the relative humidity, in those situations, is highest in the morning before sunrise, and lowest, or farthest removed from the point of saturation, at the hour of the greatest diurnal heat. Corresponding results have been obtained in this country.

HYGROMETER.

64. Those instruments by which the humidity of the atmosphere is measured are called hygrometers, from the Greek words ugros, moist, and metron, measure. Of these there exists a great variety, differing both in form and principle; but those are esteemed the most accurate in the indications, that are constructed upon the principle of condensation, to which allusion has already peen made, (Art. 62,) but a more extended explanation is here required.

65. Imagine a brightly polished metallic vessel, partially filled with water, at the temperature of 60° Fah., to be placed in a room at the same temperature. If pieces of ice are now thrown into the vessel, the water is gradually cooled down, and as this reduction proceeds, the lustre of the exterior surface will be dimmed, at a certain moment, by a fine dew. This is caused by the deposition of moisture from the atmosphere, which, in contact with the cold surface of the vessel, is now cooled down just beyond the point of saturation. The temperature of the water at this instant, which is the same as that of the vessel, is termed the dew-point.

66. By marking the difference, in degrees, between the temperature of the air and the dew-point, the relative dryness of the atmosphere, or its remoteness from saturation is obtained. But observations, like these, lead also to other important results; for, by the aid of tables, giving the elastic force of aqueous vapor, at different temperatures, the absolute weight of the vapor, diffused through a given volume of air can be determin

Wha did Kaemtz observe in respect to relative humidity?
What is a hygrometer? Explain the principle of condensation.
What is the dew-point? How is the relative humidity obtained?
What other results can be deduced?

ed, and likewise the proportion it contains, to that which would be required to saturate it. 67. The hygrometer of Prof. Daniell, which is extensively used, is thus constructed.

A glass tube, e i, figure 5., is bent twice at right angles, and terminated by two bulbs, b and f, of the same material. The bulb b is partly filled with ether, into which is inserted the ball of a delicate thermometer, d, enclosed in one arm of the instrument. All air is excluded from the tube, (d which is filled with the vapor of ether; the other bulb, f, is covered with a piece of fine muslin, a, and upon the pillar, g h, a second thermometer, kl, is fixed.

Fig. 5.

HYGROMETER.

68. Observations are thus made. The instrument being placed by an open window, or out of doors, a few drops of good ether are suffered to fall upon the muslin-covered bulb, which, from the rapid evaporation of the ether, soon becomes cool, condensing the ethereal vapor within. In consequence of this effect, the ether in b evaporates, thus causing, not only in the ether, but also in the enclosing bulb, a reduction of temperature, which is measured by the interior thermometer, e d.

As the evaporation at a proceeds, the temperature of b still continues to fall, and, at a certain point, the atmospheric vapor will be seen gathering in a ring of dew upon the glass, and the difference in degrees, at this moment, between the external and internal thermometer, denotes the relative dryness of the atmosphere. Thus, if on one day the exterior thermometer stood at 65°, and the enclosed sunk to 50° ere the dew-ring appeared-and on another, the former was at 73°, and the latter had descended to 68° before the glass was dimmed

Describe Daniell's hygrometer, fig. 5., and explain the mode of taking observations.

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?

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 motion, 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.