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Rule of thirds origin

andybrooks

Active member
I always thought that the "Rule of Thirds" was developed by Sheck Exley. However, it seems that Bob Leakey might have a much earlier claim. In "The Caverns of Mossdale Scar", he wrote "I had practiced holding my breath for as long as I could counting up to 79, 80, 90 a hundred and so on. I used to have this very strict system that I could just go one third of how much breath I could hold, and then I had to turn around which sometimes took a whole third, another third and then get out as quick as I could."
 
I always thought that the "Rule of Thirds" was developed by Sheck Exley. However, it seems that Bob Leakey might have a much earlier claim. In "The Caverns of Mossdale Scar", he wrote "I had practiced holding my breath for as long as I could counting up to 79, 80, 90 a hundred and so on. I used to have this very strict system that I could just go one third of how much breath I could hold, and then I had to turn around which sometimes took a whole third, another third and then get out as quick as I could."
Interesting variation on the theme!
 
Certainly the case in open circuit open water deep diving as a contingency plan. May have been discussed by Tom Mount though I cant remember now. There is a move away fron the psychological aspect of diving now rebreathers are often used. A shame really. The Incident Pit, Stress Loading, Pre Dive Visualisation. Gas Conservation and Diaphramatic Breathing. Nominally Scuba counts the O/C reserve as 50 bar but thats not sufficiant for deep diving. Having a third in reserve makes sense anyway. "The 'rule of thirds' means that you should use a third of your air for descending, a third for return, and then have a third left in reserve for your ascent. " Or put another way by SCUBA classes " the rule of thirds is, 1/3 of gas out, 1/3 of gas back, 1/3 for your buddy..." It seems likely that the Rule of Thirds in open water diving predates cave diving.
 
To put a date on it , in his own "Basic Cave Diving : a blue print for survival", Sheck Exley claims "The author devised the third rule in 1968 based on two assumptions..."

That document was first published in 1979, but was predated by the "Dixie Cavern Kings Cave Diving Manual" (Exley 1969) and further manuals by others.

Sheck Exley learnt to dive in 1966 and clearly had a steep and experimental learning curve!
 
Mike Boon is generally attributed to the devising a gas management strategy with regards to cave diving on open circuit though in his case he advocated using a quarter of his air for going in and then turning around. This evolved into the modern "thirds rule".
 
I remember a conversation with the late John Buxton about this very thing. He was also an early adopter of the Aqualung, as opposed to wartime surplus "Frogman" equipment. I remember him also referring to using thirds in connection with this. That would have been by the early 1960s I think (possibly even slightly earlier).

The conversation took place in the mid 1980s at a time when I was interested in correcting a few minor points I spotted in the Gaping Gill book, which had just been published in 1984. JB's memory was as sharp as a razor, so I've no reason to doubt what he said.
 
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