It is understanding how the Great Vowel Shift moved pronunciation away from spelling — and how that older spelling was fixed and fossilized by the 15th-century introduction of printing.
Three quick items:
1. A 10-minute radio discussion of the Great Vowel Shift, from the CBC’s Sunday Edition.
2. A website devoted to the Great Vowel Shift, with discussion of Chaucer, Shakespeare, etc. Check out the dialogs (top of page), particularly the one for Middle English — pre-Vowel Shift.
3. The technology of printing came to England from countries that did not use particular English letters such as thorn (Þ) and edth (ð) — and several others as well. Compromises were made, and some are still confusing people.
The lingering offender is thorn. If you read this, you can now sneer at anyone who pronounces “ye olde” with a Y sound as being inadequately educated. (You can also sneer at anyone who names a business, etc. Ye Olde Whatever in the first place, on general principles.)
3.5. You will also understand the Scottish pronunciation of the name Menzies.
That was interesting!
I live in Newport, RI, where the “th” in Thames St. is pronounced like the “th” in thing and rhymes with “frames.” We get no end of grief from vistors about this — and since it’s a tourist destination, that’s saying something. Vistors from the UK can get downright snarky about it. But the explanation (at least, the only one I’ve ever heard) is simple. The city was founded in 1639. At that time, the pronunciation of the Thames in England was exactly as written (the way we say it). In the early 1700s, George I of England, who was originally from Hanover, struggled with certain English pronunciations. It soon became fashionable to mimic his pronunciations, so the river Thames was pronounced “tems.” The change stuck, and as you pointed out, spelling didn’t keep up. Here in New England, people were apparently blissfully unaware what was cool back in the mother country and kept the original pronunciation.
So both pronunciations are correct, but not interchangeable. I think that’s so much more interesting than one group having to be “wrong.”
I suspect that you are right about “Thames,” and the mid-17th century is when some American English speech and also cultural customs began to diverge from those in England. Some put that down to the disruption of the English Civil War (1642–1651) and its aftermath.