Transit compasses need two sighting vanes to allow an accurate “transit” to be observed. One of these could be a prism to allow the easy reading of the transit sight bearing, alternatively two sighting vanes can be fitted. When the compass needs to be locked after alignment it is often fitted on a tripod for stability.
These twin vaned compasses come in a variety of sizes and can be as large as several inches in diameter.
A variation of the transit compass has a rear siting vane and a lid with a sighting hole and smaller front vane, typically they include an inclinometer and in this case a damping mechanism.
There is a lot of scope for collecting here since both prismatic and marching compasses can be included and they can date back a hundred to two hundred years.
Marching compasses are an evolution of the transit compass and designed to allow a quick check to be made on the direction.
