CHAP. XVIII. A BAROMETRIC LEVEL. 331 



sidBrable improvement in our rate, had the opinion confirmed that 

 man can become habituated to low pressures. The ascent from 

 the fifth camp (15,811 feet) to the summit (20,498 feet), excluding 

 halts, occupied 445 minutes, and was therefore executed at the 

 rate of 632 feet per hour. The descent, excluding halts, was made 

 in 145 minutes, or at the rate of 1939 feet per hour, — the mean of 

 these being 1280 feet per hour ; a speed which, although comparing 

 unfavourably with the superlative rates quoted upon pp. 31-2, was 

 a distinct advance upon our first essay. 



On the 4th of July we continued the circuit of the mountain 

 by a high-level route, intending to stop for the night at the 

 position of the First Camp ; and the march was made in a trifle 

 over five hours (for most of the way at an elevation of 14,500 

 to 15,000 feet), without coming across any impediment worth 

 mention ! The bearing of the First Camp was not known at 

 starting, and I undertook to lead the caravan by the guidance of 

 the barometer. 



On the 26th of December, at the First Camp on Chimborazo, 



the barometer stood at 17*9 inches, and from daily observation of 



it I knew the great improbability that atmospheric pressure would 



be so much as one-tenth of an inch either higher or lower at the 



same spot. I proposed to intersect the Vallon de Carrel, a little 



higher than the First Camp, by keeping on a level with a pressure 



of 17*8 inches. For this purpose Aneroids were more useful than 



the Mercurials, inasmuch as they could be read on horseback while 



in movement, without checking the march of the caravan ; and I 



trusted to them alone, after having ascertained their Index-errors 



by comparison with the Mercurial.^ Sometimes the nature of the 



ground drove us a little up or down, and pressure fell or rose as 



the case might be ; but at the earliest opportunity the level of 



17 '8 inches was resumed, and no other means were employed to 



find the desired place. 



' It was indispensable to do this, in consequence of the large errors they had 

 acquired. See How to use the Aneroid Barometer, § 66. 



