424 TEMPERATURES IN ECUADOR. appendix. 



were observed with a registering spirit minimum thermometer, by Casella, 

 which, as a rule, was placed about four feet on the windward side of the 

 tent, in free air, 3 feet 6 inches to 4 feet from the ground. The most dis- 

 cordant of the readings is the minimum of the night of December 26 1879 

 [See pp. 40, 41.] 



If the difference of altitude of the extremes of the series is divided by 

 the difference of the observed minima it will be found that the diminution 

 of temperature is at the rate of one degree Fahrenheit per 296 feet.^ If the 

 observation near the summit of Cotopaxi should be rejected (on account of 

 the possibility that it was considerably influenced by radiation), ^ and the 

 remainder alone accepted, it will be seen that the diminution is at the rate 

 of one degree for about 267 feet.^ It would seem probable that the observa- 

 tion near the summit of Cotopaxi was not greatly affected by the warmth of 

 the cone. 



It cannot, however, be doubted that the circumjacent air is often con- 

 siderably affected by radiation from heated rock-surfaces rising high above 

 the line of perpetual congelation. Bare rock, much above the snow-line, 

 when exposed to sunshine, often becomes heated to such an extent as to be 

 almost painful to the touch. This warmth is subsequently parted with, and 

 nmst affect the temperature of the neighbouring air to a material extent, — 

 in calm weather, I imagine, to a much greater distance than four feet, the 

 recognised distance for placing thermometers from the soil. As the object, 

 when observing for minimum temperatures, is to register the lowest temper- 

 atures that occur, the end in view is most likely to be attained by placing the 

 thermometer to windward, and as far as possible from the earth ; and should 

 this be done on high and isolated summits the observations will not perhaps 

 be much influenced by terrestrial radiation, though they can scarcely fail to 

 be affected by it to some small extent. I acted upon this supposition, and 

 hence the thermometer was not placed in one invariable position. 



1 "We possess a great accumulation of observations of mountain temperature, but 

 the results are only loosely accordant, and appear to indicate that the rate of increase 

 depends in some coneiderable degree on the season of the year and the local situation of 

 the place of observation. If we assemble the most accordant, and especially those cases 

 where the heights ascended have been considerable, and trigonometrically determined, 

 we find an average decrement of 1° of Fahrenheit's thermometer for every 100 yards of 

 ascent. ... As a general average deduced from balloon ascents, 400 feet per degree of 

 Fahrenheit would seem to be preferable to 300.'" Meteorology, by Sir John F. W. 

 Herschell, pp. 22-3, 2nd ed., Edinburgh, 1862. 



2 See the chapter upon Cotopaxi. 



8 This is a considerably less value per degree than has been found by Mr. Glaisher 

 upon his balloon ascents (where the observations have not been affected by terrestrial 

 radiation). See the Reports of the British Association for the Advancement of Science, 

 1862-66. 



