On 2 Mar, 15:42, Bruce wrote:

I'm sorry of my reply appeared a little terse,

Not at all. Merely that it's probably worth many posters to UK DIY
buying a cheapish commonplace text on bricklaying, but far fewer of us
will get as much benefit from a book on surveying.

but I was trying to make the point that the Watts level should be
adjusted

It probably _shouldn't_ be adjusted (for most construction
instruments) but there will be a procedure for _checking_ it. This
checking is necessary, is simpler to carry out, and can be done by
muddy-footed numpties in the field. If it needs anything more then
breathing on though, let alone serious "adjustment", then someone has
dropped it and the whole lot needs to go back to a clean bench indoors
for checking.

With a Dumpy there aren't so many degrees of freedom, so it is
possible to adjust the collimation of the bubble and the optics
relatively safely (i.e. fiddling won't put something else out of
whack!). You need two marks that are distant and adjustable to be
level, with the level itself moving between them. "Distant" means
"within useful range" and "further than the working distance" whilst
"level" means "precisely measurable to my acceptable tolerance" but
_doesn't_ need to be accurate, as you refine this as you work.

If the level is in adjustment, your marks will appear to be level no
matter which end you're sighting from. If your level is inaccurate and
you adjust the marks to appear "level" one way, then they'll appear to
be out by twice the level's error when you measure from the opposite
end. So then point the level's telescope to midway vertically between
the two points, adjust the bubble to be "level" again (a bit more
accurate this time) and repeat the process end-over-end until the
error is no longer measurable.


in the same way as any other Dumpy level,

Oddly I don't own a functional Dumpy level. An old decorative one, a
few theodolites, a Cowley, a SiteSquare and the handiest one of all
(for practical jobs) which is some Yank hand-held pocket thing with a
pinhole rather than an eyepiece and no objective lens other than a
stadia graticule.

My nearest thing to a "Dumpy" is a Watts Autoset. It has the weird
semi-circular tripod mount so I still don't have a decent tripod for
it and it's basically untrustworthy. Anyone with a little nous can
check that a Dumpy is behaving itself, but with an Autoset you can
never tell for sure unless you trust that the pendulum mechanism is
doing what it ought. Any stickiness in there can be very awkward for
you, unless it becomes obvious. Some sort of "Incabloc" mechanism
would help too.