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SOME RESULTS OF OBSERVATIONS MADE WITH A 

 BLACK BULB THERMOMETER IN VACUO. 



By J. R. Sutton, M.A., F.R.Met.S. 



(Read February 22, 1905.) 



The object of this investigation was chiefly to ascertain some of 

 the effects of various meteorological influences upon the indications 

 of a black bulb thermometer in vacuo. No attempt is made here to 

 discuss the suitability of the instrument for purposes of physical 

 research, beyond expressing the opinion that it does seem to have 

 been underrated in many quarters from the time of Sir John 

 Herschel downwards. * The investigation was prompted as much 

 by what has been urged against it by English physicists and others, 

 as by the inconclusive nature of the supposed results obtained by 

 some of those who approve of it. 



The black bulb thermometers used here have been, by preference, 

 of the ordinary pattern without a test gauge, and have given fairly 

 comparable readings. During 1903 an instrument with a test gauge 

 was used, but the glass sheath was defective, and the readings 

 averaged with fair consistency 7° too low. A correction has been 

 applied to the 1903 readings on this account. The readings are in 

 every case for an altitude of 4 feet above a grass lawn. 



Mean and extreme values for the seven years 1897-1903 are given 

 in Table I. It appears from this that the highest mean temperature 

 in the sun comes near midsummer, the lowest near midwinter ; the 

 mean monthly values ranging from 118°*8 in June and July to 153 0, 3 

 in December ; the difference between these two giving an annual 

 variation some 9° greater than that of the mean monthly maxima in 

 the shade. Readings exceeding 170° have been noted once or twice 



* See, e.g., J. Herschel, "Meteorology," p. 12, 1862. Q. J. R. Met. S., July 1886, 

 p. 193. W. M. Davis, " Elementary Meteorology," p. 61. J. Eliot, Indian Met. 

 Mem., xii., p. 32. The late G. M. Whipple used to read the black bulb thermometer 

 at X., and the shade maximum at XXII., and got differences which suggested that 

 the black bulb thermometer should be set aside in favour of the Sunshine Recorder ! 



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