[ 156 ] 

 air to be examined, then one mea- 

 sure of nirrous air : at the moment 

 the two airs come into contadl with 

 one another, he makes the great 

 tube in the water till both airs are 

 thoroughly mixed together. This 

 being done, the tube is put in the 

 water-trough, in a poiition nearly 

 vertical, to allow time for the wa- 

 ter to run down along the infide of 

 the tube, and to leave the column 

 of air free. He then Hides the 

 brafs fcale upon the glafs tube, till 

 the o, or the mark where the fub-* 

 divifions begin, correfpond with 

 that fpot w T here the two columns of 

 water and air meet. He obferves 

 with what fub-diviflon of the fcale 

 the next mark upon the glafs tube 

 above the column of water coin- 

 cides ; which number he writes 

 down. He then lets up another 



meafure 



