Scientists and engineers tend to want to answer simple relational questions with a blackboard covered with equations, even when questioners just want a rough estimate to gauge whether a change would cause more problems than it would fix. The "rules of thumb" on the following pages provide the framework for this kind of "guesstimate," with shortcuts to relatively accurate answers. Readers, staff writers, article authors and contributing editors have submitted most of these helpful hints. They are not designed to replace the equations and data sheets that you'll need to produce commercial or laboratory instruments, systems and components, but to provide a starting point for determining whether a project is feasible…