When displacement increases，the free surface corrections for slack tanks ______．A.increase

A. This indicates the design feature of().

B.cultural transmission

C.productivity

D.displacement

E.duality

e Earth's Gravity(重力) pulls it. But every cloud is made of water droplets or ice crystals. Why doesn't rain or snow fall constantly from all clouds？ The droplets or ice crystals in clouds are exceedingly small. The effect of gravity on them is minute. Air currents move and lift droplets so that the net downward displacement is zero, even though the droplets are in constant motion.

Droplets and ice crystals behave somewhat like dust in the air made visible in a shaft(束) of sunlight. To the casual observer, dust seems to act in a totally random fashion, moving about wildly without fixed direction. But in fact dust particles are much larger than water droplets and they finally fall. The cloud droplet of average size is only 1/2,500 inch in diameter. (78)It is so small that it would take sixteen hours to fall half a mile in perfectly still air, and it does not fall out of moving air at all. Only when the droplet grows to a diameter of 1/125 inch or larger can it fall form. the cloud. The average raindrop contains a million times as much water as a tiny cloud droplet. The growth of a cloud droplet to a size large enough to fall out is the cause of rain.

What is the main topic of the passage？

A．The mechanics of rain.

B．The climate of North America.

C．How gravity affects air current.

D．Types of clouds.

Displacement is the first and foremost striking feature of human language. ()

第二组

Displacement pumps are those where the volume of the pump chamber is alternately increased to draw the liquid in from the suction pipe and then decreased to force the liquid out into the delivery pipe. Centrifugal pumps are those wherein an impeller rotating at high speed throws the liquid by centrifugal force from the center to the periphery of the impeller where the liquid is discharged through the delivery outlet.

Gear pump consists of two or more meshing gears (spur, single or double helical teeth) enclosed m a close-fitted housing. These are used extensively for pumping fuel oil, lubricating oil and hydraulic oil. When gears are unmeshed on the inlet side, a pressure drop occurs that helps the atmospheric pressure acting on the surface of the oil in the reservoir to force it up the pipe to fill the void, the oil is then transferred around the periphery of the pump housing. As the gear teeth mesh again on the outlet side, they form. a seal that prevents oil from backing up to the inlet, the oil in the void is then be forced out into the discharge line.

All vane pumps have a rotor driven within a ring (cam ring) by a drive shaft coupled to a prime mover. A cylindrical rotor with sliding vanes, generally in radial slots, rotates within the ring. The center of the rotor is offset from the center of the ring. In a simple vane pump, as the rotor turns, the vanes are forced outward against the inner surface of the cam ring by centrifugal force. This outward radial movement of the vanes and turning of the rotor causes the chamber between the vanes to increase as the vanes pass further away from the inlet port. This increase in volume results in a lowering of pressure until the atmospheric pressure is sufficient to force oil from the reservoir into the inlet chamber. Oil from the inlet is swept away by the vanes toward the outlet port through a decreasing series of chambers until it is forced through the outlet port. A vane pump of balanced design has an elliptical cam ring so that two pumping chambers are formed. This pump is in hydraulically balance, since the two intake and two outlet ports are diametrically opposed to each other. Therefore the side loads exerted on the rotor cancel out, thereby increasing bearing lift and permitting high operating pressure.

According to the last sentence of paragraph 1, from where the liquid is discharged through the delivery outlet？

A.from the eye of the impeller

B.from passage of impeller

C.from the periphery of the impeller

D.from the impeller

A.duality of structure

B.genetic feature

C.arbitrariness

D.displacement

The amount of fluid delivered by a positive displacement pump can be regulated by _____.

A.throttling the pressure line

B.changing the speed of the driving rotor

C.either A or B

D.None of the above is tree

债务置换 （debt displacement) 名词解释