average result, for the rate of decrease is very rapid near the earth, after which it proceeds more slowly, but at the loftiest heights is again accelerated.

52. During the winter of 1838, the French scientific commission stationed at Bossekop, in West Finmark, 69° 58 N. lat., found this law partially reversed, amid the rigors of a polar clime; the temperature of the atmosphere increasing, nearly, 3° Fah. for the first 328 feet in height; beyond this limit it began to decrease, at first slowly, but afterwards with greater rapidity. During the summer, the temperature decreased with the altitude.

53. As a consequence of this gradual reduction of heat, a point at length may be attained, in any latitude, if we continue to ascend, where moisture, once frozen, always remains congealed. Hence, arise the eternal snows and glaciers, that crown the summits of the highest mountains.

54. Since the mean temperature of the air is highest at the equator, and sinks towards either pole, the points of perpetual congelation are farthest removed above the ocean-level within the torrid zone, and gradually approach nearer the general surface of the earth, with th increase of latitude; as the following table shows.

55. A striking departure from the rule exists, how ever, in India; for while on the south side of the Himmalehs, the snow line occurs at the height of about

Was it found true at Bossekop?

What results from this gradual loss of heat?

Where are the points of perpetual congelation nearest to the ocean?
Where farthest from it? Give examples.

!

13,000 feet, on the northern acclivity it rises to the alttude of 17,000. Many explanations of this singular fact have been given, which admit not of discussion here.

HUMIDITY OF THE ATMOSPHERE.

56. At all temperatures moisture resides in the atmosphere, self-sustained, in an invisible state. Between the particles of air intervals are believed to exist, which are, either partially, or wholly, filled with the vapor that constantly rises from the earth.

57. This peculiarity in the constitution of the atmosphere is termed the capacity of the air for moisture, and when the intervals are full of vapor, it is said to be saturated. An increase of temperature, by dilating the air, separates the particles farther from each other; the intervals are thus enlarged, and the capacity of the air increased. A diminution of temperature is followed by contrary effects; the size of the intervals is then redu ced, and the capacity lessened.

58. The capacity increases, however, at a faster rate han the temperature. A volume of air, at 32° Fan. is capable of containing a quantity of moisture, equal to the 160th part of its own weight; but for every twenty seven additional degrees of heat, this quantity is doubled..

Thus a body of air can contain,

At 32° Fah. the 160th part of its own weight.

66 59°

86°

"113°

66

80th 66

From this it follows, that while the temperature advances in an arithmetical series, the capacity is accel erated in a geometrical progression.

What departure from this rule exists?

What does the atmosphere contain at all temperatures?
What is meant by the capacity of the air for moisture?

When is the air said to be saturated?

What is the effect of heat upon the capacity?

What is the effect of cold?

Which increases at the fastest rate, temperature or capacity?

Give instances. What is the rule in respect to temperature and capacity

59. ABSOLUTE HUMIDITY. From the cause just mentioned, it would naturally be inferred, that the quantity of atmospheric vapor, or the absolute humidity, is greatest in the equinoctial regions, and diminishes towards either pole; a conclusion abundantly supported by facts as will be shown hereafter.

60. The air over the ocean is always saturated, and upon the coasts, in equal latitudes, contains the greatest possible amount of vapor; but the quantity decreases as we advance inland, for the atmosphere of the plains of Oronoco, the steppes of Siberia, and the interior of New Holland, is naturally dry.

61. The absolute humidity diminishes with the altitude, but the rate of reduction is not fully known. By comparing different seasons and hours, it is found to be greater in summer than in winter, and less in the morning than at about mid-day.

62. RELATIVE HUMIDITY. This must not be confounded with absolute humidity. By relative humidity is understood the dampness of the atmosphere, or its proximity to saturation; a state dependent upon the mutual influence of its absolute humidity and temperature; for a given volume of air may be made to pass from a state of dampness, to one of extreme dryness, by merely elevating its temperature, without altering, in the least, the amount of moisture it contains.

Thus one hundred and sixty grains of air, containing one grain of vapor, would be damp at 36° Fah., but hot and withering at the temperature of 90°. By the reverse of this process, a body of hot air will not only become humid, but will even part with a portion of its original moisture, if it is cooled down to any great extent.

63. From the numerous observations of Kaemtz, at Halle, and on the shores of the Baltic, it appears that

What is absolute humidity? Where is absolute hnmidity the greatest?
How does it diminish? Where is the air always saturated?
What is said of inland regions? What is the effect of altitude?
Compare summer and winter, morning and mid-day.

What is relative humidity? Upon what does it depend? Illustrate the effects of a change of temperature, the absolute humidity being the same.

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 their indications, that are constructed upon the principle of condensation, to which allusion has already been 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 dif ferent 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, 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, k l, 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.