That is, except,unless, separate this fact, that infirmity had seized his ability,he had measured the lands and waters.

In this use but, butan, is a participle equivalent to excepting, and may be referred to the person speaking, or more naturally, it is equivalent to excepted,and with the following words, or clause,forming the case absolute.

In the last three examples, that is omitted after but.

In these and all similar phrases,but denotes separation, exception.