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is firmly fixed to the base of the instrument. The air is partly removed from the box and it is sealed air-tight.

FIG. 158

Variations in the atmospheric pressure cause corresponding changes in the position of the elastic upper face of the vacuum chamber, and these

changes are transmitted

by a system of delicate levers to a pointer which moves around a graduated dial. The readings of this dial are made to correspond to those of a standard mercury barometer.

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These instruments are very delicate and will show a variation in reading on being raised from the floor to a table; for the atmospheric pressure varies not only with the weather but also with the elevation above sea level. Small aneroids, of about the size and shape of a watch, are used in taking the heights of mountains, and by the government surveyors in taking elevations for contour maps. The scales are made to read in feet above or below sea level, as well as in inches of mercury; but aneroids are useful in determining changes in elevation rather than in giving absolute height.

FIG. 159. Aneroid Barometer

The barograph (Fig. 160) is a self-recording aneroid barometer. The record is made by a pen which is attached to the aneroid by a system of levers, and which traces a graph on a slowly rotating cylinder covered with cross section paper, thus making a permanent record.

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174. Weather Indicated by the Barometer. A constant use of the barometer is made in the Weather Bureau in fore

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casting changes in the weather. The relation of barometric readings to the state of the weather may be stated as follows:

I. A rising barometer precedes fair weather.

II. A falling barometer precedes foul weather.

III. A sudden fall in the barometer precedes a storm.

IV. An unchanging high barometer indicates settled fair weather.

175. Cyclonic Storm Pres

sure. The relation be

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tween the readings of the barometer and the direction of the wind in circular or cyclonic storms may be studied in connection with the weather map, Fig. 161. At the center, or eye of the storm (marked Low),

FIG. 161

the pressure is least; while at the outside the pressure is greater, and the air therefore rushes toward the center.

In

the northern hemisphere winds are deflected to the right by the rotation of the earth. In approaching the center of low pressure, therefore, the direction of each air current is to the right of the center. This gives to the storm a rotary motion in a counter-clockwise direction; and if a person stands with his back to the wind, the storm center, or region of lowest barometer, will be on his left hand.

The observations of the Weather Bureau on barometric pressures for a series of years indicate that there is a well-defined movement of low pressures, or storm centers, across the continent. These areas of low pressure generally enter the United States on the northwest boundary, coming from British America, move southeastwardly until they have crossed the Rocky Mountains, and then turn northeastwardly and disappear on the Atlantic coast, or pass down the St. Lawrence River. The storms that come into the country from the Gulf of Mexico usually travel northeast along the Altantic coast.

The compressibility of

176. Height of the Atmosphere. the air is so great that the layer in contact with the surface of the earth is more dense than the layers above it. Though the density constantly decreases as the distance from the earth increases, no uniform rule can be given that will show the relation between barometric readings and the corresponding heights of the atmosphere. However, a fall of one inch in the mercury column, from the reading at sea level, indicates an elevation of about 900 ft.

Figure 162 is a graph showing the relation (in fair weather) between height above the surface of the earth in feet and the pressure of the atmosphere as measured by inches of mercury. It is seen from this curve that at a height of 20,000 ft. the pressure is reduced from 31 in. of mercury to

less than 15, which means that the density of the air is not half so great at that height as at the surface.

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Balloon ascensions have exceeded this height, notably that of Mr. James Glaisher, undertaken for the purpose of making scientific observations for the British Association for the Advancement of Science. Both Mr. Glaisher and his aëronaut, Mr. Coxwell, became unconscious, but before losing consciousness succeeded in letting enough gas escape to bring the balloon down into the denser atmosphere. The pressure recorded by the instruments indicated a height of 37,000 ft.

If the air were of uniform density, we should have what is called the homogeneous atmosphere. Its height would be about 27,500 ft.

177. Boyle's Law. - Since an increase of pressure reduces the volume of a gas, it is important to know whether there is a definite relation between the pressure exerted upon a gas and the resulting volume. This was experimentally deter

mined independently by two physicists, Boyle and Mariotte. The results obtained were formulated in what is called Boyle's Law, which may be stated as follows:

The temperature remaining the same, the volume of a given mass of gas varies inversely as the pressure acting upon it. This may be expressed by the proportion V: V' = P' : P, from which we get

PV =

P'V'

i.e., PV a constant quantity.

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The mass of the air remaining the same, it is evident that the density must increase as the volume diminishes; hence, At a constant temperature the density of a gas is directly proportional to the pressure acting upon it.

Very careful measurements show that gases do not obey Boyle's Law exactly, and that different gases behave differently in this respect. But for practical purposes the law may be considered to hold true, except at temperatures so low that the gas is about to liquefy.

20

86.

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178. Verification of Boyle's Law. (a) For Pressures Greater than One Atmosphere. Demonstration. Bend a glass tube as shown in Fig. 163, the long arm being open and the short one closed. Fix this to a vertical support and place a graduated scale between the two arms. Pour mercury into the long arm by means of a long funnel, and tip the tube in such D a way as to let bubbles of air pass from the short tube into the long one, and thus bring the mercury to the same level AB in both. This line is chosen at a convenient position, say 20 cm. below the closed end. On pouring mercury into the long tube it will be found necessary to fill it to a height of about 760 mm. above the mercury in the short tube to reduce the volume of the gas one half. The pressure upon

C

A

10

FIG. 163

B

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