The mechanic should sit down among levers, screws, wedges, wheels, etc., like a poet among the letters of the alphabet, considering them as an exibition of his thoughts, in which a new arrangement transmits a new idea.

【直译】 机械师应坐到杠杆，螺丝，楔子，轮子等器具当中，就像诗人沉浸在字母表的字母中（实指“语言文字”）那样，把它们看作自己思想的展示，其中任何一种新的组合都传达出一个新的观念。

【分析】我们发现，整个句子表达的重心，应是like之前的部分。like介词短语仅是一个比喻而已，like a poet among the letters of the alphabet作方式状语，considering them as an exibition of his thoughts作伴随状语,都是修饰谓语动词的。

你的主要疑问是：in which定语从句中，which 的先行词是什么。根据句子意思，考虑到被修饰词的位置，which应是指代前面的exhibiton（展示），in which 表示“在这个思想的展示中”。