How does this bit work then?

[Falling Star]

I've got one of these Portland Speed-Time-Distance calculators. Great little slide rule thingy, and I have no problem using the speed/distance basic functions.
What is driving me nuts is the bit on the back. I don't get it!

It says "To convert departure into D. Long or vice versa, set Mid Lat. against arrow on back of converter. Then against known Departure on upper scale of slide read D. Long. on upper scale of converter or vice versa"

Can someone please explain this function please? What is it for?

http://www.amazon.co.uk/Blundell-Harling-Speed-Distance-Calculator/dp/B0057FKHSS

 

[cliff]

Distance between lines of Longitude vary from approx 60nm at the equator (0° Latitude) to SFA at the poles (90° Latitude).
Set your mid latitude to the arrow on the back of the slide rule then on the front you can read distance between °'s of Longitude for that particular Latitude or you can read °'s Longitude for any particular distance between lines of longitude.
It might help if you think distance rather than departure - here departure refers to "distance from" or distance travelled E-W.
Clear?

 

[Falling Star]

Ah yes! Thank you. Got it now.
Tried a chart comparison and it agrees, although probably not relevant to my boating ambitions!

 

[Vara]

Didn't even know they still made these, always an essential tool in my Nav bag, pre GPS, but I mislaid it.
Just about to order a new one, but I suspect like all my other oldnav tools it will just decorate the chart table.

 

[nigelmercier]

A calculator to work out speed/distance/time given any two? I find the concept odd.

 

[Vara]

A calculator to work out speed/distance/time given any two? I find the concept odd.

Trust me, it was a useful gizmo, rather superseded by the TTG function on a plotter though.