that the rate of action did not deteriorate, and remained stable for long periods. At the time, only vegetable oil was available, which degraded quickly compared to modern lubricants. This chronometer, 60mm in diameter, is housed in a gold case, and miraculously has survived in perfect and original condition. It can be seen in the collections of the National Maritime Museum, Greenwich, London, having been saved for the nation in 1993. In Britain, prior to Harrison's marine watch, a small, or very small watch (such as Arnold's ring watch) was generally regarded as the ultimate test of watchmaking skill, especially with regard to complex and accurate watches. Both Harrison and Arnold however, demonstrated that an accurate watch had to be of large diameter, so by the end of the 18th century, a watch of large size was considered the primary characteristic of a well-made and superior watch. Arnold's second patent of 1782 In 1782, Arnold took out another patent to protect the latest and most important inventions, which were potentially lucrative. Several other watchmakers, most notably Thomas Earnshaw, had started to copy Arnold's work. Around 1780, Earnshaw modified his detent escapement by mounting the detent on a spring to create the spring detent escapement. During the same period, between 1779 and 1782, Arnold finalized the form of his chronometer watches. Through continuous experimentation, he worked out a way to make an effective but simple form of compensation balance. At the same time, he discovered a simple modification to his helical balance spring that let develop concentrically and, also, confer the property of isochronism on the oscillating balance. Not only this, but adjustments to the compensation balance and the balance spring could be carried out in a simple, calculated way. These were the main subjects of the patent, which he took out in 1782. The balance consisted of a circular steel balance wheel with two bimetallic strips attached diametrically. Each bimetallic strip terminated with a screw thread mounted with a weight or balance nut. The further along the strip this nut was screwed, the greater the compensating effect. Another part of the patent concerned an addition to the form of the balance spring—a coil of smaller radius at each end of the helical spring, which offered increasing resistance to the rotating balance as it turned towards the end of each vibration. This was an important invention, as it largely eliminated the