Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/401

Rh was given in half-lines in Russia, which, equalling the twentieth of an English inch, were readily reduced to English inches by dividing by 20. The metric barometric scale is now used in Russia. In a few countries on the Continent the French or Paris line, equalling - OSSS14 inch, still continues to be used. Probably millimetre and English inch scales will soon be exclusively in use. The English measure of length being a standard at G2 Fahr., the old French measure at 61 2, and the metric scale at 32 3, it is necessary, before comparing observations made with the three barometers, to reduce them to the same temperature, so as to neutralize the inequalities arising from the expansion of the scales by heat. The barometer is a valuable instrument as an indicator of coming weather, provided its readings be interpreted with intelligence. High pressures generally attend fine weather, but they not unfrequently accompany wet stormy weather ; on the other hand, low pressures, which usually occur with wet and stormy weather, not unfrequently ac company fine mild weather, particularly in winter and in the northern parts of Great Britain. The truth is, the barometer merely indicates atmospheric pressure directly, whilst it indicates weather only inferentially. The chief points to be attended to are its fluctuations taken in con nection with the wind and the state of the sky, but above all, the readings of the barometer as compared with those at neighbouring places, since it is difference of pressure, or the amount of the barometric gradient, which deter mines the strength of the wind and the weather generally.

Barometrical Measurements of Heights.

The decisive experiment by which Pascal established the reality of atmospheric pressure suggested to him the method of measuring heights by means of the barometer. The first attempts to effect this were necessarily rude and inaccurate, since they went on the assumption that the lower mass of air is of uniform density. The discovery, however, of the actual relation sub sisting between the density of air and its elasticity by Boyle in England, and about the same time by Mariotte in France, laid a sure foundation for this branch of atmospheric physics the relation being that, at the same temperature, the pressure of a gas is exactly proportional to its density.

The truth of this law may be shown by the following experiment. Take a glass tube, of equal bore throughout, closed at one end, and bent in the form of a siphon (fig. 1), and let us suppose that it contains in the closed limb a portion of air AB, shut off from the atmosphere by mercury filling the lower portion of the tube, and that the enclosed portion of air exists at the ordinary pressure of the atmosphere or 30 inches. In this case the mercury in each limb, being subject to the same pressure, will stand at the same level. If we now pour mercury into the long limb (fig. 2) till the level in this limb stands 30 inches above the level in the closed limb, the additional mercury will tend to compress the air in A B with a pressure equal to that exerted by a column of 30 inches of mercury. In the latter case, therefore, the air is subjected to a pressure of two atmospheres, or GO inches, while in the former it was only subjected to a pressure of one atmosphere or 30 inches. It will be found that the space A B under the pressure of two atmospheres is only half the space AB where the pressure is only one atmosphere. If mercury had been filled in till the difference of level of the mercury in the two limbs was GO inches, or a pressure of three atmospheres, the space occupied by the air in the closed limb would have been only a third of the original space when the pressure was only that of one atmosphere. Generally, Boyle s law or Mariotte s law is this : The volume of a gas varies inversely as the pressure. Since the same quantity of air has been experimented with, it follows that the density is doubled with a pressure of two atmospheres, and trebled with that of three, and hence the pressure of a gas is proportional to its density. This law, however, only holds provided the temperature is the same. The familiar illustration of a bladder, partially filled with air, expanding on being placed near a fire, shows that if the pressure remains the same, the pressure in this case being that of the atmosphere, the gas will occupy a larger space if its temperature be raised. If the temperature be increased and the air be confined so as to occupy the same space, the pressure will be increased. The relation between the temperature and pressure of gases was first discovered by Gay-Lussac; and more recently our knowledge of this branch of the subject has been greatly enlarged by the beautiful and accurate experiments of Regnault. From those experiments it has been concluded that the co-efficient which denotes increase of elasticity for 1 Fahr. of air whose volume is constant equals 002036 ; and that the co-efficient which denotes increase of volume for 1 Fahr. of air whose elasticity is constant equals 002039. It may further be added that the co-efficient of expansion for carbonic acid gas, hydrogen, and all other gases, is as nearly as possible the same. When a fluid is allowed to evaporate in the exhausted receiver of an air-pump, vapour rises from it until its pressure reaches a certain point, after which all further evaporation is arrested. This point depends on the nature of the fluid itself and on the temperature, and it indicates the greatest vapour pressure possible for the fluid at the particular temperature. Regnault has shown the amount of the vapour pressure of water at different temperatures, thus—

Temp. Fahr. Max. Pressure of Vapour, inch. Temp. Fahr Max. Pressure of Vapour inch. 0-044 6*0 0-361 10 0-068 60 0-518 20 0-108 70 0733 30 0165 80 1-023 40 C 248 90 1-410 If gases of different densities be put into the same vessel it is found that they do not arrange themselves according to their densities, but are ultimately diffused through each other in the most intimate manner. Each gas tends to diffuse itself as in a vacuum, the effect of the presence of other gases being merely to retard the process of their mutual diffusion. As regards the atmosphere, evaporation goes on until the maximum vapour pressure for the tem perature has been attained, at which point the air is said to be saturated, and whilst the temperature remains the same further evaporation is arrested. Thus, at a tempera ture of 50 evaporation goes on until the vapour pressure reaches 361 inch, but if the temperature were raised to GO the process of evaporation would be renewed, and go on till the vapour pressure rose to 518 inch. If at a vapour pressure of 518 inch the temperature were to fall from 60 to 50, the air would no longer be capable of retaining the whole of the aqueous vapour in suspension, but the surplus part would be condensed and fall as rain. In the change from the aeriform to the liquid state a quantity of latent heat is given out. The yet uncertain effect of these changes, particularly the change of form from the aeriform to the liquid state, on the pressure, temperature, and movements of the air, renders it peculiarly desirable that barometeric observations for the determination of 