E ae 4 
air to be examined, then one mea« 
fure of nitrous air: at the moment 
the two airs come into contaét with 
one another, he fhakes the great 
tube in the water till both airs are 
thoroughly mixed together. This 
being done, the tube is put in the 
water-trough, ina pofition nearly 
vertical, to allow time for the wa- 
ter to run down along the infide of 
the tube, and to leave the column 
of ait free. He then flides the 
brafs fcale upon the glafs tube, till 
the o, or the mark where the fub- 
divifions begin, correfpond with 
that {pot where the two columns of 
water and air meet. He. obferves 
with what fub-divifion of the fcale_ 
the next mark upon the glafs tube 
# 
above the column of water coin- _ 
cides ; which number he writes 2 
S = down. ~.He_ then lets up. another 
meafure 
a4 
