state; it is found that the temperature at which this change of state takes place diminishes when the pressure of the atmosphere is decreased. Under the ordinary pressure of the atmosphere, water boils at 212 degrees of Fahrenheit's thermometer; if the pressure is reduced so as to be measured by 23 inches of mercury, water boils at 200 degrees. Thus if water which is hot, though much below the ordinary boiling temperature, be placed under a receiver and the air exhausted the water soon begins to boil furiously. By observing the temperature at which water will boil on the top of a mountain we may form a good idea of the height of the mountain, supposing that we have a Table in which are recorded the results of trials already made at various elevated places. Thus at the summit of Mont Blanc water boils at 187 degrees; so that if we found water to boil on any mountain at that temperature we might assume the height to be equal to the height of Mont Blanc. It is found that a diminution of about one tenth of an inch in the height of the barometer corresponds to a diminution of about one sixth of a degree in the temperature at which water boils. It is obvious that when water boils at a low temperature inconvenience may arise from the fact that we cannot easily obtain water at so high a temperature as we require; it is said that the monks at the monastery of St Bernard cannot make good soup or good tea, because on account of their high situation water boils at too low a temperature. But by boiling water in closed vessels it is possible to produce so great a pressure as to carry the boiling point far above the ordinary temperature of 212 degrees. Other processes besides that of boiling are promoted by diminishing the pressure of the atmosphere; if a bottle of champagne is opened on the top of a high mountain the wine may burst forth and be almost entirely lost.

532. The experiment of making water boil at a low temperature, by diminishing the pressure on its surface, can be performed in a striking manner without the aid of the air pump. Water is put into a glass flask, so as to occupy about half of it; then the water is boiled by placing a lamp beneath the flask, so that the upper part of the flask becomes full of steam, the air being expelled.

The flask is now stopped with a cork, removed from the lamp, and allowed to cool down to a temperature below 212 degrees. By pouring cold water on the upper part of the flask the steam is cooled and some of it is condensed, so that the pressure on the surface of the water is much diminished; and in consequence the water begins to boil again. The experiment requires great care to prevent accidents.

533. A celebrated illustration of the pressure of the atmosphere is called the experiment of the Magdeburg hemispheres. Two hollow hemispheres are constructed of brass; they fit accurately together so as to be air tight and to form a hollow sphere: the two parts can however be separated with ease. A small pipe furnished with a stop-cock is fixed to one of the hemispheres; this pipe can be connected with an exhausting cylinder, such as we have described in Art. 521, and so the air can be withdrawn from the interior of the hollow sphere: the stopcock is then closed to prevent the entrance of fresh air, and the sphere may be removed from the cylinder. Now if we attempt to pull the hemispheres apart we find that there is a great resistance to prevent the separation; this is due to the pressure of the atmosphere on the external surface, and its amount may be readily assigned. We must find the number of square inches in the area of a section of the sphere through its centre, and multiply it by 15 to obtain the pressure in pounds. The experiment was devised by Otto Guericke of Magdeburg, the inventor of the air pump. He constructed such a pair of hemispheres one foot in diameter; the area of the section in this case is about 113 square inches, and multiplying this by 15 we obtain about 1700 for the number of pounds. Thus if we hang up the sphere when the air is exhausted, and attach a weight of about 1700 pounds to it, the two hemispheres will not be separated. This supposes the air to be completely exhausted, but even with only partial exhaustion a very great force is necessary in order to separate the two hemispheres. The rule we have given for estimating the amount of the pressure which urges one hemisphere against the other may be easily justified. Imagine one hemisphere placed mouth downwards on a

smooth horizontal table, and the air exhausted from the space between it and the table; then the resultant pressure on the external surface of the hemisphere is in fact the weight of the column of the atmosphere which stands on the portion of the table covered by the hemisphere, and reaches up to the limit of the atmosphere. The amount of this we know to be 15 pounds on each square inch of the circular area of the table which is covered by the hemisphere.

534. The air pump is a machine for withdrawing air from an enclosed space; there is also a machine called the condenser by which air may be forced into an enclosed space to any amount we please. But this instrument does not furnish us with any very important experiments, and so a brief notice of it will suffice. Take the diagram of Art. 521, and suppose the valves to open downwards instead of upwards. Let the piston be in its highest position; then when it is forced down, the pressure of the air between D and C opens the valve C, and being greater than that of the atmosphere keeps the valve D closed. Thus when the piston has reached the bottom, the air which was originally in the cylinder has been forced through C into the pipe E, and the receiver with which the pipe is connected. While the piston is being drawn up, the valve C is closed by the pressure of the air below it, while the valve D is opened by the pressure of the atmosphere. Thus when the piston is at the highest point the cylinder is again full of air, and the whole process may be repeated. Every complete operation forces into the receiver and pipe as much air as would fill the cylinder under the ordinary pressure. Suppose, for example, that the volume of the pipe and the receiver together is nine times the volume of the cylinder; then at each descent of the piston, air equal in quantity to of that originally in the pipe and the receiver is forced through C. Thus at the end of five operations the air in the pipe and the receiver consists of the original air together with more; and at the end of nine operations there is just twice the original quantity of air in the pipe and the receiver.

1 9

9

535. The air gun is an instrument of no practical importance, but which may be noticed as its action depends on the condensation of air. A strong chamber is constructed into which air is condensed until the elastic force of the whole is very great. The chamber is connected with a tube in which a bullet is placed; by opening a valve the condensed air rushes out and sweeps the bullet along the tube, from which it issues with great velocity: the force which drives the bullet along the tube is the excess of the pressure of the condensed air behind, above the ordinary pressure of the atmosphere in front.

XLIX. PUMPS.

536. There are various machines for raising water from one level to another which is higher; and we will now describe some of them.

D

537. The common pump sometimes called the suction pump. AB is a cylinder having at the bottom a valve C opening upwards. A piston works up and down in the cylinder, having a valve D opening upwards. A pipe BE passes from the bottom of the cylinder, and the end of it is below the surface of the water in a well; let E denote the level of the water in the well. Suppose the piston to be at C, and the pipe to be full of air. Let the piston be raised to A; then the pressure of the atmosphere keeps the valve D closed, and the pressure on the valve C being lessened the air in the pipe opens this valve and fills the cylinder below the piston. The pressure of the air in the pipe is now

C

B

M

E

less than that of the atmosphere, and accordingly the pressure of the atmosphere on the surface of the water in the well forces water up the pipe EB to such a height as to make the pressure at E equal to that of the atmo

sphere. When the piston descends the valve C closes, and the air between C and the piston escapes through D. The water will rise in EB each time this operation is repeated until at last it passes through C; and now when the piston descends to C the water passes through D and is then carried up by the piston as it ascends and discharged through the spout at A.

538. It will be observed that the ascent of the water consists in general of two distinct processes. The water is received from E to B by the pressure of the atmosphere, and in consequence of this EB must not be higher than the column of water which this pressure would support, that is about 34 feet. But the length AB may be as great as we please, provided that we have a cylinder and a piston rod of sufficient strength, and force enough to do the requisite work. For when the water reaches to a point in the cylinder the height of which above E is greater than the standard 34 feet, the pressure of the atmosphere will take it no further, and it must be lifted by the ascending piston from this point up to the spout. If the height of the spout above E is not greater than the standard 34 feet, then we have not the two processes but only the first of those just noticed.

539. An error is frequently made with respect to the amount of force which must be used to work the piston; it seems to be imagined that the pressure of the amosphere renders, or ought to render, any application of force to the piston unnecessary. Let us suppose that the piston is at some point between A and B, and that the water in the pipe has risen to the level M; so that between M and the piston there is air. Then above the piston we have the pressure of the atmosphere; and below the piston we have this pressure diminished by so much as corresponds to the height of the column EM. Thus on the whole the piston is urged down by a pressure which is measured by the height of the column EM of water; and so force must be applied sufficient to overcome this. But if the height of the piston above E is greater than the standard 34 feet, then below the piston there is a vacuum, and the pressure above it is the pressure of the atmosphere increased by the weight of the water which is to be lifted.