Back Aa

Reason Why

<h2>CHAPTER XVII.</h2> 341. Since all bodies expand by heat and contract by cold, why does water, when it reaches the freezing point, expand? Because, in freezing, water undergoes crystallization, in which its particles assume a new arrangement occupying greater space. 342. Why does water never freeze to a great depth? Because the covering of ice which is formed upon the surface of [Pg 86] the water prevents the cold air from continuing to draw off the caloric of the water. <hr class="bible-verse" /> "For he saith to the snow, Be thou on the earth; likewise to the small rain, and to the great rain of his strength."—Job xxxvii. <hr class="bible-verse" /> 343. Why has this exceptional law of the expansion of water, when freezing, been ordained? Because, but for this, deep waters might be frozen through their whole depth. This would destroy the myriads of fish and other living things that inhabit the water. Parts of the earth, now clad in verdure, would be lost in eternal winter; and even in the most temperate zones it would take months to effect a thaw; and thawing would be attended with such floods and subterranean commotion as are terrible to contemplate. 344. Why are bed-room windows sometimes covered with crystalline forms on winter mornings? Because the vapour of the breaths of the inmates has condensed upon the window-panes, and formed water. The water has frozen with the cold, and exhibits the beautiful crystalline forms into which its particles are arranged. Here we have another domestic illustration of the great laws of nature. It is the same law which locks the arctic regions in ice and decorates our window-panes. This beautiful phenomenon is usually witnessed by us on frosty mornings when we rise from our beds. It has a story which the observer of nature may read in its sparkling eyes. It tells that, although without the air is biting cold, God has wrapped a mantle around the face of nature to keep it from injury; and that the earth and the waters, though looking chilled and dead, have still the warmth of life preserved in their bosoms. 345. What is dew? Dew is watery vapour diffused in the air, condensed by coming in contact with bodies colder than the atmosphere. 346. Why does the air become charged with watery vapour? Because, during the day, under the influence of the sun's rays, vapours are exhaled from all the moist and watery surfaces of the earth. These vapours are held in suspension in the atmosphere until, by a change in the temperature of the earth, and of bodies on the surface of the earth, they are condensed, and deposited in translucid drops. 347. What causes the decline of temperature that favours the deposition of dew? [Pg 87] The earth, which during the day received heat from the solar rays, radiates the heat back into the air, and therefore becomes itself colder. All the various objects upon the face of the earth also radiate heat in a greater or lesser degree. And dew will be found to be deposited upon the surfaces of such bodies in proportion to the fall of their temperature through radiation. <hr class="bible-verse" /> "The Lord is my shepherd, I shall not want. He maketh me to lie down in green pastures."—Psalm xxiii. <hr class="bible-verse" /> 348. Why is there little or no dew when the nights are cloudy? Because clouds act as secondary radiators; and when the earth radiates its heat towards the clouds, the clouds again radiate it back to the earth. <div class="figcenter"> <ANTIMG src="images/i-087.jpg" width-obs="500" height-obs="172" alt="" /> <div class="caption">Fig. 3.—ILLUSTRATING THE FORMATION OF DEW.</div> </div> If plates of glass be laid over grass-beds, as in the engraving Fig. 3, no dew will be deposited on the grass underneath the glass plates, although all around the grass will be completely wetted. The explanation is that the glasses, being radiators of heat, act in the same manner as the clouds, returning the heat to the bodies underneath them, and preventing the formation of dew thereon. 349. Why does dew form most abundantly on cloudless nights? Because the heat which is radiated by the earth does not return to it. The temperature of the earth, and the air immediately upon its surface, is therefore lowered, and dew is formed. It has been observed that sheep that have lain on the grass during the formation of dew have their backs completely saturated with it, but that underneath the line where their bodies turn to the earth, their coats will be dry. In the same manner glass globes suspended in the air, on dew forming nights, will be found loaded with globules of dew upon the top, but there will be no appearance of moisture underneath. [Pg 88] <hr class="bible-verse" /> "Dost thou know the balancings of the clouds, the wondrous works of him which is perfect in knowledge."—Job xxxvii. <hr class="bible-verse" /> 350. Why are star-lit nights usually colder than cloudy nights? Because heat is radiated from the earth, and passes away into the utmost regions of the atmosphere. 351. Why is there little dew under branches of thick foliage? Because the foliage acts as a screen, which prevents the radiated heat of the earth from passing away. 352. Why is there no dew formed on windy nights? Because, as winds generally consist of dry air, they absorb and bear away the atmospheric moisture. 353. Why are valleys and low places chiefly subject to dew? Because the elevated lands around them prevent the disturbance of the air in which the moisture is held. 354. What bodies are most likely to be covered with dew? All bodies that are good radiators of heat, such as wool, swansdown, grass, leaves of plants, wood, &c. 355. What bodies are likely to receive little dew? All bad radiators of heat, such as polished metal surfaces, smooth stones, and polished surfaces generally. Dew will be found to lie more abundantly upon rough and woolly leaves than upon smooth ones. 356. At what period of the night is the largest amount of dew usually formed? It is generally supposed that dew is formed most copiously in the mornings and evenings. But such is not the case. It is deposited at all hours of the night, but most plentifully after midnight. 357. Why is dew formed most plentifully after midnight? Because, as radiation has been going on for some time, the temperature of the earth, and of various bodies upon it, has been considerably reduced. [Pg 89] <hr class="bible-verse" /> "Out of whose womb came the ice? and the hoary frost of heaven, who hath gendered it?"—Job xxxviii. <hr class="bible-verse" /> 358. In what parts of the world is the maximum of dew formed? In warm lands near the sea, or in the vicinity of rivers or lakes, as the localities of the Red Sea, the Persian Gulf the coast of Coromandel, in Alexandria, and Chili. 359. In what parts of the world is the minimum of dew formed? It is quite absent in arid regions, in the interior of continents, such as Central Brazil, the Sahara, and Nubia. 360. Why is dew seldom formed at sea? Because of the defective radiating quality of the surface of water. 361. Why is a heavy dew regarded as the precursor of rain? Because a heavy formation of dew indicates that the air is saturated with moisture. 362. What is hoar-frost? Hoar-frost is frozen dew. 363. Why is hoar-frost said to foretell rain? Because it shows that the air is saturated with moisture, and the temperature of the air being low, the vapours are likely to condense, and produce showers. 364. What is honey-dew? Honey-dew is the name applied to a sweet and sticky moisture occasionally deposited upon the leaves of plants. It is, however, an error to call it dew, as it is procured by a class of insects termed aphides. 365. What are fogs? Fogs are clouds formed near the earth's surface; but London fogs are distinguished from clouds by the fact that they embrace in their vaporous folds the smoke and volatile matters imparted to the air by the operations of man. This is also the case with fogs generally that arise near large towns. [Pg 90] <hr class="bible-verse" /> "Hath the rain a father? or who hath begotten the drops of dew?"—Job xxxviii. <hr class="bible-verse" /> 366. Why are certain coasts liable to almost perpetual fogs? Because of local or geographical agencies which contribute to their production. The coasts of California are almost constantly wrapped in fog; and, almost as constantly, the western coast of the American continent, as far south as Peru. Newfoundland, Nova Scotia, and Hudson's Bay, are all subject to dense and frequent fogs arising from the condensation of vapour from the water flowing from the hot Gulf-stream, coming in contact with the colder air. 367. What are dry fogs? Dry fogs are characterised by a dull opaque appearance of the atmosphere. They are most common in certain parts of North America, though they sometimes occur in Germany and in England. They are generally referred to the electrical state of the atmosphere, but the theory of them is still a matter of doubt. 368. What is a mist? The term mist is generally applied to vapours that rise over marshy places, or the surfaces of water, and roll or move over the land. 369. What is the difference between a mist and a fog? Fogs, as they are known to us, generally arise over the land, and are usually mingled with the smoke of large towns. Mists generally arise over water, or wet surfaces. 370. Why do mists and fogs disappear at sunrise? Because the condensed vapours are again expanded and dispersed by the heat of the sun's rays. 371. Why do fogs frequently rise in the morning and fall again in the evening? Because, warmed by the sun's rays, they become more rarefied, and fly away at an altitude where they appear to be altogether dispelled; but at night, when the earth cools by radiation, the vapours near the earth again condense, and settle in the form of fog. 372. Why do fogs sometimes rest upon a given locality for several days together, and then disappear? [Pg 91] They are probably kept near to the surface of the earth by a superstratum of cold air. A cold air lying above, or a cold air lying below, might equally contribute to keep a fog near the surface of a particular part of the earth, until a flow of wind, or a fall of rain, altered the atmospheric condition. <hr class="bible-verse" /> "He bindeth up the waters in his thick clouds; and the cloud is not rent under them."—Job xxvi. <hr class="bible-verse" /> There are many interesting facts connected with the history of dew. It has attracted the attention of natural philosophers in all ages. But its true theory was never understood until recently. The ancients imagined that dews were shed from the stars; and the alchemists and physicians of the middle ages believed that the dew distilled by night possessed penetrating and wonder-working powers. The ladies of those times sought to preserve their beauty by washing in dew, which they regarded as a "celestial wash." They collected it by placing upon the grass heaps of wool, upon the threads of which the magic drops clustered. <hr class="chap" /> <div style="break-after:column;"></div>

1 of 2

2 of 2

Update Required To play the media you will need to either update your browser to a recent version or update your Flash plugin.