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the principal and abfolutely necei% 

 fary. One is a glafs cylindrical tube, 

 or the great meafure, i8--r2o Pa- 

 ris inches long, of an equal bore 

 throughout its whole extent, whofe 

 diameter rauft be of about f inch, 

 or not much lefs, though it may be 

 larger. This glafs tube has divi- 

 sions marked upon it, each of ex- 

 actly three Paris inches. The in- 

 fide of this tube ous;ht to be rubbed 

 with fine emery, to take off the 

 fmooth furface of the glafs ; for, 

 if the furface of the eiafs be not 

 a little rougb, the water will remain 

 here and there in the form of drops 

 adhering to the iniide of the tube, 

 when air is let up into it; and thus 

 fo much of the fpace deftined for 

 the air is occupied by thefe drops, 

 which renders the column of air 

 longer than it would have been if 



the 



