1) Many British dialects, including the “Received Pronunciation”, stopped pronouncing many r’s, beginning in the 18th century (though seeds of r-loss may go back several centuries). These are called “non-rhotic” (non-r-pronouncing) dialects (as opposed to “rhotic” dialects).

By the way, note that this happened AFTER the founding of the American colonies in the 17th century, which is why most American dialects did not experience this change (except for those, esp.in parts of New England, who maintained closer contacts with Britain and imitated some of the British changes).

2) USUALLY the /r/ was lost ONLY at the very end of a phrase, or when it appears before a consonant. But when the sound is followed immediately be a vowel it IS pronounced. In other words, a Bostonian might say “The cah just pulled out”, but “the car is in the street.” This r — which is maintained because it helps one move from one vowel to the others is called a “linking r”

Note that, according to this “rule” the R in the MIDDLE of a word like “mineral” or “worry” would remain.

3) By analogy or ‘rule extension with the “linking r”, in other situations where one is transitioning from one vowel to another — as in “I saw it” or “the idea is a good one”– many of these non-rhotic dialects began to insert r’s in the pronunciation that were not there to begin with. This non-original or “intrusive r” is frowned upon in Received Pronunciation, but it still happens.

4) Some dialects carried this further and pronounced the intrusive r in other contexts, so that some words with no final r came ALWAYS to be pronounced with one. “Idear” and “drawring” in various British accents, Australian, etc., are some of the best-known examples.

Now, not all dialects that lose the /r/ have the other changes, and not all who say “idear” speak non-rhotic dialects, but the two most often appear together, by way of the process outlined above.

http://en.wikipedia.org/wiki/Rhotic_and_non-rhotic_accents

http://en.wikipedia.org/wiki/Linking_consonant

http://www.randomhouse.com/wotd/index.pperl?date=19981211