80 Mr. G. M. Whipple. Standard Barometers. [Feb. 7, 



tude, and therefore have not been considered in the discussion, although 

 they were made. 



As the position of the Greenwich standard barometer precludes its 

 being read by daylight, two gas burners are used to illuminate it at 

 the time of observation, one being fixed at the level of the pointer in 

 the cistern, the other at the barometric height. The flame of these 

 burners is raised only at the instant of observation. 



The gas lights being immediately behind the barometer, and 

 distant about six inches, somewhat heat it, and it was thought that 

 probably the thermometer in front of the instrument might not, 

 perhaps, indicate the true temperature of the mercurial column and 

 scale. 



With a view to determine the amount- of this heating effect, on 

 October 8th I took six standard thermometers to Greenwich, and, 

 attaching them to the barometer in close contact with the tube and 

 scale, three in front and three behind, made a series of observations in 

 order to investigate approximately the distribution of temperature 

 throughout the instrument. 



This was found to be as follows when the gas burners were turned 

 low, as in their ordinary condition. 



Mean of Lower thermometers. . 



,, Middle „ 



Upper 



„ Whole „ 



,, Greenwich attached thermometer 61° "42 

 Correction for Index error — o- 90 



Difference 



Mean of Greenwich cistern thermometer 



= 59°v0 

 = 61°-07 

 = 62°-46 



= 61°-06 



60°-52 



= 0°-54 

 = 58°-26 



"H.ien, however, the gas burners were turned on full, the tem- 

 p rature of the upper part of the barometer became much increased, 

 a ise of 7'1° taking place in five minutes, whilst the thermometer 

 attached to the barometer was only affected to the extent of o, 8. 



Great care was exercised in turning down the lights immediately 

 after reading during the periods of comparison, so that it is assumed 

 that no instance of such a great irregularity in the distribution of 

 temperature ever occurred. 



As from the table it appears that the temperature indicated by the 

 barometer's attached thermometer was probably 0°'5 lower than the 

 average temperature of the air surrounding the instrument, a correc- 

 tion of — o, 4 has been applied to its readings instead of — o, 9, the 

 true correction for index error as determined by Mr. Ellis by direct 



