depth/a depth
（1）It can dive to a depth of 1,000 metres. On one ship which had equipment for measuring the depth of the water, the captain noticed a sperm whale diving to a depth of nearly 2,250 metres.
(2) The water temperature must not fall below C and there must be plenty of light, so corals are found in water with a depth of less than 60 metres.
查《现代英语用法词典》, depth 是个不可数名词, 为什么上面出现了 a depth 的说法?
作“深度”(the distance down) 解的 depth 虽然通常用作不可数名词,但有时也可与不定冠词连用,如句(2)中的 water with a depth of 60 metres(深度为 60 米的水),其用法相当于 a speed of thirty miles an hour。下面是作“深度”解的 depth 的实例:
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What is the depth of this lake? 这个湖有多深?
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The river froze to a depth of over a metre. 河水冻到一米以上的深度。
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The lake has a depth in the middle of 300 feet. 湖中间的深度达300英尺。
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It extends to a depth of 3723 feet. 洞穴延伸的深度达3,723英尺。
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Loosen soil to a depth of 8 inches. 把土松到 8 英寸的深度。
当 depth 作空间或感情上的“某个深处”解时, 就转化为可数名词了:
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He lifted the manhole cover and peered into the depths beneath. 他提起了检修孔的盖，往下面的深处查看。
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She was in the depths of despair. 她陷入深深的绝望之中。
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in the depths of the sea 在海洋的深处
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depths of the earth 地球的深处