In article .com, "Harry K" wrote:

seniorgeezer wrote:
If you look at a long level there are usually two "bubbles", side by
side ... one concave up and the other concave down. I have always read
the one that is concave up in any application. No one at Home Depot or
Lowes home improvement stores seems to know the difference. Would you
please explain the difference, and which to use?

The two bubbles are for convenience. Use the one with the concave down
for the position you are holding the level. You would use one if
leveling something on the floor, the other if holding the level against
something overhead.

Nonsense. You always use the one with the convex side up (or concave down, if
you prefer), regardless of where you are taking your measurements. There are
two vials so that you can use either side of the level. Makes no difference
whether you're checking a floor or a ceiling, you *always* use the upper vial.
The vials are slightly convex for two reasons: (1) so that the bubble will
move slowly and gradually to the center as the level approaches level, and (2)
to *keep* it centered. But this works only if you're using it properly: convex
up.

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.