Dr. ingenuous z on Air . 259 
nicating with the canal of the other tube. This lateral 
tube has alfo an air-tight cock, which opens or fhuts up 
the communication with the long tube, and has a female 
fcrew to receive the male fcrew of another fhort tube, 
which ferves to receive a glafs tube bent at right angles 
and of two feet or more in length ; the diameter fomewhat 
more than that of a large quill. This glafs tube is to be 
divided into any number of equal parts. I ufe the inftru- 
ment in the following manner. The elaftic gum-bottle 
being well tied to the brafs tube, all the cocks fhut, and 
the glafs tube fixed to its place, I pour a certain quantity 
of aquafortis (v. g. g f>) into the glafs veffel, taking care 
that none of it touches the brafs cover: then I put 
into it a certain quantity of iron filings (v. g. Jj) wrapt 
up in a bit of paper to prevent its being immediately cor- 
roded. This being done, I fcrew the glafs vefTel to the 
brals tube, fo that no air can get out. When the red 
fumes begin to rife, I open the two cocks of the brafs 
tube, which open the communication between the 
glafs veffel and the elaftic gum-bottle. By fqueezing 
the elaftic gum-bottle, I force the two airs to mix to- 
gether. The diminution of the air is foon perceived 
by the elaftic gum-bottle becoming flaccid. When I 
judge the air is as much diminifhed as it can be, I put 
the extremity of the glafs tube into a veffel with water, 
and open the cock of the fide tube: the water imme- 
diately rifes in the glafs tube to a height proportioned 
to the diminution of the two airs. By repeating feveral 
times the experiment in the fame place, I found the 
L, 1 2 rife 
