142 REPORT OF THE 



the arrangement, however, I was gratified by finding on refe- 

 rence to Dr. Young's Natural Philosophy, that the principle 

 had before been suggested, and that he considered it free from 

 error, provided the distance between the surfaces in contact 

 with the mercury was not less than one-seventh of an inch. 

 It will be recollected that I had adopted a distance rather 

 greater, one-sixth of an inch. I am not aware that the plan 

 was ever before put in practice, but considering this good 

 authority, I felt justified in so doing. 



Feeling much honoured by your confidence, 



I am your obedient servant, 



James Green, 

 Philosophical Instrument Maker.'' 



The comparative thermometric observations were very sim- 

 ple. The bulb of one of the thermometers being surrounded 

 with a piece of muslin, was a few minutes before each obser- 

 vation immersed in water, which had been allowed to acquire 

 in standing the temperature of the atmosphere. This ther- 

 mometer together with a dry bulb was suspended, and gendy 

 swung until the mercury became stationary. The stand of 

 both thermometers was then noted, and recorded in the table. 



The Committee present herewith a Table which they have 

 calculated, exhibiting the amount of correction necessary for 

 variations of temperature, applied to different heights of the 

 barometric column. The calculation has been based upon the 

 apparent dilatation of mercury in glass and not the actual 

 dilatation of mercury only, because in our case the scale is 

 fixed to the glass and of course partakes of its variations. 



Lewis Brajvtz, 

 James Green, 

 J. H. Alexander, 

 William R. Fisher, 



Committee. 



