b<ts¢ J 
the..water had run down.equally 
along the fide of the tube; or at 
Jeaft it renders the column_.of. air 
uncertain in length: each divifion 
of this glafs tube is fub-divided into 
Ioo equal parts, which are not ex; 
preffed upon the glafs tube itfelf, 
but engraved for convenience upon 
a brafs flider or cylinder. moving 
along the glafs tube. This flider — 
mutt 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 tecond neceflary inftrument 
is the Ztte meafure, confitting ofa 
Slats - tube of »a fimilar .di ; 
with the great tube,-and: only. three 
inches long. This fmall tube mutt 
alfo be made rough on the. -infide 
| With | fine emery, This little mea- ee 
=< = re is fixed in a brafs focket bow Sis 
* 
