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-caufc of this effect, we are to confider, that the whole force with 

 which the plate ?« n is prefled down, is the weight of a column of 

 water equal to emnf, together with the weight of a column of 

 air of the fame bafe, reaching to the top of the atmofphere; and 

 the whole force with which it is prefled up, is the weight of an 

 equal column of air, diminiflied by the weight of a column of 

 water equal to mnrs ; therefore the a6lual force with which the 

 plate mn is prefTed down, is, the weight of a column of water 

 equal to efrs; the velocity therefore with which the plate mn 

 will iflTue through the orifice mn, wiU be the fame as through the 

 orifice r s in the velTel KbcTi; that is, equal to the velocity which 

 a heavy body would acquire in falling through the altitude e r ; 

 and all the plates of water in the tube mnr s will defcend with 

 the fame velocity ; for they cannot defcend fafl:er, becaufe other- 

 , wife there would be a vacuum left in the tube, which is pre- 

 vented by the upward prefTure of the atmofphere. And the 

 velocity of the effluent water will be the fame, whatever be the 

 prefigure of the atmofphere, provided the weight of a column of 

 air of the fame bafe with rj, and whofe height is equal to that 

 of the atmofphere, be either greater than or equal to the weight 

 of the pillar of water mfirs. This might be proved experi- 

 mentally by a veflTel of water with a pipe inferted in the bottom, 

 placed under an exhaufted receiver. But as the operation of ex- 

 hauftion is obflrudled more by the evaporation of water than of 

 mercury, it will be better to ufe mercury in thefe experiments. 

 Now if D be the defed of the gage from the ftandard altitude, it 



will 



