index continues stationary. Thus, the highest point to which the index rises in the arm, a b, indicates the least temperature, and that in cd the greatest, that happens in any interval of time, as a day, or a year; and the scale, as is evident from the figure, is graduated accordingly.

45. After every observation, each index requires to be adjusted; this is done by means of a magnet, which, being moved down the side of the arm, draws the index after it.

Another instrument of this kind was invented by Rutherford, (C. 575.)

MEAN DAILY TEMPERATURE.

46. The mean or average temperature of the day, would be accurately found by observing the thermometer at intervals of an hour during the whole twentyfour, and dividing the sum of the temperatures by the number of observations, viz., twenty-four. This method is however too laborious, and meteorologists have endeavored to arrive at the same result from two or three daily observations.

47. According to Kaemtz, a celebrated German meteorologist, if, in Germany, the thermometer is noted at 6, A. M., 2, P. M., and 10, P. M., and the sum of the temperatures divided by three, the quotient will differ but little from the true mean. The rule adopted in the State of New York, under the direction of the Regents of the University, is as follows:

Mark the temperature, first, between daylight and sunrise; secondly, between 2 and 4, P. M.; thirdly, an hour after sunset: add together the first observation, twice the second and third, and the first of the next day, and divide the sum by six; the result will be the

mean.

The mean daily temperature at Philadelphia has been found, from the hourly observations of Capt. Mor

What is understood by the mean daily temperature?
How is it obtained?

decai, to be one degree less than the temperature at 9, A. M.

48. By taking the average of all the mean daily tem peratures throughout the year, the mean annual temperature is ascertained. It is also obtained by the aid of the self-registering thermometer, the average of the two extreme temperatures being regarded as the mean of each day.

49. VARIATIONS OF TEMPERATURE IN LATITUDE. By comparing situations differing widely in latitude, it is found that the average annual temperature of the atmosphere diminishes from the equator towards either pole. This will be seen from the annexed table, which presents the results at the sea level, for nine places.

50. From this table it is also evident, that places having the same latitudes, in the two hemispheres, do not necessarily possess the same average temperature. This is owing to a great variety of local causes, the effect of which cannot always be accurately estimated.

51. VARIATIONS IN ALTITUDE. The temperature of the air diminishes with the altitude, but the law of decrease is very irregular, being affected by the latitude, seasons, hours of the day, and a diversity of local circumstances. It may however be assumed, as a general rule, that a loss of heat occurs to the extent of one degree Fah. for every 343 feet of elevation. This is an

How is the mean annual temperature found?

How does the temperature of the atmosphere vary in respect to latitude? Give examples.

Do like latitudes in different hemispheres have the same temperature? How is the temperature affected by altitude?

What is the general law of decrease?

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 the increase of latitude; as the following table shows.

55. A striking departure from the rule exists, however, 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 altıtude 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 than 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-sever 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°

66 86°

"113°

66

80th 66

From this it follows, that while the temperature advances in an arithmetical series, the capacity is accelerated 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 humidity 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.