THE FOURTH AND FIFTH LONGITUDE TIME-KEEPERS

Harrison's No. 4 and No. 5

On completion of H.3 Harrison immediately began work on H.4 which was to become his most famous and important time-keeper. The diameter of H.4 was only 5.25 inches. Its general appearance greatly contrasted to the three preceding time-keepers and mechanically it also has little in common with its predecessors. To enable the radical reduction in size of H.4 Harrison could no longer rely on either anti-friction wheels or wooden lignum vitae bearings. For the first time he used oil as a lubricant for his instrument. To minimise the ill-effects of ageing oil Harrison concentrated on producing a particularly fine finish to his work which, "coupled with the provision of wheels and pinions with a high number of teeth, promoted mechanical efficiency to a high degree." (Quill, 1966:79).

The trial for H.4 commenced in October 1761 when it left Britain on HMS Deptford for Jamaica. The trip lasted two months. At the end, H.4 was determined to be only five seconds slow. This corresponded to an error in longitude of only 1.25 minutes, about as many miles. Harrison had qualified for the award. However, he did not receive any money from the board for another ten years, and only then when King George III intervened on Harrison's behalf after he had seen for himself how successful Harrison’s final longitude time-keeper H.5 (which had been completed in 1772 and was mechanically very similar to H.4) actually was.