Q, and thus there is really a slight force in addition to the pressure of the atmosphere exerted to sustain the column of mercury, namely, the excess of the weight of P over the weight of Q. Moreover when the pressure of the atmosphere is diminished, so that the mercury in the shorter branch has a tendency to ascend, the ball must be raised; and thus a change of pressure so minute as not to be sufficient to raise P would not be exhibited by the instrument. 514. It is said that during the great earthquake at Lisbon in 1775 the mercury in the barometer in England fell so low as to disappear from that portion of the tube which is usually left uncovered for observation. The rapid fall of the barometer at sea has sometimes given warning of a coming storm when the most experienced sailors would not otherwise have had any suspicion of danger. The barometer is often popularly supposed to serve as a weather glass, and the makers of wheel barometers are in the habit of putting such words as Rain, Fair, Changeable, against parts of the graduated circle over which the end of the pointer ranges. But experience shews that there is no very close correspondence between the weather actually occurring and the contemporaneous position of the pointer. By Arts. 508 and 512 it is easy to see that there may be 5 inches difference on the circumference of a wheel barometer between the position of the end of the pointer at the same epoch at the level of the Thames and at the top of St Paul's Cathedral, while the weather is probably of the same character at the two stations. The changes of the atmospheric pressure may however be made to give some suggestions as to the weather likely to follow such changes, especially with regard to a particular place, if observations are carefully made and studied there. The following rules are given in Dr Lardner's Treatise on Hydrostatics and Pneumatics as in general fairly trustworthy. (1) Generally the rising of the mercury indicates the approach of fair weather; the falling of it shews the approach of foul weather. (2) In sultry weather the fall of the mercury indicates coming thunder. In winter, the rise of the mercury indi cates frost. In frost, its fall indicates thaw, and its rise indicates snow. (3) Whatever change of weather suddenly follows a change in the barometer may be expected to last but a short time. Thus, if fair weather follow immediately the rise of the mercury there will be very little of it; and, in the same way, if foul weather follow the fall of the mercury it will last but a short time. (4) If fair weather continue for several days, during which the mercury continually falls, a long succession of foul weather will probably ensue; and again, if foul weather continue for several days, while the mercury continually rises, a long succession of fair weather will probably succeed. (5) A fluctuating and unsettled state in the mercurial column indicates changeable weather. 515. One of the most general laws which has been observed in meteorology is expressed thus: the barometer usually falls when the thermometer rises, and the barometer usually rises when the thermometer falls. The reason is simple. When the thermometer rises the air expands and consequently overflows into the neighbouring regions; and so the pressure of the atmosphere is diminished, and the barometer falls. On the other hand, when the thermometer falls the air contracts, and this produces an influx from the neighbouring regions, and consequently an increase of pressure, and so the barometer rises. 516. An instrument called the Aneroid Barometer has been introduced in recent years for measuring the pressure of the atmosphere. It consists of an elastic metallic chamber either in the form of a flat box or of a short tube; the air is exhausted from the chamber which is then closed so as to be air-tight. When the pressure of the atmosphere increases the chamber slightly contracts, and when the pressure of the atmosphere decreases the chamber slightly expands. These changes are transmitted by a system of wheels and levers to a pointer which moves over an index; this index is graduated by trial, by the maker of the instrument, so that the reading against the pointer in any position records the corresponding pressure of the atmosphere. The instrument is recommended by its portability, for it can be made to take up little more room than a watch; but it is liable to gradual changes, so that it ought to be compared from time to time with a good mercury barometer. An Aneroid is a serviceable companion for an Alpine tourist, as it enables him to form a good estimate of the height he has reached. Suppose the tourist to make several observations with it in the course of a day; then even if the instrument is a little wrong the error will probably be the same at every station, so that the difference of the heights of any two stations will be correctly given : and the amount of error of the instrument at the time will be known if one of the observations is made at a station of which the actual height has been well settled by other means. 517. It appears as we have stated in Art. 491, that the pressure of the atmosphere is about 15 pounds on every square inch; and thus it may at first sight seem very strange that we are not conscious of this great pressure. But the fact is that the air is all around us; and also the internal parts of the body are filled with fluids, in the liquid or gaseous state, which exert a pressure from within equal to that of the atmosphere from without. 518. The applications of the pressure of the atmosphere are numerous and important. As we shall see hereafter, pumps for raising water act by means of this pressure. In a common pair of bellows when the upper board is raised. the pressure of the atmosphere forces air through a hole in the lower board; a small valve prevents the air from escaping through this hole when the upper board is pressed down, and thus the air is driven through the nozzle of the bellows. Most persons must have seen the large constructions in which the gas used for lighting the shops and houses of a town is stored for consumption. They consist of pits lined with iron and containing water; the gas is confined between the water and a large covering vessel, in the same manner as the air in the experiment of Art. 486 is retained in the upper part of the inverted tumbler. When beer is drawn from a cask it flows at first because a little air is left at the top of the cask, and this, though expanding as beer is drawn out, still exerts for a time sufficient pressure, with the assistance of the weight of the liquid, to overcome the pressure of the air which opposes the issue of the beer from the tap. After a time the air at the top of the cask is so much expanded that it no longer exerts sufficient pressure, and so the beer will not flow. Then a little more air is let in through a hole at the top of the cask provided for the purpose, called the vent-peg, which is kept closed until it is thus necessary to open it for a short time. The gurgling sound which is heard when we pour water from a decanter that is nearly full, arises from the air forced in by the pressure of the atmosphere to supply the place of the water withdrawn; the sound continues as long as the neck of the decanter is choked by the water escaping. But as the water is gradually withdrawn room is obtained in the neck of the decanter for water to pass out through part of the neck and for air to enter through the rest of the neck. A B 519. An interesting process for estimating the magnitude of the pores of bodies as compared with that of the solid parts depends on the use of the pressure of the atmosphere. Some substances, as charcoal and pumice stone, contain an immense number of small cavities, and to these the process may be applied. Take a long glass tube open at both ends, and fill a portion AB with charcoal, supporting the charcoal at B by a perforated partition which will allow air to pass through. Plunge the tube in a vessel of mercury to the level B, then cover the end A, and withdraw the upper part of the tube from the mercury. If there had been no air in the cavities of the charcoal the mercury would remain in the tube at the usual height of 30 inches above the level of the mercury in the vessel. But when the pressure of the atmosphere is diminished the air in the cavities of the charcoal issues from the cavities and expands, and by its elastic force compels the mercury to stand at a lower height than it would otherwise reach. Suppose the mercury stands at C at the height of 15 inches above the level in the vessel. Then the air in the pores and in BC, being under half the atmospheric pressure, occupies just double the space it did formerly; and thus the space denoted by BC is just equal to the volume of all the pores. It is found in this way that the solid part of charcoal is really about four times as heavy as water, bulk for bulk, although charcoal is usually taken to be about half as heavy as water, bulk for bulk. The solid matter of pumice stone is found to be as heavy as marble, bulk for bulk. XLVII. AIR PUMPS. 520. It is important to be able to examine the consequences which result when bodies are withdrawn from the influence of the pressure of the atmosphere. Accordingly machines are constructed by the aid of which we can withdraw the air almost entirely from certain closed vessels, and perform various interesting experiments in the empty space. These machines are known as Air Pumps. 521. The construction of Air Pumps may vary a little as to details, but the principles are the same in every case. A plate of brass or other metal, made exactly plane, is provided, and on that is placed a strong glass bell with its mouth downwards; this vessel is called the Receiver. The glass at the mouth is ground very smooth, so that it may fit exactly on the metal plate. To ensure that the contact between the two shall be air-tight, it is usual to smear the mouth of the glass with lard or some other unctuous substance. The air is then withdrawn from the glass vessel by a pipe which passes through the metal plate ; and we shall now describe the way in which this is effected. AB is a cylindrical vessel in which a piston can move up and down. At the bottom of the cylinder there is a valve C which opens upwards. There is also a valve D in the piston which opens upwards. A pipe E passes from the bottom of the cylinder and communicates with the receiver. Suppose the piston to be at the bottom of the cylinder, and that the receiver and the pipe contain air of B |