The test to which chronometers are subjected is not merely one of rate, but one of rate under carefully altered conditions. Thus they may be tried with the XII pointing in succession to the four points of the compass, or, in the case of chronometer watches, they may be laid flat down on the table or hung from the ring or pendant, or with the ring right or left, as it would be likely to be when carried in the waistcoat pocket. But the chief test is the performance of a chronometer when subjected to considerable heat for a long period. This is a matter of great consequence, since a chronometer travelling from England to India, Australia, or the Cape, would necessarily be subjected to very different conditions of temperature from those to which it would be exposed in England. They are therefore kept for eight weeks in a closed stove at a temperature of about 85° or 90°. At one time a cold test was also applied, and Sir George Airy, the late Astronomer Royal, in one of his popular lectures, drew a humorous comparison between the unhappy chronometers thus doomed to trial, now in heat and now in frost, and the lost spirits whom Dante describes as alternately plunged in flame and ice. The cold test has, however, been done away with. It is perfectly easy on the modern ship to keep the chronometer comfortably warm even on an Arctic expedition. The elaborate cold testing applied to Sir George Nares' chronometers before he started on his polar journey was found to have been practically quite superfluous; the chronometers were, if anything, kept rather too warm. The exposure of the chronometer in the cooling box, moreover, was found to be attended with a risk of rusting its springs. [Illustration: THE CHRONOMETER OVEN.] Once the determination of the longitude at sea became possible, it was clearly the next duty to fix with precision the position of the principal places, cities, ports, capes, islands, the world over. Of all the work done in this department none has ever been done better, in proportion to the means at command, than that accomplished by Captain Cook in his celebrated three voyages. As has already been pointed out, it is the extent and thoroughness of the hydrographic surveys of the British Admiralty which have largely contributed to the honour done to England by the international selection of the English meridian, and of English standard time, as in principle those for the whole civilized world. The generosity and public spirit therefore which led the second Astronomer Royal to help forward and support his rival, has almost directly led to this great distinction accruing to the Observatory of which he was the head.