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140

ACOUSTICS.—134

Propagation of sound,
135/Æolian harp,

141

Reflection of sound,
137 Wind,

141

Whispering gallery,
139 Sea and land breezes,

142

Musical instruments,
139 Trade winds,

143

Vibration of musical strings,

OPTICS.-144

Light,
145, Angle of vision

179

Luminous bodies,
146 Optical instruments,

182

Refraction of light,
147 Microscope,

183

Reflection of light,
150 Compound microscope,

183

Mirrors,
151 Solar microscope,

184

Plane mirror,
152 Telescope,

185

Convex mirror,
154 Refracting telescope,

186

Concave mirror,
161 Reflecting telescope,

188

Principal focus,
162 Gregory's telescope,

189

Refraction by lenses,
167 Herschel's telescope,

190

Various kinds of lenses,
168 Camera obseura,

190

Convex lens,
169 Magic lantern,

192

Concave lens,
172 Chromatics,

193

Vision,
173 Prism,

193

Eye,
174 Rainbow,

195

Spectacles,
178 Colors of things,

199

ASTRONOMY.-202

Solar system,
203/Poles of the earth,

232

Ecliptic,
206 Day and night,

233

Zodiac,
206 Seasons of the year,

234

Density of the planets,

209 Causes of heat and cold of the seasons, 239

210 Summer and winter rays,

241

Mercury.
212 Figure of the earth,

242

Venus,
212 Sular and siderial time,

245

The earth,
214 Equation of time,

247

The moon,

214 Precession of the equinoxes,

Mars,

214 Moon, her periods and changes, 255

Vesta, Juno, Pallas, and Ceres,
215 Her phases,

258

Jupiter,
216 Her surface and mountains,

260

Saturn,
218 Eclipses,

261

Herschel,
220 Lunar eclipses,

263

Diagram of the solar system,
221 Solar eclipses,

264

Comparative dimensions of the planets, 222 Tides,

266

Motions of the planets,
222 Latitude and Longitude,

268

Circular orbits of the planets, 223 Latitude, how found,

270

Elliptical orbits of the planets, 224 Longitude, how found,

271

Earth passes equal areas in equal times, 226 Fixed stars,

273

Motion of the earth round the sun, 227 Comets,

275

Circles of the earth,

2291

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NATURAL PHILOSOPHY.

THE PROPERTIES OF BODIES.

A Body is any substance of which we can gain a knowledge by our senses. Hence air, water and earth, in all their modifications, are called bodies.

There are certain properties which are common to all bodies. These are called the essential properties of bodies. "They are Impenetrability, Extension, Figure, Divisibility, Inertia, and Attraction.

Impenetrability. By impenetrability, it is meant that two bodies cannot occupy the same space at the same time, or, that the ultimate particles of matter cannot be penetrated. Thus, if a vessel be exactly filled with water, and a stone, or any other substance heavier than water be dropped into it, a quantity of water will overflow, just equal to the size of the heavy body. This shows that the stone only separates or dis. places the particles of water, and therefore that the two sub. stances cannot exist in the same place at the same time. If a glass tube open at the bottom, and closed with the thumb at the top, be pressed down into a vessel of water, the liquid will not rise up and fill the tube, because the air already in the tube resists it ; but if the thumb be removed, so that the air can pass out, the water will instantly rise as high on the inside of the tube as it is on the outside. This shows that the air is impenetrable to the water.

If a nail be driven into a board, in common language, it is said to penetrate the wood, but in the language of philosophy, it only separates, or displaces the particles of the wood. The

What is a body? Mention several bodies. What are the essential properties of bodies ? What is meant by impenetrability.? How is it proved that air and water are impenetrable?

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