I 've been on the road for a week and have just logged on from offsite, so I apologise for the lateness and brevity of this reply.
With respect to universal ring dials (aka equinoctial ring dials), the bridge carried a pierced slider which was set for the sun's declination. This is what makes this type of sundial self-orienting. One contributor to the sundial list wrote that the dial was adjustable for longitude. This was a typo--he meant latitude. Moreover, sundials _were_ used for navigation from the mid- 17th to the 19th century. I cannot stress this enough. All nautical manuals included a section on sundials, particularly the universal ring dial, which has recently been discussed, and the azimuth compass (which is a combined sundial and magnetic compass, set in gimbals). The universal ring dial was favored by mariners because it was self-orienting, could be used to find north during sunny hours and serve as a check on the ship's steering compass, and did not have to be gimbal-mounted to be effective on board ship. The back of the mariner's univ. ring dial often carried a nautical quadrant, which was used to determine his latitude. [More will be said in my forthcoming catalogue of sundials and timefinding instruments at the Adler Planetarium, Chicago. For more information about this project, or to be notified about its publication, please contact me.] I hope this helps. Sara Schechner Genuth Committee on the History and Philosophy of Science Department of History University of Maryland at College Park Francis Scott Key 2115 College Park, MD 20742 [EMAIL PROTECTED]
