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CONVERSATION V.

ON THE MECHANICAL POWERS.

54

Of the Power of Machines. Of the Lever in general. Of the Lever of the

first kind, having the Fulcrum between the power and the weight.—Of the

Lever of the second kind, having the Weight between the power and the

fulcrum.-Of the Lever of the third kind, having the Power between the

fulcrum and the weight.-Of the Pulley.-Of the Wheel and Axle.-Of

the Inclined Plane.- Of the Wedge. Of the Screw.

CONVERSATION VI.

ASTRONOMY.

CAUSES OF THE MOTION OF THE HEAVENLY BODIES.

70

Of the Earth's annual motion. Of the Planets, and their motion.-Of the

Diurnal motion of the Earth and Planets.

CONVERSATION VII.

ON THE PLANETS.

80

Of the Satellites and Moons.-Gravity diminishes as the Square of the Dis-

tance. Of the Solar System.-Of Comets.-Constellations, signs of the

Zodiac.-Of Copernicus, Newton, &c.

CONVERSATION VIII.

ON THE EARTH.

-

91

Of the Terrestrial Globe.-Of the Figure of the Earth. Of the Pendulum.

-Óf the Variation of the Seasons, and of the Length of Days and Nights.-

Of the Causes of the Heat of Summer.-Of Solar, Sideral, and Equal or

Mean Time.

Of the Moon's Motion.--Phases of the Moon.-Eclipses of the Moon.—

Eclipses of Jupiter's Moons.-Of Latitude and Longitude.Of the Transits

of the inferior Planets. Of the Tides.

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CONVERSATION X.

HYDROSTATICS.

ON THE MECHANICAL PROPERTIES OF FLUIDS.

118

Definition of a Fluid.-Distinction between Fluids and Liquids.-Of Non-

Elastic Fluids, scarcely susceptible of Compression.-Of the Cohesion of

Fluids. Of their Gravitation.-Of their Equilibrium.-Of their Pressure.

-Of Specific Gravity. Of the Specific Gravity of Bodies heavier than

Water. Of those of the same weight as Water. Of those lighter than

Water. Of the Specific Gravity of Fluids.

CONVERSATION XI.

OF SPRINGS, FOUNTAINS, &c.

128

Of the Ascent of Vapour and the Formation of Clouds. Of the Formation

and Fall of Rain, &c.-Of the Formation of Springs.-Of Rivers and Lakes.

-Of Fountains.

CONVERSATION XII.

PNEUMATICS.

ON THE MECHANICAL PROPERTIES OF AIR.

136

Of the Spring or Elasticity of the Air.-Of the Weight of the Air.-Experi-

ments with the Air Pump.---Of the Barometer.---Mode of Weighing Air.

---Specific Gravity of Air.---Of Pumps.---Description of the Sucking Pump.

---Description of the Forcing Pump.

CONVERSATION XIII.

ON WIND AND SOUND.

146

Of Wind in General.---Of the Trade Wind.---Of the Periodical Trade

Winds.---Of the Aerial Tides.---Of Sound in General.---Of Sonorous Bo-

dies.---Of Musical Sounds.---Of Concord or Harmony, and Melody.

CONVERSATION XIV.

ON OPTICS.

157

Of Luminous, Transparent, and Opaque Bodies.---Of the Radiation of Light.

---Of Shadows.---Of the Reflection of Light.---Opaque Bodies seen only

by Reflected Light.---Vision Explained.---Camera Obscura.---Image of

Objects on the Retina.

CONVERSATION XV.

OPTICS-continued.

OF THE ANGLE OF VISION, AND REFLECTION OF MIRRORS. 168

Angle of Vision.---Reflection of Plain Mirrors.---Reflection of Convex Mir-

rors.---Reflection of Concave Mirrors.

CONVERSATION XVI.

ON REFRACTION AND COLOURS.

179

Transmission of Light by Transparent Bodies.---Refraction.---Refraction by

the Atmosphere.---Refraction by a Lens.---Refraction by the Prism.---Of

Colour from the Rays of Light.---Of the Colours of Bodies.

CONVERSATION XVII.

ON THE STRUCTURE OF THE EYE, AND OPTICAL INSTRUMENTS. 195

Description of the Eye.---Of the Image on the Retina.---Refraction by the

Humours of the Eye.---Of the use of Spectacles.---Of the Single Micro-

scope.---Of the Double Microscope.---Of the Solar Microscope.---Magic

Lanthorn.---Refracting Telescope.---Reflecting Telescope.

GLOSSARY,

205

CONVERSATION I.

ON GENERAL PROPERTIES OF BODIES.

INTRODUCTION.-GENERAL PROPERTIES OF BODIES.-IMPENETRABILITY. -EXTENSION.-FIGURE.-DIVISIBILITY. INERTIA.-ATTRACTION.

ATTRACTION OF COHESION.-DENSITY.--RARITY.-HEAT.—ATTRACTION OF GRAVITATION.

EMILY.

I MUST request your assistance, my Dear Mrs. B. in a charge which I have lately undertaken: it is that of instructing my youngest sister, a task, which I find proves more difficult than I had at first imagined. I can teach her the common routine of children's lessons tolerably well; but she is such an inquisitive little creature, that she is not satisfied without an explanation of every difficulty that occurs to her, and frequently asks me questions which I am at a loss to answer. This morning, for instance, when I had explained to her that the world was round like a ball, instead of being flat as she had supposed, and that it was surrounded by the air, she asked me what supported it. I told her that it required no support; she then inquired why it did not fall as every thing else did? This I confess perplexed me; for I had myself been satisfied with learning that the world floated in the air, without considering how unnatural it was that so heavy a body, bearing the weight of all other things, should be able to support itself.

Mrs. B. I make no doubt, my dear, but that I shall be able to explain this difficulty to you; but I believe that it would be almost impossible to render it intelligible to the comprehension of so young a child as your sister Sophia. You, who are now in your thirteenth year, may, I think, with great propriety, learn not only the cause of this particular fact, but acquire a general knowledge of the laws by which the natural world is governed. Emily. Of all things, it is what I should most like to learn; but I was afraid it was too difficult a study even at my age.

Mrs. B. Not when familiarly explained: if you have patience to attend, I will most willingly give you all the information in my power. You may perhaps find the subject rather dry at first;

but if I succeed in explaining the laws of nature, so as to make you understand them, I am sure that you will derive not only instruction, but great amusement from that study.

Emily. I make no doubt of it, Mrs. B.; and pray begin by explaining why the earth requires no support; for that is the point which just now most strongly excites my curiosity.

Mrs. B. My dear Emily, if I am to attempt to give you a general idea of the laws of nature, which is no less than to introduce you to a knowledge of the science of natural philosophy, it will be necessary for us to proceed with some degree of regularity. I do not wish to confine you to the systematic order of a scientific treatise, but if we were merely to examine every vague question that may chance to occur, our progress would be but very slow. Let us, therefore, begin by taking a short survey of the general properties of bodies, some of which must necessarily be explained before I can attempt to make you understand why the earth requires no support.

When I speak of bodies, I mean substances, of whatever nature, whether solid or fluid and matter is the general term used to denote the substance, whatever its nature be, of which the different bodies are composed. Thus, the wood of which this table is made, the water with which this glass is filled, and the air which we continually breathe, are each of them matter.

Emily. I am very glad you have explained the meaning of the word matter, as it has corrected an errroneous conception I had formed of it: I thought that it was applicable to solid bodies only.

Mrs. B. There are certain properties which appear to be common to all bodies, and are hence called the essential or inherent properties of bodies; these are Impenetrability, Extension, Figure, Divisibility, Inertia and Attraction. These are also called the general properties of bodies, as we do not suppose any body to exist without them.

By impenetrability is meant the property which bodies have of occupying a certain space, so that where one body is, another can not be, without displacing the former; for two bodies can not exist in the same place at the same time. A liquid may be more easily removed than a solid body; yet it is not the less substantial, since it is as impossible for a liquid and a solid to occupy the same space at the same time, as for two solid bodies to do SO. For instance, if you put a spoon into a glass full of water, the water will flow over to make room for the spoon.

Emily. I understand this perfectly. Liquids are in reality

1. What is intended by the term bodies? 2. Is the term matter, restricted to substances of a particular kind? 3. Name those properties of bodies, which are called inherent. 4. What is meant by impenetrability? 5. Can a liquid be said to be impenetrable?