WEIGHT OF THE ATMOSPHERE.

31. We have seen, that a column of mercury, about thirty inches in height, weighs, at the surface of the earth, exactly the same as a column of the atmosphere, possessing the same base. If then the globe was covered with an ocean of mercury, thirty inches in depth, the latter would occupy the identical base that the atmosphere does now, and their respective weights might be regarded as equal.

32. Under this supposition, the diameter of the earth would be increased five feet. The difference then, in cubic feet, between the solidity of the earth, and that of a globe, whose diameter is five feet greater, will equal the number of cubic feet in the sea of mercury. This number multiplied by the weight of a cubic foot of mer cury, viz. 848,125 lbs., will equal that of the whole mass, which is the same as the weight of the atmosphere. This calculation has been made, and amounts to more than five thousand billions of tons.

TEMPERATURE OF THE ATMOSPHERE.

33. The entire body of air surrounding the globe appears to be warmed in two ways; first by the luminous beams of the sun, secondly, by the radiation of heat from the earth.

34. According to Kaemtz and Martin, the atmosphere absorbs nearly one-half the daily amount of heat, which emanates from the sun to the earth, even when the sky is perfectly serene. The remaining portion falling upon the surface of the ground, elevates its temperature, and the earth sends back into the atmosphere rays of invisible heat.

35. Modern researches have shown, that all bodies, through which heat can pass, absorb a greater propor

How is the weight of the atmosphere computed?

How many tons does it weigh?

How is the atmosphere warmed?

tion of non-luminous, than of luminous calorific rays. The heat, therefore, that radiates from the earth, will not pierce the atmosphere, with the power of the solar ray; all will be retained by the lower strata of air, which in their turn, diffuse invisible thermic rays, in every direction.

36. We thus perceive, what all observations have proved, that the upper regions of the atmosphere must be colder than the lower. It is not, however, to be forgotten, that the rarefaction of the superior strata contributes to this condition.

THERMOMETER.

37. The temperature of the atmosphere is indicated by the thermometer, an instrument, which derives its name from the Greek words, thermos, warm, and metron, measure. It consists of a small glass tube, terminated by a bulb, and is partially filled with mercury.

This fluid is usually preferred for several reasons, the most important of which are, its uniform dilation, its quick susceptibility to any change in temperature, and the great range of its expansion in the fluid state. If the instrument is to be exposed to extreme cold, alcohol must be used.

38. As mercury, like other fluids, expands by heat, and contracts by cold, its alternate elevations and depressions within the tube, can be made to indicate the corresponding changes in the state of the air, if two fixed temperatures can be found, whence to reckon the changes. These have been discovered. If a thermometer is immersed, at different times, in melting snow, the column of mercury invariably sinks to the same place in the tube, though many months may have elapsed between the experiments; and, when exposed to the steam of boiling water, the mercury always as

Is it heated most by luminous or non-luminous heat?

Are the upper or lower regions of the atmosphere the warmest?
How is the temperature of the atmosphere measured?
Describe the thermometer. Why is mercury used?

How are the two fixed temperatures obtained?

cends to the same height, under the same atmospheric pressure.

Fig. 3.

212

Boiling point.

72

52 32

39. These invariable positions, which are termed the freezing and the boiling points, are marked upon the scale to which the tube is affixed. In Fahrenheit's thermometer, figure 3., the interval between them is divided into 180 parts, each of which is called a degree (10) and as the freezing point is marked 32°, the boiling is therefore 212°. The divisions are extended downwards from 32° to 0, or the zero point, and when extreme degrees of cold are to be measured, the range is continued to 20°, 40°, and even 60° below zero. If the air is colder than 40° below zero, a spirit thermometer must be used, since mercury becomes solid at this temperature. When Simpson, a late northern traveller, wintered, in 1838, at Fort Confidence, 67° N. lat., he cast a bullet of mercury, the temperature being 49° below zero. Upon firing the ball, it passed through an inch plank, at the distance of ten paces; but flattened and broke against the wall, three or four paces beyond. In addition to the mode of graduation adopted by Fahrenheit, several others prevail (C. 570), which it is not necessary here to discuss.

0

20

...

Freezing point.
Zero.

FAHRENHEIT'S
THERMOMETER

40. The thermometer employed for meteorological purposes, should be made as accurate as possible, and in

Into how many intervals is the space between them divided in Fahren heit's scale? What are the intervals called?

How many degrees is the freezing point?

How many the boiling point?

What is the zero point?

When must a spirit thermometer be used?

Relate Simpson's experiment.

order to ensure its perfection, many niceties must be ob served in its construction.

41. FIRST. The tube should be of equal size throughout the whole stem; else the same increase of temperature will not produce the same increase in the height of the mercury, throughout every part of the tube; and so of the decrease.

SECONDLY. The bulb should be large in proportion to the tube; for then slight changes in temperature will be rendered perceptible, and the delicacy of the instrument increased.

THIRDLY. The mercury should be pure, dry, and recently boiled, in order to free it from air; and, when in the tube, should there again be boiled, to drive off any air or moisture collected within.

LASTLY. When the mercury is at the summit of the tube, and every thing else has been expelled, the top of the tube must be perfectly closed by the fusion of the glass, leaving, when the mercury has cooled, a void space or vacuum above.

42. When a thermometer has been exposed to great changes in temperature during the course of a year, the position of the freezing point upon the scale is found to be somewhat altered; for, if the instrument is then placed in melting snow, the mercury is usually seen to stand a little higher than 32°, and less than 33°. This change would occasion a constant error in the observations, and meteorologists therefore verify their thermometers at stated intervals, in the way just mentioned.

SELF-REGISTERING THERMOMETER.

43. The object for which this instrument is constructed, is to obtain, in the absence of the observer, the highest and lowest temperature of the day, or of any other interval of time.

What precautions must be taken to construct an accurate thermometer? What change occurs in the position of the freezing point?

How are thermometers verified?

For what purpose is a self-registering thermometer used?

One of the most correct thermometers of this kind, now in use, is that invented by Mr. James Six, of Colchester, which is represented in Fig. 4.

It consists of a long glass bulb, G, narrowing into a fine tube, which is first bent downward, forming the arm a b, and then upwards, forming the arm c d, which terminates in a small cavity, L. The two arms contain mercury, which extends down from a on one side, and up to c on the other the bulb and the rest of the tube are filled with alcohol, except the upper part of the cavity L. Upon the top of the mercury in each arm rests an index (which is more perfectly seen at A), consisting of a piece of iron wire capped with enamel, and loosely twined with a fine glass thread; when the mercury descends, the index would fall, were it not for the glass thread, which, pressing like a spring against the sides of the tube, supports the index, in any position.

44. The action of the instrument is as follows: When an increase of temperature expands the spirit, the mercury is depressed in the arm a b, and elevated in c d, carrying the index up with it. If the temperature now falls, the spirit contracts, and the mercury descends in c d; but the index remains in its last position, from the pressure of the glass spring against the tube; and, as it does not fit tightly to the latter, the alcohol above it flows readily by.

As the cold augments, the mercury rises in a b, bearing up the index of this arm, until an increase of temperature occurs, when the mercury here falls, and the

Describe Six's, from fig. 4