if the farface of the elat 
f 453 3 
the principal and abfolutely necef= 
fary. One is a glafs cylindrical tube, 
or the great eS: é€, 18——-20 Pa- 
ris inches long, of an equal bore 
throughout its whole extent, whofe 
diaraiater muit be of about ! > ifth, 
or not much lefs, though it may be 
larger. This glais tube has -divi- 
fions marked upon it, each of ex- 
actly three Paris inches. The in- 
fide of this tube ought to be rubbed . 
wi fine emery, to take off the 
1 furface of the se for, 
a little rough, the water will remain = 
ere and there in the form of drops 
adhering to the infide of the tube, 
when, air is let up into it; and thus, 
fo much of the fpace: deitined for 
the air is. occupied by thele drops, 
which renders the column of air 
ee longer than it would. have ae PA. 
