The Reason Why. Robert Kemp Philp

      Because they would occur, in just the same order, in the absence of light.

230. Why are dark coloured dresses usually worn in winter, and light in summer?

      Because black absorbs heat, and therefore becomes warm; while light colours do not absorb heat in the same degree, and therefore they remain cool.

      231. Why do iron articles, even when near fire, usually feel cool?

      Because they are bad absorbers, and do not take up heat freely, unless they are in contact with a hot body.

232. How is heat diffused through the atmosphere?

      By convection. The warmth radiating from the surface of the earth warms the air in contact with it; the air expands, and becoming lighter, flies upwards, bearing with it the caloric which it holds, and diffusing it in its course.

233. How do the waters of the ocean become heated?

      Chiefly by convection. Nearly all the heat which the sun sheds upon the ocean is borne away from its surface by evaporation, or is radiated back into the atmosphere. But the ocean gathers its heat by convection from the earth. It girdles the shores of tropical lands where, being warmed to a high degree of temperature, it sets across the Atlantic from the Gulf of Mexico, and exercises an important influence upon the temperature of our latitude.

234. What is the cause of winds?

      Currents of air, and winds, are the result of convection. The air, heated by the high temperature of the tropics, ascends, while the colder air of the temperate and the frigid zones blows towards the equator to supply its place.

      "Give unto the Lord the glory due unto his name; worship the Lord in the beauty of holiness."—Psalm xxix.

235. What is the cause of sea breezes?

      Sea breezes are also the result of convection. The land, under the heat of the day's sunshine, becomes of a high temperature, and the expanded air on its surface flies away towards the ocean. As the sun goes down, the earth cools again, and the air flies back to find its equilibrium.

      Many countries by the sea are subjected to these periodical breezes, known as either "land" or "sea breezes," according to their direction. About eight o'clock in the morning an ærial current begins to flow from the sea towards the land, and continues until about three o'clock in the day; then the current takes a reverse direction, flowing from the land to the sea. This it continues to do throughout the night, until the time of sunrise, when a temporary calm ensues.

236. Why does a soap bubble ascend in the air?

      Because, being filled with warm air, it is lighter than the surrounding medium, and therefore ascends.

237. Why does the bubble fall after it has been in the air some time?

      Because the air contained in it has become cool, and, as it contains carbonic acid gas, it is heavier than the air.

      238. What became of the warmth at first contained in the bubble?

      It has been distributed in the air through which the bubble passed.

      239. What does this simple illustration of the distribution of warmth explain?

      It explains the law of convection, or heat distribution, over the surface of the globe.

240. Why does air ascend the chimney?

      Because, being heated, it becomes lighter than the surrounding medium, and therefore flies upwards, through the outlet provided for it.

241. Why does air fly from the doors and windows towards the fire-place?

      Because, as the warm air flies away, cold air rushes in to occupy its place.

      "How much better is it to get wisdom than gold? and to get understanding rather to be chosen than silver."—Proverbs xvi.

242. What does this example of the motion of the air in our rooms explain?

      It explains the movement of volumes of air by convection, and illustrates the origin of breezes and winds.

243. What is the chief effect of this law of convection?

      Under its influence air and water are the great equalisers of solar heat, rendering the earth agreeable to living things, and suited to the laws of their existence.

      Owing, also, to this law of convection, the constituents of the air are equalised. The breath of life, supplied by the purer oxygen of the "sunny south," is diffused in salubrious gales over the wintry climes of the north. And the waters, evaporated from the bosom of the central Atlantic Ocean and the Pacific, are borne across vast continents, and poured down in fertilising showers upon distant lands.

      To the educated mind, nothing is too simple to merit attention. To the ignorant, few things are sufficiently attractive to excite curiosity. Knowledge enables us to estimate the varied phenomena that are hourly arising around us, and to see, even in the most trifling effects, illustrations of those great causes and consequences that govern with mighty power the material world. Man, sitting by his fire-side, is enabled to witness the operation of some of nature's grandest laws: light and heat are around him; conduction, radiation, reflection, absorption, and convection of heat are all going on before him; little winds are sweeping by his footstool, and warm currents, with miniature clouds folded in their arms, are passing upward before his view. Chemical changes are going on; the solid rock of coal disappears, flying away as an invisible gas. The little "hills are melted," and hard stones have been converted into "fervent heat." Although some of these changes are imperceptible to the eye, they are manifest to the educated mind; and the pleasures of philosophical observation are as sweet as a poet's dreams.

CHAPTER XIII.

       Table of Contents

      "Neither do men light a candle, and put it under a bushel, but on a candlestick; and it giveth light unto all that are in the house."—Matt. v.

244. Why will a piece of paper, held three or four inches over the flame of a candle, become scorched?

      Because the hot air and gas produced by the burning of the candle ascends rapidly.

245. Why will a piece of paper held about an inch below the flame of a candle scarcely become warmed?

      Because the heat ascends; and only a little of it falls upon the paper, and that by radiation.