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the water had run down equally 

 along the ride of the tube ; or at 

 leait it renders the column of air 

 uncertain in length : each divinon 

 of this glafs tube is fub-divided into 

 loo equal parts, which are not ex~ 

 preffed upon the glafs tube itfelf, 

 but engraved for convenience upon 

 a brafs Aider cr cylinder moving 

 along the glafs tube. This Aider 

 mult be open on both fides, to fhew 

 the infide of the glafs tube, that 

 the height of the column of water 

 in the tube may be feen. 



The fecond necefTary inftrument 

 is the little meafure^ confi fling of a 

 glafs tube of a fimilar diameter 

 with the great tube, and only three 

 inches long. This fmall tube muff 

 alfo be made rough on the infide 

 with fine emery. This little mea- 

 sure is fixed in a brafs focket, hav- 

 7 ing 



