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