376 ANNUAL OF SCIENTIFIC DISCOVERY. 



BALLOON ASCENTIONS FOR METEOROLOGICAL PURPOSES. 



UNDER the superintendence of a committee of the British Associa- 

 tion, several balloon ascentions have recently been made in England 

 for the purpose of obtaining meteorological observations. The chief 

 points to which attention lias been directed, are the variations of the 

 temperature and humidity of the air due to elevation above the earth's 

 surface. The instruments used consisted of a syphon-barometer, on Guy 

 Lussac's principle, two pairs of dry and wet thermometers, and the 

 dew-point hygrometer of Regnault and Daniell. One pair of dry 

 and Avet thermometers was mounted with the bulbs protected from the 

 effects of radiation by double concentric shades, with brightly-polished 

 silver surfaces, open at top and bottom, for the free circulation of 

 the air. The second pair had their bulbs enclosed within polished 

 tubes, (also protected by polished shades,) a brisk current of air being 

 made to pass over them by the action of an aspirator. The object of 

 this arrangement was, to diminish the effects of radiation --to cause 

 the thermometer to assume more readily the temperature of the sur- 

 rounding air and to remove from the neighborhood of the wet 

 thermometer the vapor formed by evaporation from its bulb, and thus 

 to cause the instrument to indicate with more accuracy the true tem- 

 perature of evaporation. Care was taken to procure thermometers ot 

 extreme delicacy, the bulbs of those actually employed being cylindri- 

 cal, about half an inch long, and one-twelfth of an inch in diameter : 

 they were found to assume the temperature of the surrounding medium 

 with very great rapidity. The aspirator used was a pair of elongating 

 cylindrical bellows which were drawn open by weights attached to 

 their lower end the air beinij allowed to enter by means of the 



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tube which enclosed the dry and wet thermometer. The same 

 aspirator was, by means of a second stop-cock and tube, used to pro- 

 duce the current of air necessary in operating with Ilegnault's hygrom- 

 eter. On the third ascent the temperature on leaving the ground 

 was about 58 Fahr. ; at the lower surface of the cloud (1,500 feet 

 high,) it was 50 ; at the upper surface, (3,400 feet,) 50^ ; at 4,400 

 feet, or 1,000 feet after clearing the cloud, 52^ : after this height the 

 temperature decreased till it reached 25 at the elevation of 12,600 

 The thermometer indicated, in the first ascent, a fall of one degree for 

 every elevation of 808 feet; and, on the second occasion, a fall of one 

 degree for every 345 feet of ascent. This ratio of fail to height was 

 observed to be very nearly constant. 



The two points of greatest interest noticed in these ascents were- 

 1st, That the tension of vapors decreased at a regular rate for some 

 distances from the surface of the earth, and then very abruptly dimin- 

 ished by a large amount, being in fact reduced to nearly the lowest 

 value attained during the remainder of the ascent. The height at 

 which this sudden reduction in the quantity of aqueous vapor occurred 

 was different on the two days -- on the 1 7th it was about 5,000 feet, 

 and on the 2Gth nearly 3,000. 2nd, it was also noticed that at the 



