As will be seen, the instrument consists of a glass tube having a bulb at one end and closed at the other, so as to keep out the air. Before closing the upper end, the tube is partially filled with mercury, and the air above it is driven out by heating the mercury to near its boiling point when the tube above the mercury will be filled with mercurial vapor. It is now sealed, and, on cooling, the vapor condenses and a vacuum results. The expansion or contraction of the mercury by applying or withdrawing heat from the body with which the bulb is in contact, causes the highest point of the mercury column to rise or fall, and, since for equal changes of temperature the mercury rises or falls equal distances, this instrument, when properly made and graduated, indicates any change in temperature with great accuracy.

8.

Thermometer Graduations.-In order to graduate the thermometer it is placed in melting ice, and the point to which the mercurial column falls is marked freezing. It is then placed in the steam rising from water boiling in an open vessel, and the point to which the mercurial column rises is marked boiling.

210

100

200

90

190

180

80

170

180

70

د 130

140

40

30

20

There are now two fixed points, the freezing point and the boiling point. If it is desired to make a Fahrenheit thermometer, the distance. between these two fixed points is divided into 180 parts, called degrees. The freezing point is marked 32° and the boiling point 212°; 32 parts are marked off from the freezing point downwards, and the last one is marked 0° or zero. The graduations are carried above the boiling point and below the zero point as far as desired. This thermometer was invented in 1714 and was the first to come into general use.

FIG. 1

In graduating a centigrade thermometer, the freezing point is marked 0°, or zero, and the boiling point 100°; the

distance between the freezing and boiling points is divided into 100 equal parts; these equal divisions are carried as far below the freezing point and above the boiling point as desired. The reason that Fahrenheit placed the zero point on his thermometer 32° below freezing was because that was the lowest temperature he could obtain, and he supposed that it was impossible to obtain a lower one. Where there is any doubt as to the thermometer used, the first letter of the name is placed after the degree of temperature. For example, 183° F. means 183° above zero on the Fahrenheit instrument, and 183° C., that it is 183° above zero on the centigrade instrument.

In Russia, and a few other countries, another instrument is used, called the Réaumur; the freezing point is marked 0°, or zero, and the boiling point 80°, the space between these two points being divided into 80 equal parts. 183° R. means 183° on the Réaumur thermometer.

9. In order to distinguish the temperature below the zero point from that above, the minus sign (-) is placed before the figures indicating the number of degrees below zero, and the plus sign (+) for those above. Thus 18° C. means that the temperature is 18° below the zero point on the centigrade thermometer, and 25.4° F., that it is 25.4° below zero on the Fahrenheit thermometer.

10. To Convert the Reading of One Thermometer Into That of Another. It is sometimes necessary to change the reading of one thermometric scale into that of another; for instance, when the temperature is given in degrees of Fahrenheit to find the corresponding value on either Réaumur or centigrade, and conversely. By using the accompanying thermometric chart, Fig. 2, this is very readily accomplished. All that has to be done is to place a ruler, or the straight edge of a sheet of paper, between the center a and the given degree on either of the circles representing the scale of the given thermometer; the intersection of this edge with the other circles will give the corresponding degree on the respective scales.

ILLUSTRATION.-A centigrade thermometer shows a temperature of 80°. Find the corresponding temperature on a Fahrenheit and a Réaumur thermometer.

Proceed as follows: Draw the straight line a b, center a and the 80° mark on the centigrade scale. temperature, as indicated by the intersection of this line with the other scales, will then be 64° on Réaumur and 176° on Fahrenheit thermometers, very nearly.

THE BAROMETER

11. Since air has weight it is evident that the enormous quantity of air that constitutes the atmosphere must exert considerable pressure on the earth. This air pressure decreases with the increase of altitude, because there is less air abovus the higher we ascend. The pressure of

the atmosphere would be practically the same at all places

10

0

having a common altitude above the level of the sea if it were not for the disturbing influence of the solar heat and of the movement of the air caused by its unequal heating at different places on the earth.

The instrument used for measuring the pressure of the atmosphere is called the barometer, of which there are two kinds in general use— the mercurial and the aneroid.

12. The principle of the mercurial barometer, which is represented in Fig. 3, is as follows: A glass tube a, Fig. 4, about 3 feet long, and inch in diameter, closed at one end, is filled with mercury. By closing the open end with a finger the tube is turned over and inserted into a vessel or cup b. Some of the mercury will now flow downwards out of the tube into the cup until the weight of the mercury remaining in the tube is equal to the pressure of the air on the surface of the mercury in the cup. The space above the mercury in the tube will be practically a vacuum, consequently, there will be no pressure on the top surface of the mercury in the tube. It is evident, then, that when the pressure of the atmosphere on the surface of the mercury in the cup increases, the mercury in the tube is forced upwards, and that FIG. 3 when the pressure decreases, the

mercury in the tube falls. It is on this principle that the barometer shown in Fig. 3 is constructed. The tube and cup at the bottom of the instrument are protected by a casing of brass or some other metal. At the top of the tube is a graduated scale that can be read to 1 inch by means of a vernier, which is quite sufficient for nautical purposes. An accurate thermometer is usually attached to the casing for the purpose of determining the temperature of the outside air at the time the barometric reading is made. This is necessary since mercury expands when the temperature is increased and contracts when the temperature falls; for this reason a standard temperature is assumed to which all barometric readings are reduced. This standard temperature is usually taken at 32° F., when the 31 height of the mercurial column is 30 inches.

FIG. 4

32

a

FIG. 5

13. How to Read Off a Mercurial Barometer. In reading off a barometer the lower edge of the vernier is brought into contact with the uppermost point of the mercury when the eye is at an equal height and looking horizontally at the tube. For instance, let a b, Fig. 5, represent a portion of the scale of a barometer, cd the vernier, and e