An aircraft comprising a lubrication system, the lubrication system comprising: a lubricant reservoir; a lubricant supply passage fluidly connecting the lubricant reservoir and a space requiring lubrication; and a lubricant supply pump, the lubricant supply pump comprising: a piston having a first piston head slidably received in a chamber in fluid communication with the lubricant reservoir; and a second piston head slidably received in a pumping chamber; the pumping chamber being divided into a first cavity and a second cavity by the second piston head, the first cavity being arranged between the first piston head and second piston head; the first cavity and second cavity being placeable in fluid communication with one or more pressure sources externally of the pumping chamber so as to provide a pressure differential between the first cavity and the second cavity whereby, the piston may move as a result of the pressure differential so as to cause the first piston head to dispense lubricant from the lubricant reservoir to the space through the lubricant supply passage; and wherein the pressure differential between the first and second cavities (52, 54) is derived from a difference in ambient pressure at ground level and the ambient pressure at altitude. a lubricant reservoir; a lubricant supply passage fluidly connecting the lubricant reservoir and a space requiring lubrication; and a lubricant supply pump, the lubricant supply pump comprising: a piston having a first piston head slidably received in a chamber in fluid communication with the lubricant reservoir; and a second piston head slidably received in a pumping chamber; the pumping chamber being divided into a first cavity and a second cavity by the second piston head, the first cavity being arranged between the first piston head and second piston head; the first cavity and second cavity being placeable in fluid communication with one or more pressure sources externally of the pumping chamber so as to provide a pressure differential between the first cavity and the second cavity whereby, the piston may move as a result of the pressure differential so as to cause the first piston head to dispense lubricant from the lubricant reservoir to the space through the lubricant supply passage; and a piston having a first piston head slidably received in a chamber in fluid communication with the lubricant reservoir; and a second piston head slidably received in a pumping chamber; the pumping chamber being divided into a first cavity and a second cavity by the second piston head, the first cavity being arranged between the first piston head and second piston head; the first cavity and second cavity being placeable in fluid communication with one or more pressure sources externally of the pumping chamber so as to provide a pressure differential between the first cavity and the second cavity whereby, the piston may move as a result of the pressure differential so as to cause the first piston head to dispense lubricant from the lubricant reservoir to the space through the lubricant supply passage; and wherein the pressure differential between the first and second cavities (52, 54) is derived from a difference in ambient pressure at ground level and the ambient pressure at altitude.