CANARY ISLANDS. d^ 



Other points between its summit and the ocean, in order 

 to ascertain the elevation at which certain species of 

 vegetation can exist in these latitudes. My barometer 

 having become impaired during the ascent, I was obliged 

 to resort to the thermometrical barometer which consists 

 of nothing more than immersing a common thermometer 

 into boiling-hot water, which will cause the mercury to 

 rise in proportion to the specific gravity of the atmosphere. 

 At the level of the ocean, the boiling point of Fahrenheit's 

 scale is 212° ; but as we ascend into the air, it gradually 

 decreases, which depends, in a measure, on the humidity 

 and temperature of the air. In making experiments of 

 this kind, many precautions are necessary. It is im- 

 portant to have the instrument properly constructed, and 

 to make use of pure water. Care should also be observed 

 with regard to communicating the instrument to the water, 

 and to the time after it commences boiling.* 



At a quarter past seven in the evening, the thermometer 

 stood at 53°, and on being immersed into boiling-hot 

 water, the mercury indicated a depression of 17-5^ below 

 the boiling point at the level of the ocean, which gives an 

 elevation of about 9800 feet above the sea. After enjoy- 

 ing a delicious view of the nocturnal scenery and the 

 rising moon, I retired to the tent in order to pass the night 

 in tranquillity and silence ; but our merry guides were 

 seated round an enormous fire revelling over the fragments 

 of a wild goat, and kept up their noisy mirth during the 

 night with so much enthusiasm, that neither threats nor 

 persuasion would induce them to be quiet. I remained in 

 the camp until midnight, trying to catch a moment's 

 repose, but finding my efforts in vain, I was forced to dis- 

 card all thoughts of sleep, and rose to prepare for the 

 ascent. From the diminution of the weight of the atmos- 

 phere, combined with other circumstances, several of us 

 were affected with nausea, vertigo, hemorrhage, and a uni- 

 versal uneasiness. Our pulse " did not beat, but flutter ; "' 

 and so feeble as to be hardly perceived. Every breath of 



*For an account of an improved instrument of this kind constructed by Re 

 F. J H. VVollaston, see the Pliilosophical Transactiops for 1817, p. 183, 



