Here the instruments were overhauled. The mercurial barometer reading by verniers to three places of decimals was set up and read, and the two aneroids were adjusted to read with it. These two aneroids perhaps deserve a word. Aneroid A was a three-inch, three-circle instrument, the invention of Colonel Watkins, of the British army, of range-finder fame. It seems strange that the advantage of the three-circle aneroid is so little known in this country, for its three concentric circles give such an open scale that, although this particular instrument reads to twenty-five thousand feet, it is easy to read as small a difference as twenty feet on it. It had been carried in the hind sack of the writer's sled for the past eight winters and constantly and satisfactorily used to determine the height of summits and passes upon the trails of the interior. Aneroid B was a six-inch patent mountain aneroid, another invention of the same military genius, prompted by Mr. Whymper's experiments with the aneroid barometer after his return from his classic climbs to the summits of the Bolivian Andes. Colonel Watkins devised an instrument in which by a threaded post and a thumb-screw the spring may be relaxed or brought into play at will, and the instrument is never in commission save when a reading is taken. Then a few turns of the thumb-screw bring the spring to bear upon the box, its walls expand until the pressure of the spring equals the pressure of the atmosphere, the reading is taken, and the instrument thrown out of operation again--a most ingenious arrangement by which it was hoped to overcome some of the persistent faults of elastic-chamber barometers. The writer had owned this instrument for the past ten years, but had never opportunity to test its usefulness until now. So, although it read no lower than about fifteen inches, he took it with him to observe its operation. Lastly, completing the hypsometrical equipment, was a boiling-point thermometer, with its own lamp and case, reading to 165 deg. by tenths of a degree.