What's the rule of thumb for installing shelving on a load bearing wall,
inside a house built in the early 1940's? In particular, two 10x48 shelves
will be mounted to three brackets on the wall, around 8 and 20 inches off
the ground.

I'm unsure of the wall's construction (?), and am having great difficulty
finding any studs using a Zircon. Perhaps, I'd have better luck finding the
studs with some powerful earth magnets. The wall appears fairly strong, but
is it enough to hold 2 shelves containing 80 lbs. each? Any suggestions?

Thanks,
john