Bubbles are usually grouped into higher level bubbles to decrease complexity.
2.
Originally the two-inch mortar was fitted with a large collimating sight with elevating and cross-level bubbles, but this was soon dropped as unnecessary for front-line use.
3.
Second time was better because I learned to blame the inconsistency of the location of the level bubble on the fact that I have one leg an inch shorter than the other.
4.
Although the use of this sight may be novel for levels, the idea of using a mirror to sight a spirit level bubble from a more convenient position has long been used with surveyor's telescopic dumpy levels.
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