ineffective, which seems to have substantially slowed the rate of production. Even though he produced a number of pocket timekeepers, from around 1772–1778, Arnold was still experimenting with different types of compensation balance and methods of balance spring adjustment. The most difficult problem to surmount was the problem of making an effective and continuously adjustable temperature compensation device. For technical reasons, the temperature compensation for the balance spring had to somehow be incorporated into the balance itself and not act on the balance spring directly as had been done previously by Arnold and others. Arnold's first patent of 1775 In 1775, Arnold took out a patent for a new form of compensation balance with a bimetallic spiral at the centre. This spiral actuated two weighted arms, making them move in and out from the centre, changing the radius of gyration, and thus the period of oscillation. In the same patent, he included a new helical balance spring. This shape reduced lateral thrust on the balance pivots as they rotated, and reduced random errors from the "point of attachment" effect, which any balance with a flat spring suffers from. As Arnold stated rather succinctly in a 1782 letter to the Board of Longitude, "...the power in all parts of the spring is uniform." The fact that Arnold had recognized the technical advantages of a balance spring of this form clearly demonstrates a high degree of insight. The balance that was the subject of the patent appears to have been an unsuccessful design. Certainly, some marine chronometers used this balance, but none have survived. Pearson records a balance of this kind in his possession that was 2.4 inches in diameter. From 1772 to 1775, Arnold also made about 35 pocket timekeepers. Not many, about ten of these, survive and none in their original form, as Arnold was constantly upgrading their specification. They appear to have originally had a pivoted detent escapement, with a steel balance and a helical balance spring. A spiral bimetallic curb acting on this spring was intended to provide the temperature compensation, but this system evidently did not work, as every watch was subsequently altered and improved by Arnold shortly afterwards. Surviving chronometers from this series include Numbers 3, 29 and 28. Further experimentation and invention by Arnold led to a breakthrough in the late 1770s. He redesigned the compensation balance and developed two designs that showed promise. Known