Initially there is descent, which is the downward movement of the fetus to the pelvic inlet.

起初是下降，胎儿向下移动进入骨盆入口。

Fluctuation, there must be some downward movement as well.

波动，一定也有一些向下的运动。

So let's start with the first one: downward movement.

那么让我们从第一个开始：向下运动。

And the momentum from that downward movement carried the rope below the fixed end, inverting the wave.

向下运动的动量将绳索带到固定端下方，使波浪反转。

We mean also that there is some recognisable degree of regularity in the time-sequence and duration of the upward and downward movements.

我们还表示，在上升和下降运动的时间顺序和持续时间方面存在某种可识别的规律性。

But this same movement of subsidence will tend to submerge the area whence the sediment is derived, and thus diminish the supply, whilst the downward movement continues.

但同样的下沉运动将倾向于淹没沉积物的来源区域，从而减少供应， 同时继续向下运动。

The diaphragm contracts and descends with each inhalation (inspiration). The downward movement of the diaphragm enlarges the area in the thoracic cavity and reduces the internal air pressure, so that air flows into the lungs to equalize the pressure.

每次吸气（吸入）隔膜收缩并下降。隔膜向下运动扩大胸腔内的面积并降低内部气压，使空气流入肺部以平衡压力。