Barometric Measurement of' the Height ofChevwi- 71 



the attached thermometer shews it in every case, however un- 

 equally the instruments may be affected by heat from the ob- 

 server's body, or other causes. 



As barometers, however well made, even by the same artist, 

 show small differences from one another, depending upon slight 

 variations in the specific gravity of the mercury, its different 

 degrees of purity, and the perfection of the construction, it was 

 considered indispensable to institute a rigorous comparison, to 

 determine the differences, which have been denominated Index 

 Errors. If, therefore, Sir T. M. Brisbane's barometer be called 

 A, Mr Galbraith*8 B, Mr Adie's C, then A being taken as a 

 standard, the index errors will be 



in. in. in. 



A = 0.000, B -= + 0.093, and C = + 0.108 ; 



that is, if the three barometers were all standing together on the 

 same level, and having the same temperature, it would require 

 0.093 inch to be added to the height read from Mr Galbraith's 

 barometer B, to render it the same as Sir Thomas Brisbane's, 

 and it would require 0.108 inch to be added to Mr Adie's, C, 

 to make it of the same height as Sir T. M, Brisbane's. These 

 numbers were obtained by comparing Mr Galbraith's barome- 

 ter, first with Sir T. M. Brisbane's at Holy Island, and after- 

 wards at Edinburgh with Mr Adie's. Whence, these correc- 

 tions being applied, 



Sir Thomas Brisbane's will shew 



B = 29.849, r :=> 57°0, t = 57.0. 



Mr Galbraith's, corrected, will give 



b = 26.993 

 Index error + 0.043 



Corrected b = 27.086, r = 44.5, e = 44.5. 



Mr Adie's, corrected, will give 



B = 29.591 

 Index error + 0.108 



B = 29.699, r = 60.0, t = 60.0. 



With these, then, the final results are determined by the for- 

 mula investigated in this Journal for October 1831, 



