Thanks Chris, Your description was very helpful. That was one of my hypotheses, and now that I understand this it makes it more obvious why these dials were the most popular type of portable dial.
Do you or anyone else know of a source for these dials? The ones sold on the web page mentioned before are reasonably priced but don't have the sliding gnomons, and all of the others I have found are "antiques" and sell for several thousands of dollars. Does anyone know of a source that makes or can make them for a more reasonable price. Also, does anyone know how accurately these dials are at orienting to north? Are they accurate enough to be used as a replacement for a compass? They seem to have the advantage that they don't have to be adjusted for magnetic declination and that they aren't affected by other magnetic fields. (And of course the obvious disadvantage that they must have sun in order to work.) So, they seem like they would be a great companion to a compass. Thanks, Bryan
