CHAPTER XVI.

308. What are loadstones? Where do they abound? What is said of discoveries in magnetism? Whence come the terms magnetism and magnet?

309. What is said of the attraction of magnetism? What law is there in regard to it? What is said of the poles of a magnet? What of magnetism by induction?

310. What is said of attraction and repulsion in magnets? Explain the formation of the curves of iron filings in the experiment represented in Fig. 282.

311. How may artificial magnets be made? What is said of the horseshoe magnet and its armature?

312. What is said of the magnetic needle and the mariner's compass?

313. What is the declination of the needle? When was it first observed? What is said of observations after this? What is said of the dip of the needle?

314. What is said of the earth as a magnet? What of it as a magnetizer?

315. What is said of fixing magnetism? What of impairing it? In what other substances besides iron does magnetism exist? In what is magnetism like electricity? In what is it unlike it?

316. What relation has magnetism to electricity? What were the original observations of Oersted in regard to it?

317. Describe the manner of making the most powerful electro-magnets. Describe the experiment represented in Fig. 287.

318. Show the application of electro-magnetism in the electric telegraph.

319. What is the contrivance called the signal key? How is the alphabet of Morse's Telegraph constructed?

320. What is said of the communication through the earth in telegraphing?

INDEX.

[The numbers refer to the paragraphs]

Action and reaction equal, 182

Air, compressibility of, 152

Air, elasticity of, 161

Air, density of, dependent on pressure, 158

Air, pressure of, as affecting the boiling point, 171, 327

Air, currents in, from heat, 281

Air, solution of water in, 287

Air attracted by the earth, 148

Air in interstices of substances, 160

Air a non-conductor of heat, 302, 304

Air-pump, 155

Air-guns, 164

Animals, coverings suited to climate, 305

Archimedes, his lever, 229

Archimedes, his discovery in the bath, 138

Archimedes and the crown, 143

Artesian wells, 120

Aqueducts, 119

Atmosphere, pressure of, 154, 168

Atmosphere, how it moves with the earth, 188

Atomic theory, 15

Attraction, nature of, 53

Attraction, capillary, 72

Attraction between masses, 73

Attraction between solids and liquids, 69

Attraction, Newton's idea of, 54

Attraction, universality of, 77

Attraction, variety in results of, 87

Attraction, opposition between modes of, 88

Attraction, proportion of the mutual motions of, 76

Balloons, 149, 280

Balloon, hydrostatic, 159

Barometer, 169

Barker's mill, 182

Battery, electrical, 387

Berkeley, his ideas of matter, 2

Boats, why they float, 139

Boiling point as affected by the pressure of the air, 171

Brittleness, 29

Buds in winter, 308

Bulk, relation of, to resistance of liquids and gases, 193

Camera obscura, 352

Canals, 117

Capillary attraction, 72, 86

Cartesian image, 159

Centre of gravity defined, 96

Centre of gravity, support of, in animals, 106

Centre of gravity, movements of, in walking, 107

Centre of gravity in relation to attitudes, 108

Centre of gravity in floating bodies, 145

Centrifugal force, 213

Centripetal force, 213

Chimneys, draught of, 282

Clothing, object of, 306

Clouds, how formed, 288

Clouds, colors of, 364

Clouds, shapes of, 289

Clouds, latent heat in, 324

Cocoons, constructed for winter, 307

Colors of objects, 363

Colors of the clouds, 364

Cohesion, strength of, 68

Cohesion, what is essential to it, 66

Cold, absence of heat, 270

Cold at great heights, cause of, 322

Cold from evaporation, 326

Compressibility of substances, 35

Compressibility of air, 152

Condenser, 162

Conductors of heat, 298

Conductors of electricity, 375

Conduction of heat, 297

Conduction, relation of density to, 300

Conduction in liquids, 301

Conduction, influence of, on sensation, 310

Convection of heat, 296

Coverings of animals suited to climate, 305

Crystallization, 62

Daguerreotype, 369

Dams, 122

Davy's safety-lamp, 299

Dawn explained, 346

Density, 24

Dew, formation of, 315

Dew-drops, colors in, 367

Dew-point, 317

Diatonic scale, 267

Draught of chimneys, 282

Drowning, how it can often be prevented, 141

Earth, globular form of, 61

Earth, roundness of, shown, 113

Earth, attraction toward centre of, 80

Earth, thickness of its air-covering, 150

Earth, shape of, influenced by centrifugal force, 218

Earth, rotation of, affecting winds, 284

Elasticity, 39

Elasticity, definition of, 42

Electricity, origin of the term, 370

Electricity, attraction and repulsion in, 371

Electricity, vitreous and resinous, 372

Electricity, galvanic, 394

Electricity, universality of, 378

Electricity, discharge of, from points, 383

Electricity, sound of, 389

Electricity, heat produced by, 391

Electricity, mechanical injuries by, 390

Electrics and non-electrics, 377

Electrical battery, 387

Electro-magnetism, 413

Electro-magnets, 414

Electric telegraph, 415

Equestrian feat, 185

Equilibrium, stable and unstable, 109

Evaporation, 286

Evaporation, cold produced by, 326

Expansion by heat, 274

Expansion, exception to it, 330, 331

Extension, a property of matter, 44

Eye, description of, 353

Eye, images in, inverted, 356

Feathers, how a non-conductor of heat, 305

Fire-engine, 179

Flexibility, 29

Flies, feet of, 157

Force, relation of, to velocity, 194

Forcing-pumps, 178

Franklin, his theory of electricity, 373

Franklin, his discovery, 392

Freezing, process of, described, 330

Freezing point, why at 32°, 332

Freezing in the midst of boiling, 327

Freezing mercury, 318

Freezing mixtures, 325

Friction, 196-198

Friction in machinery, 245

Frost, 44

Fur, how a non-conductor of heat, 305

Fur, how altered by climate, 305

Fusee of a watch, 238

Galvanism, 394

Gases, movements of the particles in, 10

Gases, space in, 21

Gases, compressibility of, 38

Gasometer, 163

Glass, annealing of, 32

Glass-making, centrifugal force used in, 216

Governor, steam, 217

Gravitation, 79, 86

Gravitation, limiting size, 90

Gravitation, action of, on solids in a liquid, 136

Gravity, specific, defined, 135

Gridiron pendulum, 210

Gunnery, retardation by condensed air in, 166

Hail, how produced, 290

Hardness, 28

Harmony, 266

Hearing, mysteries of, 269

Hearing, trumpet, 262

Heat, nature of, 271

Heat, sources of, 272

Heat, relations of, to forms of matter, 13

Heat, relation of, to bulk of liquids, 37

Heat, relation of, to light, 273

Heat producing expansion, 274, 275, 279

Heat, communication of, 295

Heat, convection of, 296

Heat, conduction of, 297

Heat, radiation of, 311

Heat, reflection of, 314

Heat, connection of, with light, 312

Heat, production of, by electricity, 391

Heat, degree of, endurable by man, 328

Heat, latent, 319

Heat, capacity of different substances for, 320

Heat, relation of, to density, 321

Heat, relation of, to forms of substances, 323

Heat and cold, 270

Heights measured by barometer, 170

Hume, his ideas of matter, 3

Hydrometer, 144

Hydrostatics, what it teaches, 110

Hydrostatic balloon, 159

Hydrostatic bellows, 132

Hydrostatic paradox, 131

Hydrostatic press, 133

Ice, formation of, 329

Ice, force of its expansion, 333

Ice-crystals, colors in, 367

Icebergs, centre of gravity in, 145

Impenetrability, 45

Imponderable agents, 16

Inclined plane, 241

Induction in electricity, 379

Induction in magnetism, 400

Inertia, 48

Inertia shown in the communication of motion, 183

Inertia shown in disposition of motion to continue, 184

Insulating stool, 382

Jupiter, mountains in, 92

Kaleidoscope, 343

Laggan stones, 100

Land-breeze, 283

Lenses, 349

Lever of first kind, 224

Lever, no gain of power in it, 227

Lever of second kind, 231

Lever of third kind, 232

Leyden jar, 384, 385

Life-boats, 139

Light, nature of, 334

Light, sources of, 335

Light, diffusion of, 337

Light, velocity of, 338

Light, reflection of, 340

Light, refraction of, 345

Light moves in straight lines, 336

Light shown to be compound, 360

Light, colors in, 360

Light, recomposition of, 362

Light, chemistry of, 369

Lightning-rods, 393

Liquids, movability of their particles, 9, 57

Liquids, incompressibility of, 36

Liquids, attraction in, 56

Liquids, globular shape of drops of, 58

Liquids, cause of level surface of, 111

Liquids, pressure of, in proportion to depth, 121

Liquids, lateral pressure of, 123

Liquids, pressure of, equal in all directions, 125

Liquids, upward pressure as depth, 127

Liquids, in what like gases, 153

Liquids, friction of, in tubes, 197

Liquids, friction of, in streams, 198

Liquids, expansion of, by heat, 275

Loadstones, 397

Machines not sources of power, 222

Machinery, friction in, 245

Magic lantern, 351

Magdeburg hemispheres, 156

Magnetism, origin of term, 397

Magnetism, attraction of, 398

Magnetism, repulsion of, 401

Magnet, poles of, 399

Magnets, artificial, 403

Magnets, horseshoe, 404

Magnetic needle, 405

Malleability, 28

Man a tool-making animal, 247

Mariner's compass, 405

Matter distinguished from spirit, 1

Matter, effects of, on senses, 6

Matter, forms of, 7

Matter, nature of, 14

Matter, constitution of, 14

Matter, variety in properties of, 17

Matter, divisibility of, 18, 51

Matter, minute division of, 19

Matter, pores and spaces in, 22

Matter, relation of heat to spaces of, 23

Matter, inertia of, 48

Matter, impenetrability of, 45

Matter equally inclined to rest and motion, 50

Mechanical powers, real advantages of, 246

Mercury, freezing of, 318

Microscopes, 350

Microscopical animals, 19

Mirages, 347

Mirrors, 342

Mirrors, curved, 344

Moth's wing, 19

Motion, causes of, 181

Motion, universality of, 180

Motion, absolute and relative, 191

Motion, obstacles to, 192

Motion, reflection of, 206

Motion, compound, 212

Motion, curved, 213

Motion in orbits, 221

Motion disposed to be straight, 211

Motion, that of falling bodies projectile, 220

Motion and rest, 189

Momentum, 201

Mountains, size limited, 92

Nature, order in, 65

Near-sightedness, 355

Needle, magnetic, 399

Needle, declination of, 406

Needle, dip of, 407

Noise, how it differs from musical sound, 263

Non-conductors of heat, 298

Non-conductors of electricity, 375

Notes, musical, how produced, 264

Odors, minuteness of their particles, 19

Order in nature, 65

Papin's digester, 292

Pendulum, 208-210

Perpetual motion attempted, 118

Persia, accident to, 274

Pisa, tower of, 104

Pitchers, why they have lips, 89

Pneumatics, what it teaches, 146

Pneumatic trough, 167

Pop-guns, 164

Pottery, centrifugal force used in, 216

Powder, explosive force of, 165

Prince Rupert's drops, 33

Projectiles, 219, 220, 221

Pulleys, 239

Pumps, 176

Pumps, forcing, 178

Radiation of heat, 311

Radiation, its relation to absorption, 313

Rain, how it is caused, 290

Rainbow, 365, 366

Rarity, 24

Reaction equal to action, 182

Reflection of heat, 314

Reflection of light, 340

Reflection of motion, 206

Reflection of sound, 260

Remora, 157

Rest, merely relative, 189

Rivers, 115, 116

Rivers, bends in, 215

Roemer, observations of, in regard to the velocity of light, 339

Rope-dancers, skill of, in managing centre of gravity, 109

Safety-lamp, 299

Scales, 98, 225

Screw, 243

Sea-breeze, 283

See-saw, a lever, 228

Sensation, influence of the conduction of heat on, 310

Shape, relation of, to velocity, 195

Shot, manufacture of, 60

Signal key, 415

Silk-worm, thread of, 19

Size limited by gravity, 90

Sluice-gates, 122

Snow, how produced, 290

Snow, crystals of, 64

Snow a protection to plants, 309

Solids, constitution of, 8

Solids, attraction in, 39

Solution, 12, 22

Sound, what it is, 248

Sound, sensation of, how produced, 252

Sound, velocity of, 256

Sound, transmission of, 253

Sound, loudness of, 258

Sound, diffusion of, 259

Sound, reflection of, 260

Sound, concentration of, 262

Sound, none produced by the heavenly bodies, 255

Speaking-trumpet, 262

Specific gravity defined, 135

Specific gravity of animals, 140

Specific gravity of the human body, 141

Specific gravity of solids, how ascertained, 142

Specific gravity of liquids, how ascertained, 144

Spectrum, 360

Spider, web of, 19

Spirit distinguished from matter, 1

Spirit, origin of word, 4

Spirit-level, 114

Springs, 120

Stability of bodies, 103-105

Steam transparent and invisible, 293

Steam, expansive force of, 165

Steam-engine, 294

Steel, flexible and brittle, 30

Steel, tempering of, 31

Steelyards, 98, 225

Stereoscope, 358

Suckers, 157

Sucking, explanation of, 167

Suction, 177

Sulzer's experiment, 394

Sun as a source of heat, 272

Surface, relation of, to movability, 193

Syphon, 172

Tantalus, cup of, 174

Telegraph, 415

Telescopes, 350

Tenacity, 25

Tenacity, comparative, of substances, 26

Tenacious substances, value of, 27

Thermometer, 37, 276

Thermometer, Fahrenheit's, 277

Tides, 78

Tubes, friction of liquids in, 197

Twilight explained, 346

Unison, 268

Up and down, explanation of, 81

Vapor, influence of pressure upon the formation of, 292

Vaporization, 291

Velocity, relation of, to force, 194

Velocity, relation of, to shape, 195

Velocities, great, how produced, 203

Velocities, great, how arrested, 204

Vibration of sounding bodies, 249

Vision, distinct, what necessary to, 354

Vision, why single, 357

Vision, why erect, 356

Visual angle, 348

Voice, how produced, 265

Voltaic electricity, 394

Volta's pile, 395

Walrus, feet of, 157

Water, crystallization of, 63

Water man's first mirror, 112

Waves, how formed, 199

Waves, height of, 200

Wedge, 242

Weight, 52, 82-85

Wheel and Axle, 235

Wheel-barrow a lever, 231

Whispering galleries, 261

Windlass, 236

Windows, double, 303

Winds, 283

Winds as affected by the rotation of the earth, 284

THE END.

FOOTNOTES:

[1] I take the following from Dr. Arnot:

The reasons that so many people are drowned in ordinary cases, who might easily he saved, are the following:

1. Their believing that continued exertion is necessary to keep the body from sinking, and hence their generally assuming the position of a swimmer, in which the face is downward, and the whole head must be kept out of the water to allow of breathing. Now as a man can not retain this position without continued exertion, he is soon exhausted, even if a swimmer, and if not, the unskillful attempt will scarcely secure for him even a few respirations. The body raised for a moment by exertion above the natural level, sinks as far below when the exertion ceases; and the plunge, by appearing the commencement of a permanent sinking, terrifies the unpracticed individual, and renders him an easier victim to his fate.

2. From a fear that water entering by the ears may drown as if it entered by the nose or mouth, a wasteful exertion is made to prevent it; the truth being, however, that it can only fill the outer ear, or as far as the membrane of the drum, and is therefore of no consequence.

3. Persons unaccustomed to the water, and in danger of being drowned, generally attempt in their struggle to keep their hands above the surface, from feeling as if their hands were tied while held below; but this act is most hurtful, because any part of the body kept out of the water, in addition to the face, which must be so, requires an effort to support it which the individual is supposed at the time incompetent to afford.

4. Not having reflected that when a log of wood or a human body is floating upright, with only a small portion above the surface, in rough water, as at sea, every wave in passing must cover the head for a little time, but will again leave it projecting in the interval. The practiced swimmer chooses this interval for breathing.

5. Not knowing the importance of keeping the chest as full of air as possible; the doing which has nearly the same effect as tying a bladder of air to the neck, and without other effort will cause nearly the whole head to remain above the water. If the chest be once emptied, and if from the face being under water the person can not inhale again, the body is then specifically heavier than water, and will sink.

[2] This is true except when the tube is so small that capillary attraction exerts considerable influence.

[3] I was once consulted in regard to a smoking stove. It was an open Franklin stove, the pipe of which went through a fire-board into a monstrous chimney. I recommended that a pipe with a knee should extend from the pipe of the stove a little way up the chimney. The expedient was successful, because but a small body of air, that in the pipe, needed to be heated to establish an upward current.

[4] As in the case of many other inventions, so here the same idea was originated and put to practical use by more minds than one. George Stephenson, who from being a common engine-wright in a colliery rose step by step till he invented the locomotive, constructed a lamp which illustrated in another way the same principle as the lamp of Davy does--in other words, he invented another safety-lamp. But this does not in the least detract from the glory which the invention has given to the name of Davy, for each acted independently of the other. In Davy's case, it is to be remarked, there was a long course of scientific reasoning and investigation which led him at length to the invention, the record of which is exceedingly interesting. No invention or discovery is made without thought, though accident may suggest the thought; but here is an invention which, without any suggestion by accident, was evolved by laborious and long-continued thought, proceeding step by step to its conclusion.

[5] I will mention here a contrivance that I once adopted for a small conservatory, which I wished to keep warm from the heat of a room to which it was adjoining. In each space of the window-frames were put two panes of glass, there being nearly half an inch of space between them. In this way I secured all the benefit of double windows with less expense and a less cumbrous arrangement. In mentioning this contrivance now and then, casually, I have found that a few others have thought of it, and adopted it with the same success that I did.

[6] We are in entire ignorance of the nature of electricity, and we use the term _fluid_ merely as a matter of convenience.

TRANSCRIBER'S NOTE

Italic text is denoted by _underscores_.

Bold text is denoted by =equal signs=.

Obvious typographical errors and punctuation errors have been corrected after careful comparison with other occurrences within the text and consultation of external sources.

Except for those changes noted below, all misspellings in the text, and inconsistent or archaic usage, have been retained. For example: farther, further; India rubber, India-rubber; every where; catechetical; incloses; tricksy; pervious; motal; enrobes; subtile.

Pg 79, 'gravity made made him' replaced by 'gravity made him'. Pg 157, 'if non elastic' replaced by 'if non-elastic'. Pg 264, 'the divergency when' replaced by 'the divergence when'. Pg 297, '283. Electricity' replaced by '383. Electricity'. Pg 321, the Questions begin with number '13' and has not been changed. Pg 331, the number '174' is used twice and has not been changed. Pg 335, 'with phosporus' replaced by 'with phosphorus'.