From the most accurate observations it has been deduced that storms consist of bodies of air rotating round a centre. The little eddies that we observe upon the street, on a dusty day, are examples, on a small scale, of what we mean. Here it will be noticed that the particles of dust are carried round and round, and that, along with this motion of rotation, there is a motion of translation, as it is called, of the entire whirling mass from one place to another. In the case of great storms these two motions are also combined; the air whirls with great violence round a centre, while, at the same time, the centre itself is continually carried forward. Imagine a rotating cylinder of air thus impelled: the base of the cylinder, being in contact with the earth, encounters friction and other obstructions, is retarded, and hence the cylinder leans forward. The upper portion of the cylinder may, therefore, be over a place long before the bottom has reached it; the storm may be active above, while all is calm below. How then is the pressure of the air influenced by one of these atmospheric whirlpools? Owing to its swift rotation, the air is driven out from the centre on all sides, so that in the neighbourhood of the centre the space is partly emptied of air, and consequently cannot support a high barometric column. At the centre itself the depression is a maximum. Reflecting upon these facts, there is no difficulty in conceiving how the barometer may be affected long before the storm has reached it, and thus serve as a valuable warning to those who are able to interpret its indications.

LESSON VI.

THE RELATION BETWEEN THE VOLUME AND THE PRESSURE OF ELASTIC FLUIDS.

1. Law of Mariotte.-Having learned that air is compressible, our next step is to determine the exact relation between the amount of the compression and the force which produces it. This is done by means of the apparatus sketched in fig. 16. ABC is a bent tube of glass, open at A, and furnished at c with a good

A

stop-cock. When the cock is open, let a quantity of mercury be poured into A, sufficient to close the bent portion of the tube at B, and to rise to the level of dd', half an inch or an inch above the bent porLet the cock c be now

tion. closed

B

Fig. 16.

we have then a quantity of air enclosed in the space above the mercury surface d', and this is the air which we will submit to compression. Let mercury be poured in at A: as the column in A B heightens, the surface d' will be observed to ascend gradually, the air above it being squeezed into a smaller space by the pressure of the mercury. When the volume of air has been squeezed into exactly half its bulk, let the pouring in at a cease. Let the difference of level of the mercury in both arms of the tube be now ascertained: it will be found that this difference is about 30 inches. If A Bв be long enough, mercury can be poured in at A until the air in BC has been squeezed into one-third of its original volume; we should then find the difference of level in both arms of the tube to be 60 inches. To reduce the air to one-fourth of its volume, a column of 90 inches would be requisite, and so on. But how is it that the increase goes on by additions of 30 inches? Let us consider this state of the air in the tube BC before the cock at B was closed. It is evident that it then bore the pressure of the atmosphere above it, which, as already explained, is equal to that of 30 inches of mercury. When the tube A B is filled to a height of 30 inches above the level in BC, it is plain that two atmospheres, as they are called, are pressing on the air in B C. When the height of the pressing column is 60 inches, three atmospheres press upon the air; when 90 inches, a pressure

of four atmospheres is exerted, and so on. Now, the experiment shows us that by doubling the pressure we reduce the volume of air to one-half, by trebling the pressure we reduce it to one-third, by increasing the pressure four times we diminish the volume four times; and hence we arrive at the important law—a law first discovered by the celebrated Robert Boyle, and rediscovered by Mariotte, whose name it usually bears that the volume of a gas is inversely proportional to the pressure under which it exists.

2. For a long time it was considered that this law was perfectly true for all gases; but it is now known that this is not in strictness the case. When compressed in the manner we have described, some gases are finally reduced to the liquid condition. By cold and pressure, Mr. Faraday has liquified numbers of them. And it is found that those gases which are most easily liquifiable, or which, in other words, are nearest to their points of condensation at ordinary temperatures, exhibit very sensible deviations from the law of Mariotte. Carbonic acid and sulphurous acid are examples of gases of this class; while other gases, amongst which are the constituents of our atmosphere, have resisted all attempts hitherto made to liquify them. With these the law of Mariotte is true to a far greater extent than with the others.

3. To imprint the important principle above established upon the mind, it would be useful to work a few examples in which the law is applied. Want of space prevents us from giving more than one here.

Example 1.-Assuming the law of Mariotte to be true for all pressures, and that water is incompressible; at what depth below the surface of the sea would a bubble of air have the same density as the surrounding water, supposing the weight of the water to be 840 times that of the air at the surface, and that a column of 32 feet of water exercises a pressure of one atmosphere ?

What is required here is to know what pressure will reduce a volume of air tooth part of the bulk it possesses at the surface of the sea, where the pressure

upon it is one atmosphere. According to the law of Mariotte, a pressure of 840 atmospheres would accomplish this; and, hence, a depth of 32 feet of water being equal to one atmosphere, it might be thought that we have only to multiply 840 by 32 to find the depth sought. But it must be remembered that the air at the surface already bears the pressure of an atmosphere of air, and to these 839 atmospheres of water pressure must be added to make up the 840. Hence, multiplying 839 by 32, we obtain 26,848 feet as the depth sought.

Below this depth, if the above conditions continued to hold, the air would be heavier than the water, and would therefore sink instead of rise. It must not, however, be forgotten, that the case here supposed is an imaginary one, for water is not incompressible, and it is certain that long before the pressure referred to has been attained, the law of Mariotte ceases to be true.

4. Further Effects of Atmospheric Pressure.-In the parometer we see a column of mercury prevented by the pressure of the atmosphere from flowing out of the tube which contains it. If the top of the tube were perforated so as to permit the air to enter, the mercurial column would immediately sink. Such a perforation is made in the lid of a teapot, and if the lid fit tightly, you will observe that the water flows, or ceases to flow, according as the vent-hole is opened or closed. In water-glasses for birds, the liquid within the glass is sustained by the atmospheric pressure; the bird drinks from the bowl a (fig. 17), and finally the water within the bowl sinks, until a small bubble of air enters by the tube b. and ascends to the summit of the glass; here, by its elastic force, it presses the liquid downwards, and again partially fills the bowl a. In this way the process is continued until the whole of the water has been used. The pneumatic ink-bottle is made upon the same principle as the bird-glass: the bowl Fig. 17. into which the pen dips is supplied in the same manner as that from which the bird drinks. Many lamps are so constructed that the oil is gradually supplied to the

α

b

wick in the same manner as the water is supplied to the bird, or the ink to the pen.

P

5. In speaking of the weight of air, we referred to the case of a flask from which the air had been removed: we will now describe how this removal may be accomplished. A sketch of the exhausting syringe is given in fig. 18. c is a cylinder on which the solid piston P moves air-tight; c is a cock by means of which the external atmosphere may be admitted to, or shut off from, the space underneath the piston; d is a tube furnished with a thread at its lower extremity, on which the vessel to be exhausted, F, is sewed; e is a cock by means of which a communication may be established at pleasure between the flask and the cylinder C. The vessel being screwed on, let the cock e be opened, and c closed. When the piston P is raised, a portion of the air passes from F through e, and diffuses itself in the space below the piston. After the piston has reached its highest limit, let e be closed and c opened; in forcing P down, the air beneath it will escape through c into the atmosphere. Closing c and opening e again, the same process is repeated; at each successive ascent of the piston, a portion of the air is taken from F, and if the pumping continue long enough, it is evident that we can exhaust F almost completely.

F

Fig. 18.

We might modify the exhausting syringe by doing away with the tube e, and making an aperture through the solid piston, to be closed by a valve opening upwards, as in the case of the common pump.

6. In the air pump the exhaustion is hastened by making use of two cylinders, instead of one, the pistons of both being worked by a single winch. The instrument is shown in fig. 19. cc' are the two cylinders in