250 Mr. Alison's Ascent of the Peak of Teneriffe. 



or eight months, yet the cold is considerably less than at the 

 polar regions, where our hardy navigators were not afraid to 

 pass the winter in the midst ot the horrors of a frozen sea. 



By means of a balloon, the experiment could occasionally 

 be made, and with it they have not to fear so many of the 

 causes of local influence, which are always difficult to de- 

 termine. 



In choosing a time perfectly calm, the aeronaut could raise 

 himself almost perpendicularly, and with a velocity which he 

 could moderate according to his wish, by having a cask of water 

 furnished with a stop-cock, instead of bags of sand, which are 

 generally used. The balloon might be observed from several 

 points of its course, to be able to represent it by an equation 

 which would enable its observers to compare the movement of 

 the fixed thermometers with those which the aeronaut carried. 



I do not know of any mountain that has been so frequently 

 measured as the Peak of Teneriffe, and with results so dif- 

 ferent ; which circumstance, I think, is caused by its peculiar 

 local situation. 



Although barometrical admeasurement has been brought to 

 great perfection by the labours of Shuckburgh, Lindenau, 

 Biot, llamond, the immortal Laplace, and others, yet it is 

 sometimes affected by certain influences, which destroy the 

 fundamental supposition. The theory supposes the different 

 strata of the atmosphere to be in the regular order of their 

 density, the air to be in perfect equilibrium, and the decrease 

 of temperature to be uniform : as the weight of the atmosphere 

 decreases as you recede from the centre of the earth, and is 

 proportional to the square of the distance, any particular 

 pressure near the surface will destroy this regularity. 



Barometrical observations show that a particular accumula- 

 tion of atmosphere is above the Canaries; this is probably 

 caused by a current existing in the upper regions which is 

 opposed to a lower one. Baron Von Buch, in a memoir upon 

 the climate of the Canaries, mentions that most travellers to 

 the summit of the Peak have taken notice of a strong west 

 wind blowing there so violently as hardly to allow them to 

 stand, whilst a north-east wind prevailed below. I have fre- 

 quently observed this current from the town of Orotava, when 

 the clouds were nearly at the same elevation as the summit of 

 the Peak, where they would accumulate in a dense body on 

 the south-west side, whilst on the opposite point they would 

 be broken into small parcels, which reunited after they had 

 passed a short distance ; at the same time, in the lower regions, 

 the vapours and wind were all from the north-east. It is 



therefore 



