The Reason Why. Robert Kemp Philp

and is a non-conductor.

170. Why are summer breezes said to be cool?

      Because, as they pass over the heated surface of the body, they bear away a part of its heat.

171. Why is a still summer air said to be sultry?

      Because, being heated by the sun's rays, and being a bad conductor, it does not relieve the body by carrying off its heat.

172. Why does fanning the face make it feel cooler?

      Because, by inducing currents of air to pass over the face, a part of the excessive heat is taken up and carried away.

173. Why does perspiration cool the body?

      Because it takes up a part of the heat, and, evaporating, carries it into the air.

174. Why does blowing upon hot tea cool it?

      Because it directs currents of air over the surface of the tea, and these currents take up a part of the heat and bear it away.

175. Why does air in motion feel cooler than air that is still?

      Because each wave of air carries away a certain portion of heat and being followed by another portion of air, a further amount of heat is borne away.

      "Though I walk in the valley of the shadow of death I will fear no evil, for thou art with me."—Psalm xxiii.

176. Is the atmosphere ever as hot as the human body?

      Not in this country. On the hottest day it is 10 or 12 deg. cooler than the temperature of our bodies.

      177. What is the highest degree of artificial heat which man has been known to bear?

      A man may be surrounded with air raised to the temperature of 300 deg. (the boiling point being 212), and yet not have the heat of his body raised more than two or three degrees above its natural temperature of from 97 deg. to 100 deg.

178. Why may man endure this degree of heat for a short time without injury?

      Because the skin, and the vessels of fat that lie underneath it, are bad conductors of heat.

      And because perspiration passing from the skin and evaporating, would bear the heat away as fast as it was received.

      Because, also, the vital principle (life) exercises a mysterious influence in the preservation of living bodies from physical influences.

179. Is the air ever hot enough, in any part of the world, to destroy life?

      Yes. The hot winds of the Arabian deserts, which are called simooms, scatter death and desolation in their track, withering trees and shrubs, and burying them under waves of hot sand. When camels see the approach of a simoom they rush to the nearest tree or bush, or to some projecting rock, where they place their heads in an opposite direction to that from which the wind blows, and endeavour to escape its terrible violence. The traveller throws himself on the ground on the lee side of the camel, and screens his head from the fiery blast within the folds of his robe. But frequently both man and beast fall a prey to the terrible simoom.

      180. Why are these hot winds so terrible in their effects?

      Because, being in motion, they search their way to every part of the body, and passing over it leave some portion of their heat behind, which is again followed by additional heat from every fresh blast of wind.

      "The fear of the Lord is the beginning of knowledge: but fools despise wisdom and instruction."—Proverbs i.

CHAPTER IX.

       Table of Contents

      181. What is Radiation?

      The radiation of heat is a motion of the particles, in a series of rays, diverging in every direction from a heated body.

      182. What is this phenomena of Radiation understood to arise from?

      From a strongly repulsive power, possessed by particles of heat, by which they are excited to recede from each other with great velocity.

      183. What is the greatest source of Radiation?

      The sun, which sends forth rays of both light and heat in all directions.

184. When does a body radiate heat?

      When it is surrounded by a medium which is a bad conductor.

      185. When we stand before a fire, does the heat reach us by conduction or by radiation?

      By radiation.

186. What becomes of the heat that is radiated from one body to another?

      It is either absorbed by those bodies, or transmitted through them and passed to other bodies by conduction, or diffused by convection, or returned by reflection.

      187. How do we know that heat is diffused by radiation?

      If we set a metal plate (or any other body, though metal is best for the experiment) before the fire, rays of heat will fall upon it. If we turn the plate at a slight angle, and place another object in a line with it, we shall find that the plate will reflect the rays it has received by radiation, on to the object so placed; but if we place an object between the fire and the plate, we shall find that the rays of heat will be intercepted, and that the latter can no longer reflect heat.

      "The fear of the Lord is the beginning of wisdom: a good understanding have all they that do his commandments."—Psalm cxi.

188. Does the agitation of the air interfere with the direction of rays of heat?

      It has been found that the agitation of the air does not affect the direction of rays of heat.

      189. Why, then, if a current of air passes through a space across which heat is radiating, does the air become warmer?

      Because it takes up some portion of the heat, but it does not alter the direction of the rays.

      This is clearly illustrated by reference to rays of light which are seen under many circumstances. But they are never bent, moved, nor in any way affected by the wind.

190. Why will not a current of air disturb the rays of heat, just as it would a spider's web, or threads of silk?

      Because heat is an imponderable agent, that is, something which cannot be acted upon by the ordinary physical agencies. It has no weight, presents no substantial body, and is, in these latter respects, similar to light and electricity.

      191. What other sources of radiation of heat are there besides the sun and the fire?

      The earth, and all minor bodies, are, in some degree, radiators of heat.

192. What substances are the best radiators?

      All rough and dark coloured substances and surfaces are the best radiators of heat.

193. What substances are the worst radiators of heat?

      All smooth, bright, and light coloured surfaces are bad radiators of heat.

      Dr. Stark, of Edinburgh, has proved, by a series of experiments, the influence which the colours of bodies have upon the velocity of radiation. He surrounded the bulb of a thermometer successively with equal weights of black,