Reading Comprehension

Agriculture in ancient Egypt depended on a single annual event, the summer flood of the Nile, which was fed by monsoon rains falling far to the south in the Ethiopian highlands. As the river rose, it spread across the valley floor, soaking the soil and leaving a layer of fertile silt behind when it withdrew in the autumn. Farmers enclosed their fields with earthen banks so that the water could be held in basins for weeks before it was released, a method that required neighbors to cooperate because one neglected bank could ruin the fields beside it. The Egyptians divided their year into three seasons named for the river's behavior, inundation, growth and harvest, which shows how completely the flood organized ordinary life. The height of the flood mattered enormously, since a low rise left fields dry and people hungry, while an excessive one swept away villages and dikes. Officials therefore recorded the level of the river at gauges known today as nilometers, and the readings helped them estimate the coming harvest and set the taxes owed. Because the revenue of the state rose and fell with the water, a measuring device on the riverbank served in effect as an instrument of government.

Transmission networks are planned so that the loss of any single line or transformer leaves the rest of the system inside its limits. The rule exists because power does not stop when a path disappears; it redistributes itself over the remaining paths according to their electrical properties, without consulting anyone. If one of those paths is then loaded beyond what it can carry, its own protection removes it, and the flow shifts again onto a smaller set of survivors. A cascade of that kind runs faster than any operator can follow, since each stage is settled by relays that act in a fraction of a second. The blackout that darkened the northeastern United States and Ontario in August 2003 followed this pattern and left tens of millions of people without power. The inquiry that followed traced the event less to the initial faults than to a control room that had lost the tools that would have shown the danger developing. That conclusion shifted the emphasis of reliability work, which now treats situational awareness, and the discipline of keeping trees clear of conductors, as engineering matters of the first order. A network is judged in the end not by whether it escapes every fault, but by whether one fault can be kept local.

Air flowing across a mountain range does not resume a level path once it reaches the far side. It descends the sheltered slope, overshoots, rebounds, and goes on oscillating downwind, sometimes for a hundred miles, in a train of waves whose crests can reach into the stratosphere. Where enough moisture is present each crest condenses into a smooth, lens-shaped cloud that stands still while the air rushes through it, and such clouds look benign in a way that has misled generations of pilots, since the stillness belongs to the wave and not to the air. Underneath the crests, near the height of the ridge, the flow can break down into a rolling mass of turbulence that ranks among the most violent an aircraft can meet outside a thunderstorm, and it is often marked by a ragged cloud that a pilot busy admiring the tidy ones may disregard. The vertical currents in the wave itself are powerful enough that an aircraft at full power may be unable to climb, or may be carried upward against the intention of its pilot. The standard precautions are to cross a ridge with a generous reserve of height, to approach it at an angle that permits a turn away, and to treat a smooth ride in wave conditions as a temporary state.

An airborne assault is an exercise in delivery before it is an exercise in fighting. The troops who land cannot bring the artillery, vehicles, or supply columns that ordinary formations rely on, and everything they will use in the first days must arrive with them or be dropped to them afterward. That constraint governs the whole enterprise. Transport crews must navigate at night to a point on the ground with a precision that ordinary transport flying never demands, and hold formation while flying slowly and low enough for parachutes to open safely. They must also fly straight through whatever fire the defenders put up, because a crew that takes evasive action scatters the parachutists it carries over ground too wide for them to reassemble on. Gliders offered one answer to the scattering problem, since a glider delivers its load in one place, but they landed hard on fields chosen from photographs and met obstacles the photographs did not show. The operations of 1944 showed both faces of the method, as formations that dropped accurately achieved results out of proportion to their numbers while those scattered by weather, navigation error, or defensive fire spent days reassembling. The lesson drawn afterward was that airborne forces are only as capable as the air transport organization that carries them, a conclusion that shaped the relationship between the two arms long after the war.

Sea water is salty because rivers are very slightly salty and because the ocean has no outlet except evaporation, which removes water and leaves the dissolved material behind. Rain falling on land takes carbon dioxide from the air, becomes mildly acidic, and slowly attacks the rock it runs over, carrying the products to the sea in quantities too small for anyone drinking from a river to notice. Over the immense stretch of time the process has been running, that trickle has raised the salt content of the ocean to about thirty-five grams in every kilogram of water. Two features of that figure are worth separating. The concentration varies from place to place, since it falls where rivers empty or ice melts and rises where evaporation is strong, and it therefore serves as a label for following a body of water across a basin. The proportions among the dissolved substances, by contrast, are nearly the same everywhere in the open ocean, which tells us that the sea is mixed far faster than any local process can change its recipe. The ocean is also not growing saltier without limit, because the same elements are removed by sediments, by organisms that build shells, and by reactions with hot rock at the ridges, so the composition has stayed within narrow bounds for a very long time.