Tim Lamb explained on 20/12/2007 :
Boyes I think they are called, in Whitby.
No messing about with level bubbles, just get the head near enough so that the plumb bob inside with the laser attached hangs freely and turn it on.
I must admit I had thought it would be complete rubbish, but it is spot on and surprisingly useful/easy to use. I even used it to produce levels for my garage extension of last year.