A Guide to the Scientific Knowledge of Things Familiar. Ebenezer Cobham Brewer
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PART I.
HEAT.
INTRODUCTION.
Q. What is heat?
A. The sensation of warmth.
Q. How is this sensation produced?
A. When we touch a substance of higher temperature than ourselves, the warmer substance keeps parting with its heat, till both are of equal temperature.
Q. What is that “stream of heat” called, which flows thus, from one body, to another?
A. Calo’ric. Caloric, therefore, is the matter of heat, which passes from body to body; but Heat is the sensation, of warmth, produced by the influx of Calo’ric.
Q. What are the four principal sources of heat?
A. 1.—The Sun. 2.—Electricity. 3.—Chemical Action: and 4.—Mechanical Action.
Q. What are the principal effects of heat?
A. Expansion, Liquefaction, Vaporization, and Ignition.
CHAPTER I.
Q. What is the principal source of Heat?
A. The Sun.
Q. Why do burning glasses set fire to substances submitted to their power?
A. The rays of the sun, collected by the Burning Glass, are all bent to one point, called the “focus;” thus the heat and light, (which should be diffused over the whole glass,) being gathered together into one point, are very greatly increased.
Q. Why is there a dark rim round this focus?
A. Because the rays of light, which should have fallen there, are bent into the focus, and the space around, (being deprived of these rays) is accordingly darkened.
Q. Are all the rays bent into one point?
A. No, not quite all: and, therefore, the rim round the focus is only slightly shadowed.
CHAPTER II.
Q. What is the second chief source of heat?
A. Electricity.
Q. What is lightning?
A. Lightning is only an Electric Spark, taken from the clouds.
Q. What causes the discharge of an electric cloud?
A. When a cloud, overcharged with electric fluid, approaches another which is under-charged, the fluid rushes from the former into the latter, till both have the same quantity.
Q. Is there any other cause of lightning, besides the one just mentioned?
A. Yes; sometimes mountains, trees, and steeples, will discharge a lightning cloud floating near; and sometimes electric fluid rushes out of the earth, into the clouds.
Q. What produces electricity in the clouds?
A. 1st—The evaporation from the earth’s surface.
2ndly—The chemical changes perpetually going on: and
3rdly—Currents of air of unequal temperature, excite electricity by friction, as they pass by each other.
Q. How high are the lightning-clouds from the earth?
A. Electrical clouds are the lowest of all clouds; they are rarely more than 700 yards above the ground; and sometimes, they actually touch the earth with one of their edges.
Q. How high are the clouds generally?
A. In a fine day, the clouds are often 4 or 5 miles above our head; but the average height of the clouds is from 1–½ to 2 miles.
Q. Why is lightning sometimes forked?
A. When the lightning-cloud is a long way off, the resistance of the air is so great, that the electrical current is diverted into a zig-zag course.
Q. Why does the resistance of the air make the lightning zig-zag?
A.