Cabin pressurisation. Do a Google on that - 707.
Done it for you -
http://www.answers.com/topic/boeing-707
The 707's engines could not supply sufficient bleed air for pressurization without a serious loss of thrust, so the aircraft instead used engine-driven turbocompressors to supply high-pressure air for this purpose. On many commercial 707's the outer port (#1) engine mount is distinctly different from the other three, as this is the only engine not fitted with a turbocompressor.