In practice ,a rotor weighing about 10 tons and rotating at 3,000 r .p . m . may be so well balanced that the motion ,except for sound ,is just perceptible.

实际上，一根据重约10吨、每分钟3000转的转子可以平衡得非常好，以致除了响声外，人们几乎觉察不出它是在转动。 [分析]此处just =almost not, hardly 曾误译为“正好觉察得出……”，这样意思就反了

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In practice, a rotor weighing about 10 tons and rotating at 3, 000 r. P. M. May be so well balanced that the motion, except for sound, is just perceptible.

实际上，一根据重约10吨、每分钟3000转转子可以平衡得非常好，以致除了响声外，人们几乎觉察不出它是在转动。

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