Today in Postal History

This folded letter was sent from the French Post Office in Constantinople.
The French Post Office opened in Constantinople in 1812.

The cover is franked with a single 1872 30c. brown on yellowish paper Ceres.
There are two strikes of the French Constantinople CDS.

The cover was destined for London.
The sender requested service via Marseilles.
This was the choice for mail bound for northwestern Europe.
Mail for central Europe would have been via Brindisi.

The transit markings help us trace its progress.
The first stop was Marseilles on May 23.
From there the letter went to Paris where it arrived and then departed on May 24.
Note the DEPART in the Paris CDS.
The letter arrived in London E. C. on May 25.
It was fully paid to its destination.

The transit times sound pretty good when the only transportation modes
  available were steamships, ferries, and railroads.

The sender, C. Zwicky in Constantinople, used an oval stamp dated May 14.