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ing a flat flider at the orifice of 

 the tube, which, being pufhed in 

 when the tube and the focket are 

 full of air, cuts off exactly the co- 

 lumn of air within the tube, and 

 at once fhuts out that quantity of 

 air which is more than the three 

 inches wanted. All that part of 

 the column of air which is thus cut 

 off by the flat Aider, is let out by 

 turning or inverting the whole 

 meafure under the fur face of the 

 water. Thus the quantity of air 

 fhut up in this little meafure will 

 be conftantly the fame, whatever 

 change may afterwards happen to 

 the expanfive force, or to the elas- 

 ticity of the air within this mea- 

 fure. 



Abbe Fontana ufes this Eudiome- 

 ter in the following manner : he 

 firft introduces two meafures of the 



air 



