One reason that viruses can be so hard to find is that they're so small - typically a few millionths of an inch across.
病毒很难被发现，其中一个原因是因为他们都很微小——通常只有百万分之一英寸大小。
He had learned to read the millionths of an inch on the micrometer and to shape the grinding wheel with an instrument set with a diamond.
他学会了在千分尺上辨识一百万分之一英寸，会用镶嵌着金刚石的工具制作砂轮。
He had learned to read the millionths of an inch on the micrometer and to shape the grinding wheel with an instrument set with a diamond.
他学会了在千分尺上辨识一百万分之一英寸，会用镶嵌着金刚石的工具制作砂轮。
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