308 



REPORT 1864. 



August 29, 1864. 



On August 31, at the height of 7000 and 8000 feet high, the blackened 

 bidb thermometer exposed to the full influence of the sun, read 3° only higher 

 than the shaded thermometer. 



On September 29, at the height of 14,000 feet, the excess of reading of the 

 blackened bulb thermometer was 2^° only under a bright sun, and the in- 

 crease of readings of the actinometer was 3 divisions to 5 divisions only ; 

 at 13,000 feet the excess of blackened bulb readings increased to 4° and 5°, 

 and the increase in one minute of the actinometer readings were 7 to 8 divi- 

 sions. At the height of 3000 and 4000 feet the influence of the sun increased, 

 raising the blackened bulb to 7° and 8° in excess of the readings of the shaded 

 thermometer ; the scale readings of tlic actinometer increased to 20 and 25 

 divisions in one minute, and on reaching the ground the increase in the same 

 time was from 48 to 50 divisions. 



On January 12 the readings of the exposed and shaded thermometers were 

 nearly always alike. 



On April G I was unable to use the actinometer, and never succeeded in 

 placing it properly. The excess of reading of the blackened bulb thermometer 

 was but small during the cloudy state of the sky, and increased to 5° and 6° 

 at 10,000 feet, this excess becoming larger on descending into the lower 

 atmosphere, until cloud was entered. 



On June 13 the excess was at all times small. 



On June 20, at many inspections the readings of the two thermometers 

 were identical. 



On June 27 the exposed thermometer nearly always read lower than the 

 shaded thermometer ; on examination of these instruments afterwards, they 

 were both found to read correctly. 



On Augiist 29 the blackened biilb thermometer read lower than the shaded 

 thermometer, when 6000 feet were passed ; it then read higher, increasing to 

 7° at 14,000 feet high. 



From all these experiments it seems that the heat-rays from the sun for 

 the small bulb of a thermometer, communicate very little or no heat to it, 

 and the heat is less in proportion to the less density of the atmosphere ; si- 

 milar results being sho'syn by the use of Herschel's actinometer. 



