of which is dependent upon the size of the interior diameter of the tube, and a correction for this must be added to the apparent height, in order to obtain the true altitude. In tubes of a small bore, the error from this source is considerable; but when the diameter exceeds half an inch, it becomes so small, that it may safely be neglected. This will be rendered evident by the inspection of the following table, which gives the amount of depression for tubes of various sizes.

19. When the instrument is not stationary, but is carried from clime to clime, and to different heights above the sea-level, two other corrections are necessary; one for the varying force of gravity, in different latitudes, and the other for the change of pressure, which diminishes with every increase of altitude above the ocean.

Such is the barometer, an instrument of great practical use, and of the highest value in meteorological researches.

PRESSURE OF THE ATMOSPHERE.

20. VARIATION IN LATITUDE. The mean or average pressure of the atmosphere, as indicated by the barometer, is found to be nearly the same in all latitudes, when every essential correction is made. It increases a little from the equator to about the 30th degree of latitude, where it is greatest; it then decreases to nearly the 64th degree, where it is least; after this it again increases, and between the 75th and 76th degrees, the pressure is equal to that of the equatorial climes. All

Is it greater in tubes of a small or large bore?

When the barometer is portable, what other corrections are necessary? What is said of the barometer ?

In what manner does the pressure of the atmosphere vary in latitude ?

this is obvious from the following table, founded upon observations, where corrections are made for gravity, altitude above the sea-level, and temperature.

21. The pressure of the atmosphere at any given spot is not invariable; for the height of the barometer is perpetually changing throughout the year. The extent of its fluctuations is, however, by no means the same in all places, being least at the equator, and greatest towards the poles. Thus its range within the tropics is but a little more than one-fourth of an inch; at New York, 40° 42′ 40′′ N. lat., 2.265 inches, from the observations of five years; at St. Johns, Newfoundland, 47° 34' 3" N. lat., 2.54 inches, during the same period; while in Great Britain it amounts to three inches. The greatest fluctuations occur between the 30th and 60th degrees of latitude.

22. There is also a constant daily variation in the atmospheric pressure, for the barometer, as a general rule, falls from 10 o'clock, A. M. to 4, P. M.; it then rises until 10, P. M., when it again begins to descend, reaching its lowest point at 4, A. M.; from this time it rises, until it once more attains its highest elevation, at 10, A. M. These variations are exceedingly minute, and contrary to the annual range, are greatest at the equator, and decrease with the latitude; disappearing about the parallel of 60°.

23. This variation amounts at

Give examples.

Where are the annual fluctuations of the barometer least?
Where greatest?

Give examples.

Describe the diurnal variations. Where greatest? Where least?

In the tropical regions, according to Humboldt, so regular are the diurnal changes, that the barometer indicates true time, within a quarter of an hour.

These daily fluctuations, in the atmospheric pressure, for a long time, perplexed meteorologists, but their cause has, at length, been discovered, by means of the late observations, at the English observatories. They are found to arise from the stated variations in temperature, that occur during the day.

24. VARIATIONS IN ALTITUDE. As we ascend above the surface of the earth, we leave a portion of the atmosphere below us, and are freed from its pressure. This fact is denoted by the fall of the barometer. When De Luc, a French philosopher, ascended to the height of 20,000 feet, his barometer sunk to twelve inches. In 1838, the aëronaut Green, rose from Vauxhall gardens, in London, to an elevation of nearly three miles and three quarters; the mercury in the barometer gradually descending, from thirty inches to fourteen and seven-tenths.

25. As a general rule, this depression, near the surface of the earth amounts to one-tenth of an inch for every eighty-seven feet in altitude; but where perfect accuracy is required, several corrections must be made. The barometer then becomes, in the hands of skillful observers, an important instrument for determining altitudes, and so exact are its indications, that two inde

Give examples.

What is said by Humboldt of their regularity in the tropics?

How are they caused?

How is the pressure of the air influenced by the altitude?

What instrument indicates the changes of pressure?

In the instances given, how low did the mercury sink?

What is the law of depression?

For what purpose is the barometer sometimes employed?
Give instances.

pendent estimates of the height of Mount Etna, made by means of this instrument, differ only one foot; that of Capt. Smyth being 10,874 feet, while Sir John Herschel's is 10,873 feet.

DENSITY OF THE ATMOSPHERE.

26. When one portion of the atmosphere is said to be more dense than another, all that is meant is simply this; that a given volume, or bulk, of the first portion, as one gallon, contains more aërial particles than an equal volume of the second; thus, if it contains twice as many particles, it is said to be twice as dense.

Fig. 2.

27. The density of the atmosphere decreases with the altitude. This result is caused by the diminished pressure of the air, and the decreasing force of gravity. Imagine the atmosphere to be divided into a vast number of thin, concentric strata, which in figure 2, are represented by the spaces between the lines 1-2, 2-3, 3-4, 4-5, &c.

Now it is clear, that the particles in

ATMOSPHERIC STRATA.

each layer are pressed together by the whole weight of the atmosphere above them, while, at the same time, they are drawn together by the force of gravity. Variations in the latter power are only appreciable at great distances from the earth, and the observed changes in density, at two or more stations, may therefore be ascribed to the difference in the weight or pressure, of the superincumbent atmosphere. The height of the barometer, at different elevations, thus denotes the density of the air at these points.

When is one portion of the atmosphere denser than another?

What two causes principally influence its density? Describe figure 2. Which cause may be neglected?

What instrument measures the density?

28. The density, however, is not exactly proportioned to the pressure, slight modifications arising, from several causes; the most important of which is temperature. The heat of the atmosphere decreases with the altitude, and since heat expands, and cold contracts, a given volume of air,th part of its bulk, at 32°, for every degree Fah.; or in other words, thus lessens and increases its density, a correction must be made for this influence.

29. It has been found by calculation, combined with observations, that, if the altitudes are represented by an increasing arithmetical series, the densities of the atmosphere decrease in a geometrical progression. Thus, if at the height of 18,000 feet the air, as the barometer indicates, is but half as dense, as at the surface of the earth; at 36,000 feet it will be reduced to one-fourth, and at 54,000 feet to one-eighth of its original density.

30. The rarefaction of the air at lofty elevations, lessens the intensity of sound, impedes respiration, and causes the minute veins of the body to swell and open. Thus, at a short distance, the report of a pistol upon the summit of Mont Blanc, can scarcely be heard. Gay Lussac and Biot, ascending from Paris, in a balloon, to the height of 25,000 feet, breathed with pain and difficulty, and upon the high table lands of Peru, the lips of Dr. Tschudi, cracked and burst; while the blood flowed from the veins of his eyelids.

In consequence of this diminution of pressure, water boils, in such situations, at a comparatively low temperature; thus, at Quito in Equador, 9,537 feet above the sea level, ebullition takes place at the temperature of 196° Fah.

In what manner does temperature affect the density?

What is the law of decrease in reference to altitude? Illustrate.
What are the effects of a rarefied atmosphere? Give instances.