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<h2>CHAPTER XXXII.</h2> 691. What is a barometer? A barometer is an instrument which indicates the pressure of the atmosphere, and which takes its name from two Greek words signifying measurer of weight. 692. Why does a barometer indicate the pressure of the atmosphere? Because it consists of a tube containing quicksilver, closed at one end and open at the other, so that the pressure of the air upon the open end balances the weight of the column of mercury (quicksilver), and when the pressure of the air upon the open surface of the mercury increases or decreases, the mercury rises or falls in response thereto. 693. Why is a barometer called also a "weather-glass"? Because changes in the weather are generally preceded by alterations in the atmospheric pressure. But we cannot perceive those changes as they gradually occur; the alteration in the height of the column of mercury, therefore, enables us to know that atmospheric changes are taking place, and, by observation, we are enabled to determine certain rules by which the state of the weather may be foretold with considerable probability. 694. Why are barometers constructed with circular dials, and an index to denote changes? Because that is a convenient mechanical arrangement, by which [Pg 158] the alterations of the relative pressures of the air and the mercury are more clearly denoted than by an inspection of the mercury itself. <hr class="bible-verse" /> "Fair weather cometh out of the north: with God is terrible majesty."—Job xxxvii. <hr class="bible-verse" /> <div class="figcenter"> <div class="figsub"> <ANTIMG src="images/i-158a.jpg" width-obs="203" height-obs="500" alt="" /> <div class="caption">Fig. 20.—BAROMETER.</div> </div> <div class="figsub"> <ANTIMG src="images/i-158b.jpg" width-obs="133" height-obs="500" alt="" /> <div class="caption">Fig. 21.—TUBE OF BAROMETER, WHEEL, AND PULLEY.</div> </div></div> 695. Why does the hand of the weather dial change its position when the column of mercury rises or falls? Because a weight, which floats upon the open surface of the mercury, is attached to a string, having a nearly equal weight at the other extremity; the string is laid over a revolving pivot to which the hand is fixed, and the friction of the string turns the hand, as the mercury rises or falls. [Pg 159] <hr class="bible-verse" /> "Thou visitest the earth, and waterest it: thou greatly enrichest it with the river of God, which is full of water: thou preparest them corn, when thou hast so provided for it."—Psalm lxv. <hr class="bible-verse" /> 696. Why does tapping the face of the barometer sometimes cause the hand to move? Because the weight on the surface of the mercury frequently leans against the sides of the tube, and does not move freely. And, also, the mercury clings to the sides of the tube by capillary attraction; therefore, tapping on the face of the barometer sets the weight free, and overcomes the attraction which impedes the rise or fall of the mercury. Fig. 21 illustrates the mechanism at the back of the barometer. A is a glass tube; between A and E there exists a vacuum, caused by the weight of the mercury pressing downwards. This space being a vacuum, makes the barometrical column more sensitive, as there is no internal force to resist or modify the effects of the external pressure. E represents the height of the column of mercury; C the open end of the tube; F the weight resting on the surface of the mercury; P the pivot over which the string passes, and upon which the hand turns; W the weight which forms the pulley with the weight F. 697. Which is the heavier, dry or vaporised air? Dry air is heavier than air impregnated with vapours. 698. Why is dry air heavier than moist air? Because of the extreme tenuity of watery vapours, the density of which is less than that of atmospheric air. 699. Why does the fall of the barometer denote the approach of rain? Because it shows that as the air cannot support the full weight of the column of mercury, the atmosphere must be thin with watery vapours. The fall of the mercury in the long arm of the tube would cause the weight F to be pressed upwards. This would release the string to which the weight W is attached; it would, therefore, fall, and turn the hand down to Rain or Much Rain. 700. Why does the rise of the barometer denote the approach of fine weather? Because the external air becoming dense, and free from highly elastic vapours, presses with increased force upon the mercury upon which the weight F floats; that weight, therefore, sinks in the short tube as the mercury rises in the long one, and in sinking turns the hand to Change, Fair, &c. [Pg 160] <hr class="bible-verse" /> "He caused an east wind to blow in the heaven; and by his power he brought in the south wind."—Psalm lxxviii. <hr class="bible-verse" /> 701. Why does the barometer enable us to calculate the height of mountains? Because, as the barometer is carried up a mountain, there is a less depth of atmosphere above to press upon the mercury; it therefore falls, and by comparing various observations, it has been found practicable to calculate the height of mountains by the fall of the mercury in a barometer. 702. To what extent of variation is the weight of the atmosphere liable? It may vary as much as a pound and a half to the square inch at the level of the sea. 703. When does the barometer stand highest? When there is a duration of frost, or when north-easterly winds prevail. 704. Why does the barometer stand highest at these times? Because the atmosphere is exceedingly dry and dense, and fully balances the weight of the column of mercury. 705. When does the barometer stand lowest? When a thaw follows a long frost; or when south-west winds prevail. 706. Why does the barometer stand lowest at those times? Because much moisture exists in the air, by which it is rendered less dense and heavy. 707. What effect has heat upon the barometer? It causes the mercury to fall, by evaporating moisture into the air. 708. What effect has cold upon the barometer? It causes the mercury to rise, by checking evaporation, and increasing the density of the air. <hr class="bible-verse" /> "For so the Lord said unto me, I will take my rest, and I will consider in my dwelling place like a clear heat upon herbs, and like a cloud of dew in the heat of harvest."—Isaiah xviii. <hr class="bible-verse" /> In noting barometrical indications, more attention should be paid to the tendency of the mercury at the time of the observation, than to the actual state of the column, whether it stands high or low. The following rules of barometric reading are given as generally accurate, but liable to exceptions:— [Pg 161] Fair weather indicated by the rise of the mercury. Foul weather by the fall of the mercury. Thunder, indicated by the fall of the mercury in sultry weather. Cold, indicated by the rise of the mercury in spring, autumn, and winter. Heat, by the fall of the mercury in summer and autumn. Frost, indicated by the rise of the mercury in winter. Thaw, by the fall of the mercury during a frost. Continued bad weather, when the fall of the mercury has been gradual through several fine days. Continued fine weather, when the rise of the mercury has been gradual through several foul days. Bad weather of short duration, when it sets in quickly. Fine weather of short duration, when it sets in quickly. Changeable weather, when an extreme change has suddenly set in. Wind, indicated by a rapid rise or fall unattended by a change of temperature. The mercury rising, and the air becoming cooler, promises fine weather; but the mercury rising, and the air becoming warmer, the weather will be changeable. If the top of the column of mercury appears convex, or curved upwards, it is an additional proof that the mercury is rising. Expect fine weather. If the top of the column is concave, or curved downwards, it is an additional proof that the mercury is falling. Expect bad weather. <hr class="chap" /> <div style="break-after:column;"></div>

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