It is not that the action of the scales in the one case, and the balance in the other, differ in the principles of their construction or manner of working; but the beam of one is set on an infinitely finer axis than the other, and of course turns by the addition of a much smaller weight.
[原译]这并不是说一方面使用的秤，同另一方面使用的天平，在结构和工作方式的原理上有差别；但是安装天平横杆的那个轴，要比秤杆的轴精细得多，所以即使加上一个非常小的重量，天平的横杆也会转动。（《科技英语理解与翻译》，p.95）
我认为,把上句中的 but 译为“但是”是很费解的。请分析。
大家知道,有时候 it is that = it is because。例如:
If I find fault, it is that I want you to do better in future.
如果我指责你的缺点,那是因为我希望你将来做得更好些。
not that ... but (that) 则是肯定一个原因, 否定另一个原因, 或者肯定一个判断, 否定另一个判断。一般可以译为“不是因为……而是因为”“(并) 不是……而是”。
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He felt a bit worried, not that comrades were not working hard, but that they did not pay enough attention to safety. 他有些着急，倒不是因为同志们不努力工作，而是因为他们不够注意安全。
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Not that I loved Caesar less, but that I loved Rome more. 并非我爱凯撒浅，而是我爱罗马深。
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Not that I am unwilling to go with you, but that I am busy now. 并不是我不愿意跟你一起去，而是我现在很忙。
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Not that I dislike the work, but that I am unequal to it. 并不是我讨厌这个工作，实在是力不胜任。
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It was not that I actually remembered anything at all. But my father had described over and over again what we should see at every milestone after leaving the nearest town. 这倒不是因为我还记得家乡的什么情况，而是因为父亲曾三番五次地向我描述过离开邻近的镇子之后，在每块里程碑旁边应该看到的景物。
[试译]……并不是前者所使用的秤和后者所使用的天平，在结构原理或工作方式上有什么不同，而是安装天平横杆的轴比秤杆轴精细得多……
not the least 见 not least
not to mention 见mention