单项选择题

The first device men had for measuring time was the sundial, which was invented around 700 B.C. The early sundial was a hollow half bowl with a bead (有孔小珠) fixed in the center. As the sun traveled across the sky, the shadow of the bead traveled in and is across the face of the bowl. The bowl was divided into 12 equal parts called hours. The length of these hours varied with the seasons, as days were longer or shorter. In the summer an hour might have been half again as long as our hours now, in the winter only half as long. For 1,600 years this way of measuring hours by dividing the daylight into 12 parts didn’t change.
A minute is the sixtieth part of an hour and a second is the sixtieth part of a minute. Both of these measurements are for convenience in dividing time into useful sections. The ancient Babylonians reckoned time more accurately than the people who came after than for several thousand years. They used a water clock, the water running through a hole of a very carefully calculated size from one jar into another. The time it took for the water to drip completely through was the length of the day of the equinox. Day and night are equal at that time, each lasting 12 hours.
Our modem industry depends on clocks and timing. Assembly lines run on exact time schedules. In the manufacture of almost every article around you there are certain processes that must be timed precisely. China must be baked for an exact length of time, glass hardened, paint dried electrically, canned food processed. If you look around your room, you will probably see dozens of other things that had to be timed when they were made, some of them to a millionth of a second. Parts of radio tubes and light bulbs must be timed as exactly as this.
Our whole world runs on a time schedule. Trains and planes, schools and business, radios, traffic lights, and the cake for dessert all depend on the clock.
Flyers make a clock out of the sky, so they can call directions. They imagine it to be a huge clock face with their plane at the center of the dial. The nose of the plane points to 12 o’clock. Then if one man yells "see gull at 2 o’clock", everybody knows exactly where to look. Measured by the sundial, an hour in the summer is______.our hours now.