NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Do you know this instrument ?
From: Don Seltzer
Date: 2024 Oct 28, 18:23 -0400
From: Don Seltzer
Date: 2024 Oct 28, 18:23 -0400
On Mon, Oct 28, 2024 at 5:16 PM NavList Community <NavList@navlist.net> wrote:
Re: Do you know this instrument ?
From: David Pike
Date: 2024 Oct 28, 13:54 -0700After playing around with diagrams, I’m still struggling, so I’m taking a more logical approach. With the mirrors perpendicular to the line of sight and parallel you would see two images next to each other. If you moved the arm (for want of a better name) until the top of one image is inline with the bottom of the other, the amount the mirrors move apart or together will be a function of 'a known', the height of a lighthouse say, and 'an unknown', the distance off. If you take care of the known value first by winding it onto that drum thingy. Then the movement of the arm appears to give you the distance off directly.
Something like that was done with refracting telescopes in the 18th century. One of the lenses was split into two halves and a micrometer mechanism was used to separate the two halves by precise amounts. This created two images side by side, but displaced by an angle that could be calibrated to the number of micrometer turns.
Astronomers used it to measure the variations in the sun's semidiameter. Naval officers came up with a different use. Calling it a 'Coming Up Glass', they would use it to view a distant ship being chased. After initially setting the two images, they would check if the two images were slowly separating or overlapping. Separating images indicated that the chase ship was pulling away. Overlapping images meant that they were gaining on the chase.
Don Seltzer
