Modern high altitude expeditions use liquid fuel stoves such as Optimus, Primus and MSR with special burner heads to take account of the much lower pressure. The earlier Himalaya expedidtions, such as Mallory and co in 1920 tried Primus stoves but were not very successful because basically, the climbers were "gentlemen" who never used any kind of stove, having servants to do it for them. At the base camp at 20,000 feet they used wood fired stoves because they had enough porters (referred to as "coolies") to bring large quantities of wood up to the camp, and servants to do the cooking. The less gentlemanly climbers of the 1953 expedition used Primus no 3 stoves which Hilary in particular had become expert in maintaining and modifying (different jets and burner heads), at up to 27000 feet.