What is the name for this search algorithm?
You may be interested to a related algorithm, known as Interpolation Search, which achieves on average the complexity of $O(\log \log n)$ for searching $n$ keys. This is the usual search you do when you scan a phone book for example.

This paper discusses the algorithm, which was originally introduced earlier (1957) by Peterson.

Related data structures: dynamic interpolation search and interpolation search for non independent data.