{"language": "en", "segments": [{"text": " [...]", "start": 0.0, "stop": 29.98, "id": 0}, {"text": " [...]", "start": 30.0, "stop": 59.98, "id": 1}, {"text": " [...]", "start": 63.02, "stop": 89.98, "id": 2}, {"text": " Formations like these require long hours of intensive", "start": 110.6, "stop": 114.44, "id": 3}, {"text": " drilling and careful judgment.", "start": 114.44, "stop": 115.94, "id": 4}, {"text": " When the troop goes around the corner, the riders on the outside of the turn have", "start": 116.56, "stop": 120.88, "id": 5}, {"text": " to adjust their speed to keep even with the riders on the inside.", "start": 120.88, "stop": 124.52, "id": 6}, {"text": " The man on the outside has to ride a lot farther and a lot faster", "start": 126.08, "stop": 129.64, "id": 7}, {"text": " in order to keep up with the parade.", "start": 129.64, "stop": 131.46, "id": 8}, {"text": " The outside wheels must spin faster than the wheels on the inside because they", "start": 133.22, "stop": 138.86, "id": 9}, {"text": " have a greater distance to travel in the same length of time.", "start": 138.86, "stop": 142.1, "id": 10}, {"text": " When a wagon turns a corner, the wheels can travel at different speeds because", "start": 142.62, "stop": 148.06, "id": 11}, {"text": " each one can turn freely on the axles.", "start": 148.06, "stop": 150.84, "id": 12}, {"text": " And in the early automobiles, the rear wheels turned separately and only one", "start": 151.3, "stop": 156.46, "id": 13}, {"text": " wheel was connected to the engine.", "start": 156.46, "stop": 158.04, "id": 14}, {"text": " But when only one wheel was driven by the engine, it had to do all the work and", "start": 159.08, "stop": 164.6, "id": 15}, {"text": " it couldn't get a good enough grip on the road to do its job properly.", "start": 164.6, "stop": 168.64, "id": 16}, {"text": " So the one-wheel drive was soon out of date.", "start": 169.04, "stop": 171.76, "id": 17}, {"text": " But if two wheels are locked on an axle so that they are not free to turn", "start": 172.86, "stop": 177.58, "id": 18}, {"text": " separately, one or the other has to slide.", "start": 177.58, "stop": 181.14, "id": 19}, {"text": " So engineers had to find a way to connect both rear wheels to the engine without", "start": 182.04, "stop": 187.08, "id": 20}, {"text": " sliding and slipping on turns.", "start": 187.08, "stop": 189.0, "id": 21}, {"text": " The device which makes this possible is a part of the rear axle.", "start": 189.96, "stop": 194.22, "id": 22}, {"text": " It is called the differential because it can drive the rear", "start": 194.56, "stop": 199.42, "id": 23}, {"text": " wheels at different speeds.", "start": 199.42, "stop": 201.36, "id": 24}, {"text": " The differential looks complicated, but once we understand its principle,", "start": 202.56, "stop": 206.74, "id": 25}, {"text": " it is amazingly simple.", "start": 207.12, "stop": 208.62, "id": 26}, {"text": " These two wheels are mounted on separate axles and supported by a frame so that", "start": 209.48, "stop": 215.4, "id": 27}, {"text": " they can revolve freely at different speeds.", "start": 215.4, "stop": 218.8, "id": 28}, {"text": " Let's fasten a spoke on the inner end of each axle so that", "start": 219.58, "stop": 226.58, "id": 29}, {"text": " by turning the spokes, we can turn each wheel separately.", "start": 226.58, "stop": 231.7, "id": 30}, {"text": " With a bar or cross piece, we can turn both wheels in the same", "start": 235.06, "stop": 241.94, "id": 31}, {"text": " direction at the same rate of speed.", "start": 241.94, "stop": 245.22, "id": 32}, {"text": " Let's get something to hold this bar in place so that it", "start": 246.2, "stop": 249.92, "id": 33}, {"text": " will press against the spokes.", "start": 249.92, "stop": 251.58, "id": 34}, {"text": " Notice that this support is not locked to the axle. It turns freely.", "start": 252.94, "stop": 257.34, "id": 35}, {"text": " Now we can spin the wheels by rotating the support.", "start": 258.66, "stop": 262.42, "id": 36}, {"text": " This is fine as long as both wheels are able to turn at the same speed.", "start": 263.58, "stop": 268.46, "id": 37}, {"text": " But let's see what happens when we go around the corner.", "start": 269.18, "stop": 272.74, "id": 38}, {"text": " With this arrangement, we cannot drive one wheel faster than the other.", "start": 273.18, "stop": 278.48, "id": 39}, {"text": " And if we stop one wheel,", "start": 280.68, "stop": 285.14, "id": 40}, {"text": " the other wheel won't budge.", "start": 285.86, "stop": 288.4, "id": 41}, {"text": " Let's put this bar on a pivot so that it can swing in either direction.", "start": 291.0, "stop": 296.88, "id": 42}, {"text": " Now, the bar can still turn both wheels at the same speed.", "start": 303.44, "stop": 308.44, "id": 43}, {"text": " And because it pivots, it lets one wheel turn", "start": 309.22, "stop": 314.12, "id": 44}, {"text": " even when the other is stopped.", "start": 314.12, "stop": 316.58, "id": 45}, {"text": " But if turned too far, the bar will swing around until it won't drive the spokes", "start": 318.08, "stop": 324.62, "id": 46}, {"text": " that turn either wheel.", "start": 324.62, "stop": 326.2, "id": 47}, {"text": " We need another crossbar and more spokes to carry on the job.", "start": 327.08, "stop": 331.94, "id": 48}, {"text": " When we stop one wheel, the crossbars will continue to push the spokes", "start": 332.88, "stop": 338.74, "id": 49}, {"text": " of the free wheel around.", "start": 338.74, "stop": 340.4, "id": 50}, {"text": " As long as both wheels are free to turn, the bars do not swing on their pivot", "start": 342.24, "stop": 348.38, "id": 51}, {"text": " and the wheels move at the same speed.", "start": 348.38, "stop": 352.08, "id": 52}, {"text": " Now we have the working principles of a differential.", "start": 353.94, "stop": 357.22, "id": 53}, {"text": " To adapt the model for use in an automobile, we will have to make a few changes.", "start": 358.0, "stop": 363.36, "id": 54}, {"text": " In order to reduce the jerky action caused by wide spaces between the spokes, we", "start": 364.68, "stop": 371.4, "id": 55}, {"text": " will put in more spokes.", "start": 371.4, "stop": 372.9, "id": 56}, {"text": " Further filling in the spaces between the spokes gives", "start": 377.0, "stop": 380.72, "id": 57}, {"text": " steadier, more continuous action.", "start": 380.72, "stop": 383.06, "id": 58}, {"text": " And changing the shape gives firm, constant contact.", "start": 383.98, "stop": 388.2, "id": 59}, {"text": " Now we can make the gears thicker and stronger, and we have differential gears.", "start": 389.68, "stop": 395.26, "id": 60}, {"text": " The edges are cut so that they will fit together more smoothly and silently.", "start": 396.38, "stop": 400.98, "id": 61}, {"text": " And another gear is added to share the work of driving the", "start": 401.44, "stop": 405.28, "id": 62}, {"text": " axles. The principle is the same.", "start": 405.28, "stop": 407.52, "id": 63}, {"text": " In order to turn the support and drive the wheels, we can fasten a large gear", "start": 409.04, "stop": 414.68, "id": 64}, {"text": " here, connected by a smaller gear to a source of power.", "start": 414.68, "stop": 420.94, "id": 65}, {"text": " Notice that the power is connected to the differential at the center line.", "start": 428.02, "stop": 432.68, "id": 66}, {"text": " We can make our model more compact by moving the gears closer together.", "start": 435.06, "stop": 439.78, "id": 67}, {"text": " When we put our differential in an automobile, we have to leave room for the", "start": 441.3, "stop": 445.32, "id": 68}, {"text": " drive shaft, which carries the power from the engine.", "start": 445.32, "stop": 448.4, "id": 69}, {"text": " We may build the floor of the car above the drive shaft.", "start": 449.72, "stop": 452.98, "id": 70}, {"text": " But if we do, we won't have much room inside unless we make the", "start": 454.64, "stop": 459.8, "id": 71}, {"text": " top of the car high too.", "start": 459.8, "stop": 461.34, "id": 72}, {"text": " Of course we could lower the floor and ceiling, but the drive shaft would be", "start": 462.56, "stop": 466.44, "id": 73}, {"text": " higher than the floor. This would have disadvantages.", "start": 466.44, "stop": 469.6, "id": 74}, {"text": " A shaft in the middle of the floor of an automobile would be inconvenient for", "start": 470.72, "stop": 475.38, "id": 75}, {"text": " passengers and would be awkward for carrying luggage.", "start": 475.38, "stop": 478.76, "id": 76}, {"text": " Today, engineers have found a way to make the car roomier and closer to the road", "start": 479.84, "stop": 484.46, "id": 77}, {"text": " without a clumsy shaft above the floor.", "start": 484.46, "stop": 487.54, "id": 78}, {"text": " The drive shaft from the engine to the differential is lowered out of the way,", "start": 488.44, "stop": 494.0, "id": 79}, {"text": " and the drive shaft is connected to the rear axle at the bottom.", "start": 494.12, "stop": 498.42, "id": 80}, {"text": " A new low center drive makes the rear axle quieter, stronger, and more durable", "start": 499.6, "stop": 505.44, "id": 81}, {"text": " because it gives better, smoother contact between the gears.", "start": 505.44, "stop": 510.32, "id": 82}, {"text": " The automobile of today with the low center drive is stronger and more rugged.", "start": 511.9, "stop": 518.22, "id": 83}, {"text": " Every part of the rear axle has been built to withstand strains far greater than", "start": 519.34, "stop": 525.14, "id": 84}, {"text": " it will ever meet on the straightaway or around the corner.", "start": 525.14, "stop": 528.6, "id": 85}, {"text": " [...]", "start": 559.6, "stop": 567.58, "id": 86}]}