Relative dating in archaeology
But you would never have any 78s in junkyards closed before they were invented.The same is true for 45s, and 8-tracks, and cassette tapes, and LPs, and CDs, and DVDs, and mp3 players (and really, any kind of artifact).Absolute dating techniques were not available to him (radiocarbon dating wasn't invented until the 1940s); and since they were separately excavated graves, stratigraphy was no use either.
Petrie's notions about Egyptology—and archaeology in general—were revolutionary.
When they became popularly available, you could find them everywhere; but then the technology changed and they became rare again. Archaeologists investigate trash, not shop window displays, so we measure things when they are discarded; in this example, we're going to use junkyards.
Archaeologically, you would expect no 78s to be found in a junkyard that was closed before 78s were invented.
There might be a small number of them (or fragments of them) in the junkyard which stopped taking junk during the first years 78s were invented.
You would expect a large number in one closed when 78s were popular and a small number again after 78s were replaced by a different technology.