of Mr. Harrison's Timekeeper" as soon as it was published, evidently with a view to encourage him to make a precision timekeeper of the same kind. Maskelyne subsequently encouraged Arnold by employing him on several occasions, mostly in connection with watch and clock jewelling. In 1769, Arnold modified Maskelyne's centre seconds watch by John Ellicott, changing the cylinder escapement from steel to one made of sapphire. He lent this watch to the Astronomer William Wales for use in assessing the practicability of Maskelyne's Lunar distance method for finding the ship's longitude during the voyage of the Transit of Venus expedition to the West Indies in 1769. Around this time, Arnold also seems to have started to think about making an accurate timekeeper to find the longitude. Arnold's first "Watch Machines" Arnold's approach to precision timekeeping was completely different from that of Harrison, whose technical ethos was rooted in seventeenth- and early eighteenth-century theory and practice. Arnold knew that as the balance and balance spring control the timekeeping in a portable watch, he only needed to find a way of giving the balance a consistent impulse with minimal interference from the wheel work, together with an effective temperature compensation. After making some experimental machines, he produced what could be regarded as a production model to the Board of Longitude in March 1771. This machine was completely different from Harrison's watch. It was a mahogany box of approximately that housed a movement that, though relatively simple, was close to the same size as Harrison's, with a balance of a similar diameter. The radical difference, however, was a newly designed escapement that featured a horizontally placed pivoted detent that allowed the balance to vibrate freely, except when impulsed by the escape wheel. The spiral balance spring also had a temperature compensation device similar to those in Arnold's watches, and based on Harrison's bimetallic strip of brass and steel. Arnold proposed manufacture of these timekeepers at 60 guineas each. Three of these timekeepers travelled with the explorers James Cook and Captain Furneaux during their second voyage to the southern Pacific Ocean in 1772–1775. Captain Cook also had Kendall's first timekeeper on board as well as one of Arnold's. Whereas Kendall's performed very well and kept excellent time during the voyage, only one of Arnold's was still running on their return to England in 1775. The performance of these clocks was recorded in the logbooks