as the "T" and "S" balances, and marked as such in Arnold's 1782 patent (probably because of their appearance), both employed bimetallic strips of brass and steel with weights attached, which changed the radius of gyration with change in temperature. Although these probably needed a lot of adjustment, they appear to have worked well compared to his previous attempts at a compensation balance. A revolution in precision timekeeping Around 1777, Arnold redesigned his chronometer, making it larger in order to accommodate the new "T" balance that worked with his pivoted detent escapement and patented helical spring. The first chronometer of this pattern was signed "Invenit et Fecit" and given the fractional number 1 over 36, as it was the first of this new design. It is generally known as Arnold 36 and was, in fact, the first watch that Arnold called a chronometer, a term that subsequently came into general use and still means any highly accurate watch. The Royal Observatory, Greenwich tested Arnold 36 for thirteen months, from 1 February 1779 to 6 July 1780. The testers placed it in several positions during the trial, and even wore it and carried it around. The watch exceeded all expectations, as it demonstrated great accuracy. The timekeeping error was 2 minutes 32.2 seconds, but the error in the last nine months amounted to just one minute. The greatest error in any 24 hours was only four seconds, or one nautical mile of longitude at the equator. Subsequently, Arnold produced a pamphlet that detailed the trial and results, with attestations of veracity from all those concerned with the tests. Maskelyne's assistant, the Rev. John Hellins, was in charge of the pamphlet. The astonishing performance of this watch caused controversy, because many thought the result was either a fluke or a "fix" of some kind, particularly as Maskelyne was, effectively, one of Arnold's patrons. From a technical point of view, however, the design was entirely sound and highly accurate over long periods. Arnold evidently learned the lessons that Harrison had learned before him—using a large, quickly oscillating balance (18,000 beats per hour) with small pivots. Arnold's detent escapement provided minimal interference with the controlling helical balance spring, since the temperature compensation was in the balance itself. Harrison had suggested this as a prerequisite, though he never developed the idea. Arnold's pivoted detent escapement did not need oil on acting surfaces, with the advantage