with moisture-in the first instance the dryness of the atmosphere would be indicated by 15°, and in the second by 5.

69. The action of this instrument is almost instantaneous, for the enclosed thermometer begins to fall in two seconds after the ether is dropped. It is usual, where great precision is required, to read off the degrees of the interior thermometer at the moment the dew-ring appears, and also at the moment it vanishes; the average of the two observations being taken as the true dew-point.

70. In England the dew-point is seldom 30° Fah. below the temperature of the air; the greatest difference at Hudson, Ohio, as given by Prof. Loomis, is 36°. In the tropical regions its range is the most extensive; for, in the burning clime of India, the dew-point has sometimes sunk as low as 29°, while the temperature of the atmosphere was 90°-a difference of sixty-one degrees.

71. HEIGHT OF THE ATMOSPHERE. Whether the atmosphere is boundless or not, is a question which natural philosophers have been unable to determine. De Luc regards it as unlimited, and imagines the planetary spaces to be filled with a medium so exceedingly attenuated as not to retard the motions of the heavenly orbs. The earth and the various celestial bodies are supposed to condense this subtil fluid around them into an atmosphere, by virtue of their respective attractions.

72. Were this true, the densities of the atmospheres thus formed would differ, on account of the variations in the size and mass of these bodies. It therefore constitutes a strong objection to this hypothesis, that the density of the atmosphere of Jupiter (as shown by the refraction of the light of his satellites, at the period of their eclipses) is not superior to that of our own; although the force of attraction at the surface of this planet is al

How far below the temperature of the air does the dew-point descend in England? in Ohio? in India? Is the height of the atmosphere known? What is De Luc's opinion? What is the objection to this hypothesis?

most three times greater than that of the earth. Moreover, when Venus passes near the sun, she exhibits no atmosphere, according to Wollaston, notwithstanding her size is nearly equal to that of the earth.

73. Those who maintain that the atmosphere is limited, suppose, that at a certain distance from the earth, the expansive energy of its particles is exactly balanced by the force of gravity, and that beyond this point, an infinite void extends. This distance has been computed to be not far from 22,200 miles from the centre of the globe.

74. Whichever theory may be adopted, it is certain that the atmosphere extends to very great heights. Dr. Wollaston has shown, by calculation, that the atmosphere, at the altitude of nearly forty miles, is still sufficiently dense to reflect the rays of the sun, when this luminary is below the horizon. It is capable of transmitting sound at a loftier elevation, for in 1783, a vast meteoric body exploded at an altitude of more than fifty miles, the sound reaching the earth like the report of a cannon. Still farther; if the combustion of meteors is truly assigned to the action of the atmosphere, the existence of the latter, at the distance of one hundred miles from the earth, may be regarded as proved.

What do the advocates of a limited atmosphere suppose?
How far is this point from the earth's centre?

At what height does the atmosphere reflect light?

At what altitude transmit sound?

66

What inference is drawn from the combustion of meteors?

PART II.

AERIAL PHENOMENA.

CHAPTER I.

OF WINDS IN GENERAL.

75. CAUSE OF WIND. Wind is air in rnotion, occurring whenever the repose of the atmosphere is broken, from any cause whatsoever. It is usually the result of a change of temperature, and consequently of density, but the rush of an avalanche, causing a sudden displacement of a vast volume of air, has been known to produce a momentary wind of great violence, along the borders of its path.

76. If two contiguous, upright columns of air, with their bases at the same level, are unequally heated, the colder is the denser, and at its base a current will flow towards the lighter column, (just as the compressed air within a bellows streams out into the rarer atmosphere,) but at the top, to supply this loss, a counter current prevails.

77. This is illustrated by Franklin's simple experiment; if a door is opened, communicating between a warm and cold room, and a lighted taper then placed at the bottom of the doorway, the flame is bent towards the warm apartment; but if held at the top, its direction is reversed.

78. On account of the unequal distribution of heat

What does part second treat of? What does chapter first treat of?
Define wind. When does it occur?

If two contiguous columns of air are unequally heated, what motion takes place? State Franklin's experiment.

over the surface of the globe, phenomena like these occur in nature, on a widely extended scale; for if two neighboring countries are unequally heated, the air above them partakes of their respective temperatures, and there arises at the surface of the earth, a wind blowing from the colder to the warmer region, while at the same time, a directly contrary current prevails in the upper strata of the atmosphere.

79. VELOCITY. Every gradation exists in the speed of winds, from the mildest zephyr, that scarcely bends the flower, to the most violent hurricane, which prostrates the giant oak, and hurls to the ground the proudest works of man. They have been classed as follows, by Smeaton, according to their rapidity and force.

Velocity of the wind, Perpendicular force on one square

miles per hour.

Common appellation of
such winds.

Hardly perceptible.

Gentle wind.

Pleasant brisk gale.

foot in lbs. avoirdupois.

Very brisk.

80. The velocity of the upper currents of the atmosphere, is as variable as that of the winds which sweep over the surface of the globe; for the aëronaut, Green, who ascended from Liverpool, in 1839, to the height of 14,000 feet, encountered a current that bore. him along at the rate of five miles per hour, but upon descending to the altitude of 12,000 feet, he met with a contrary wind, blowing with a velocity of eighty miles per hour.

How does it explain the origin of winds?

What is said of the velocity of winds?

Give the common appellations of winds, with their velocity and force. What is said of the speed of the upper currents?

Give examples.

On one occasion, his balloon was carried over the of ninety-seven miles in fifty-eight minutes.

space

81. ANEMOMETER. The velocity of the wind is estimated by the anemometer, an instrument so called from the Greek words, anemos, wind, and metron, measure. One of the best is Woltmann's. It consists of nothing more than a small windmill, to which is attached an index, in order to mark the number of revolutions per minute; the number of course increasing with the speed of the wind. Now if the atmosphere is still, and the anemometer is carried against it at the rate, for instance, of ten miles per hour, the number of its revolutions will be exactly the same as if the instrument was stationary, and the vanes revolved by the force of a breeze possessing the same velocity.

82. If then, upon a calm day, the anemometer is taken upon a railroad car, moving, for example, at the speed of twenty miles an hour, and the number of revoutions for half an hour accurately noted, we can obtain, (by dividing this result by 30,) the number of revolutions per minute, corresponding to those of a wind having a velocity of twenty miles per hour. In this manner, a table adapted to the instrument can be constructed for all winds, moving with a greater or less rapidity.

The velocity of the higher aërial currents is ascertained by the speed with which the shadow of a cloud passes over the surface of the earth.

83. FORCE. The force of the wind is obtained, by observing the amount of pressure it exerts upon a given, plane surface, perpendicular to its own directions. If the pressure-plate acts freely upon spiral springs, the power of the wind is denoted by the extent of their compression, and that weight will be a measure of its force, which produces the same effect upon the springs. This instrument, which is also termed an anemometer,

What is an anemometer? Describe Woltmann's, and the mode of computing by it the velocity of the wind.

How do we judge of the speed of the upper currents?
In what manner is the force of the wind estimated?