PART I.

OF THE ATMOSPHERE.

3. As the common properties of the air, viz., weight, Auidity and elasticity, are supposed to be already known, C. 502,) we shall proceed at once to the discussion of he entire body of air, termed the atmosphere; and first of its pressure, which is ascertained by the barometer, an instrument so called from the Greek words, baros, weight, and metron, measure.

BAROMETER.

4. This instrument is of the highest importance ir Meteorology, and requires a minute description. It is thus constructed. Into a glass tube, about three feet in length, open at one end and closed at the other, mercury is poured until it is full; the open end being now closed by the finger, or any other means, the tube is inverted, and the lower end immersed in a vessel of mercury. When beneath the surface of the fluid the end is unstopped, and the column of mercury within the tube then settles down, until its summit is about thirty inches above the level of that within the vessel. The space above the column in the tube is a void, and is called the Torricellian vacuum, from Torricelli, the name of the Italian philosopher, who first constructed this instru

ment.

5. The column of mercury within the tube is supported above the level of that in the vessel, by the upward pressure of a column of the atmosphere, having the same base as itself.

What is the atmosphere?
How is its pressure ascertained?
How is the barometer made?

What supports the column of mercury?

6. Thus, in fig. 1., the atmospheric column a a, of indefinite length, but of the same size as the barometric column Db, presses upon the mercury in the vessel. K, with a force equal to its own weight; now since any force, acting upon a fluid, is communicated in every direction, this pressure will be transmitted through the mercury, in the direction of the arrows, and acts at D, within the tube, against the mercurial column Db. This upward force will be resisted at D, by the weight of Db, and the mercury will sink in the tube until the two pressures counterpoise each other, in exactly the same manner as two equal weights in the opposite scales of a balance.

K

7. From these considerations, it is manifest, that the weight of the atmospheric column a a is equal to that of the mercurial column, Db of the same base; and this weight can be estimated in the following manner. If the base at D contains one square inch, the column Db, at its usual height, will contain, nearly, 30 cubic inches; and since one cubic inch of mercury weighs 3426.76 grains, the weight of thirty will amount to 102802.8 grains.

BAROMETER.

This product being now divided by 7004, the number of grains in a pound avoirdupois, the result will be nearly 14.7 lbs. ; a quantity equal to the weight of the barometric column, and consequently to the pressure of the atmosphere on every square inch of surface.

8. Any increase in the density of the atmosphere will be denoted by an elevation of the mercury, and a decrease by its depression. The cause of this is obvious, in the first case, a a becomes heavier, and requires more

Explain Figure 1.

How is the pressure of the atmosphere, on every square inch, computed? How does any change in the density of the air affect the height of the barometer?

mercury to balance it; therefore Db is lengthened. In the second case, a a is lighter, and, as a less quantity of mercury will then balance it, Db is shortened. Such changes are constantly occurring, but are very minute, and, in order that they may be accurately indicated, the instrument must be made with the nicest care.

9. To secure a perfect instrument, it is essential that the mercury should be free from any solid impurities, else the summit of the column will either be above, or below, its proper level, according as the foreign matter, mixed with the mercury, is lighter, or heavier, than the fluid. This end is attained by straining the mercury through chamois leather. If it is amalgamated with zinc, or lead, it is purified by washing it with acetic, or sulphuric acid.

10. When the tube is filled, moisture and small bubbles of air are found adhering to its interior surface, and are also contained in the mercury. These, if not expelled, will ascend when the tube is inverted into the Torricellian vacuum, the moisture rising in vapor. By their united elastic force, the ascent of the barometric column will be checked, whenever any increase in the density of the atmosphere tends to elevate it.

11. This source of error is removed by boiling the mercury in the tube. When all the air and vapor are expelled, the tube, if gently struck, will give forth a dry, metallic sound; but if a bubble of air remains, the sound will be dull and heavy. By connecting the open end of the tube with an air pump, during the process of boiling, Dr. Jackson, of Boston, has still more effectually removed this imperfection.

12. By these means, the air may perhaps be totally excluded, when the instrument is first constructed; but in the course of time, it will again insinuate itself between the glass and the mercurial column. To prevent this evil, Prof. Daniell, of King's College, London,

What precautions are adopted to secure a perfect barometer?

How is the mercury purified, and why?

How are moisture and air expelled from the tube, and why?
What is Prof. Daniell's improvement?

welds to the open end of the glass tube a ring of platinum, which possesses a greater affinity for mercury than glass. The mercury adheres closely to the platinum, like water, and the passage of air, according to all experiments, appears thus to be effectually prevented.

13. Since the constant changes in the weight of the atmosphere produce corresponding fluctuations in the height of the barometer, a scale is placed near the top of the tube, extending from twenty-seven to thirty-one inches, a space, which includes, at the surface of the earth, all the fluctuations of the column. This scale is divided into tenths of an inch; but, as the variations of the barometer are exceedingly minute, a contrivance, called a vernier, is annexed, by which a change, to the extent of one five hundredth of an inch, can be easily measured.

14. As the surface of the mercury, in the reservoir, is raised by the descent of the column, and depressed by its elevation, any change in the height of the barometer cannot be accurately estimated, while the scale remains. in the same position; unless this surface is always brought to the same point, before taking an observation. The necessity of so doing will be obvious, from the fol lowing illustration.

Suppose the surface of the mercury in the cistern K, figure 1., to be fifty square inches, while that of a horizontal section of the column is but one. Should the barometer sink one inch, the surface of the mercury in the cistern will rise one fiftieth of an inch, and the amount of the depression of the column, if measured from this surface, will be only forty-nine fiftieths of an inch, instead of one inch, its true depression.

15. The contrivance employed by Fortin, a celebrated French artist, to remove this error, consists in adjusting to the cistern K, fig. 1., a movable bottom, which can be elevated or depressed, by means of the screw

What is the length of the barometric scale?

How small a variation in height can be measured?

What is Fortin's contrivance, and for what purpose adopted?

P, until the surface of the mercury shall just touch the fixed ivory index L, at its lower extremity; which point is the zero of the scale, or the place from which the height of the barometer begins to be reckoned.

16. When, by adopting the previous precautions, the barometer has been so far perfected, two corrections are still necessary, before recording observations; the irst for temperature, and the second for capillarity. That of temperature depends upon the expansion of the mercury and the scale; the latter being partially corrective of the former, inasmuch as the divisions of measurement upon the scale, lengthen, at the same time, with the column of mercury.

17. TEMPERATURE. Mercury dilates, for every degree Fah. about one ten-thousandth part of its bulk, taken at the freezing point. The expansion of the scale varies with the metal of which it is composed, but its amount is, usually, so small, that it may safely be neglected in the required correction. Hence the following practical rule has been adopted, for reducing any observed altitude of the barometer, to the corresponding altitude, at the freezing point. "Subtract the ten-thousandth part of the observed altitude, for every degree above the freezing point." Thus, if the barometer stands at 29 inches, and the thermometer at 52°, the required correction is 20 x .0001 × 29 058. If the temperature is below 32°, the correction must be added. To facilitate these calculations, a thermometer is always attached to the barometer.

=

18. CAPILLARITY. By capillary attraction is (C. 53,) understood, the force exerted by the interior surface of small tubes, upon the fluids contained within them. When the fluid moistens the tube, it rises above its proper level; but when it does not, as in the case of mercury, it sinks below it. From this cause, a depression, termed its capillarity, occurs in the barometer, the extent

How is the barometer affected by a change in temperature?

Give the rule for reducing the height to the corresponding height at the freezing point.

Why is capillarity a source of error?