QUESTIONS.-1. What is the principle of the diving bell? 2. What were the dimensions of Dr. Halley's diving bell? 3. How was light let in? 4. Fresh air? 5. How do divers make known their wish to be drawn up? 6. Of what use is this invention? 7. Describe the steam-engine. LESSON 27. Nature and Properties of Air, Den'sity, the degree of closeness and compactness of the parti- THE science which treats of the mechanical properties of elastic or aëriform fluids, such as their weight, density, compressibility, and elasticity, is called Pneumatics. The air in which we live surrounds the earth to a considerable height, revolves with it in its diurnal and annual motion, and, together with the clouds and vapours that float in it, is called the atmosphere. The height to which the atmosphere extends has never been ascertained; but at a greater height than forty-five miles it ceases to reflect the rays of light from the sun. The air is invisible because it is perfectly transparent; but it may be felt on moving the hand in it, or when it moves and produces what we call wind. It is nearly nine hundred times lighter than water, but the whole atmosphere presses on all sides like other fluids, upon whatever is immersed in it, and in proportion to the depths. Its pressure upon a mountain is known to be less than in the plain or valley beneath. If a glass tumbler be completely filled with water, and covered with a piece of writing paper, so as to hold it tight, and accurately even, the water will not run out although the glass be inverted and the hand removed. The |