Truly this water which surrounded me was but another air heavier than the Earth's atmosphere, but (it is 省略)almost as clear (as the Earth’s atmosphere 省略).
解答:第1个“but”为副词用法，相当于“only(仅仅/只是)”，第2个but为连词用法引导转折关系并列句，but意思为(但是)。
以上句子语法结构分析如下供网友参考!
Truly【状语】// this water【主语】// which surrounded me 【后置定语】// was【系动词】// but another air【表语】// heavier than the Earth's atmosphere【后置定语】// , but【连词】// it【主语】// is【系动词】// almost as clear【表语】// as the Earth’s atmosphere【比较状语】// .
分句(A):Truly this water which surrounded me was but another air heavier than the Earth's atmosphere
备注(1):but为副词，相当于only(仅仅/只是)
备注(2):which surrounded me为定语从句修饰前面名词this water.关系代词which指代前名词this water,并在定语从句作主语成分。
备注(3):heavier than the Earth's atmosphere为形容词短语修饰前面名词another air,相当于定语从句which is heavier than the Earth’s atmosphere.
分句(B):but (it is 省略)almost as clear (as the Earth’s atmosphere 省略).
分句(A)和分句(B)属于表示转折关系并列句，由表示转折关系并列连词but连接。
Truly this water which surrounded me was but another air heavier than the Earth's atmosphere, but (it is 省略)almost as clear (as the Earth’s atmosphere 省略).
参考译文:说真的,我周围的水只不过是另外一种比地球的大气更重的空气，但几乎是和大气同样的清澈。
